Date:	Wed, 28 Jan 2009 19:55:57 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Roland McGrath <roland@...hat.com>, akpm@...ux-foundation.org,
	mingo@...e.hu, richardj_moore@...ibm.com, naren@...ux.vnet.ibm.com
Subject: Re: [RFC Patch 1/10] Introducing generic hardware breakpoint
	handler interfaces

On Thu, Jan 22, 2009 at 07:30:29PM +0530, K.Prasad wrote:
> This patch introduces two new files hw_breakpoint.[ch] which defines the 
> generic interfaces to use hardware breakpoint infrastructure of the system. 

Leveraging hardware breakpoints across architectures wouldbe quite nice!

A few RCU-related questions below.

							Thanx, Paul

> Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> ---
>  include/asm-generic/hw_breakpoint.h |  243 ++++++++++++
>  kernel/hw_breakpoint.c              |  722 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 965 insertions(+)
> 
> Index: linux-HBKPT-kgdb-2.6.28/include/asm-generic/hw_breakpoint.h
> ===================================================================
> --- /dev/null
> +++ linux-HBKPT-kgdb-2.6.28/include/asm-generic/hw_breakpoint.h
> @@ -0,0 +1,243 @@
> +#ifndef	_ASM_GENERIC_HW_BREAKPOINT_H
> +#define	_ASM_GENERIC_HW_BREAKPOINT_H
> +
> +#ifndef __ARCH_HW_BREAKPOINT_H
> +#error "Please don't include this file directly"
> +#endif
> +
> +#ifdef	__KERNEL__
> +#include <linux/list.h>
> +#include <linux/types.h>
> +#include <linux/kallsyms.h>
> +
> +/**
> + * struct hw_breakpoint - unified kernel/user-space hardware breakpoint
> + * @node: internal linked-list management
> + * @triggered: callback invoked after target address access
> + * @installed: callback invoked when the breakpoint is installed
> + * @uninstalled: callback invoked when the breakpoint is uninstalled
> + * @info: arch-specific breakpoint info (address, length, and type)
> + * @priority: requested priority level
> + * @status: current registration/installation status
> + *
> + * %hw_breakpoint structures are the kernel's way of representing
> + * hardware breakpoints.  These are data breakpoints
> + * (also known as "watchpoints", triggered on data access), and the breakpoint's
> + * target address can be located in either kernel space or user space.
> + *
> + * The breakpoint's address, length, and type are highly
> + * architecture-specific.  The values are encoded in the @info field; you
> + * specify them when registering the breakpoint.  To examine the encoded
> + * values use hw_breakpoint_get_{kaddress,uaddress,len,type}(), declared
> + * below.
> + *
> + * The address is specified as a regular kernel pointer (for kernel-space
> + * breakponts) or as an %__user pointer (for user-space breakpoints).
> + * With register_user_hw_breakpoint(), the address must refer to a
> + * location in user space.  The breakpoint will be active only while the
> + * requested task is running.  Conversely with
> + * register_kernel_hw_breakpoint(), the address must refer to a location
> + * in kernel space, and the breakpoint will be active on all CPUs
> + * regardless of the current task.
> + *
> + * The length is the breakpoint's extent in bytes, which is subject to
> + * certain limitations.  include/asm/hw_breakpoint.h contains macros
> + * defining the available lengths for a specific architecture.  Note that
> + * the address's alignment must match the length.  The breakpoint will
> + * catch accesses to any byte in the range from address to address +
> + * (length - 1).
> + *
> + * The breakpoint's type indicates the sort of access that will cause it
> + * to trigger.  Possible values may include:
> + *
> + * 	%HW_BREAKPOINT_RW (triggered on read or write access),
> + * 	%HW_BREAKPOINT_WRITE (triggered on write access), and
> + * 	%HW_BREAKPOINT_READ (triggered on read access).
> + *
> + * Appropriate macros are defined in include/asm/hw_breakpoint.h; not all
> + * possibilities are available on all architectures.  Execute breakpoints
> + * must have length equal to the special value %HW_BREAKPOINT_LEN_EXECUTE.
> + *
> + * When a breakpoint gets hit, the @pre_handler or @post_handler (whichever or
> + * both depending upon architecture support and assignment by user) callback is
> + * invoked in_interrupt with a pointer to the %hw_breakpoint structure and the
> + * processor registers.
> + * Data breakpoints occur after the memory access has taken place.
> + * Breakpoints are disabled during execution @triggered, to avoid
> + * recursive traps and allow unhindered access to breakpointed memory.
> + *
> + * Hardware breakpoints are implemented using the CPU's debug registers,
> + * which are a limited hardware resource.  Requests to register a
> + * breakpoint will always succeed provided the parameters are valid,
> + * but the breakpoint may not be installed in a debug register right
> + * away.  Physical debug registers are allocated based on the priority
> + * level stored in @priority (higher values indicate higher priority).
> + * User-space breakpoints within a single thread compete with one
> + * another, and all user-space breakpoints compete with all kernel-space
> + * breakpoints; however user-space breakpoints in different threads do
> + * not compete.  %HW_BREAKPOINT_PRIO_PTRACE is the level used for ptrace
> + * requests; an unobtrusive kernel-space breakpoint will use
> + * %HW_BREAKPOINT_PRIO_NORMAL to avoid disturbing user programs.  A
> + * kernel-space breakpoint that always wants to be installed and doesn't
> + * care about disrupting user debugging sessions can specify
> + * %HW_BREAKPOINT_PRIO_HIGH.
> + *
> + * A particular breakpoint may be allocated (installed in) a debug
> + * register or deallocated (uninstalled) from its debug register at any
> + * time, as other breakpoints are registered and unregistered.  The
> + * @installed and @uninstalled callbacks are invoked in_atomic when these
> + * events occur.  It is legal for @installed or @uninstalled to be %NULL,
> + * but one of the handlers - @pre_handler or @post_handler must be populated
> + * (please check support for your architecture using pre_ and
> + * post_handler_supported() routines to check for support.  Note that it is not
> + * possible to register or unregister a user-space breakpoint from within a
> + * callback routine, since doing so requires a process context.  Note that for
> + * user breakpoints, while in @installed and @uninstalled the thread may be
> + * context switched. Hence it may not be safe to call printk().
> + *
> + * For kernel-space breakpoints, @installed is invoked after the
> + * breakpoint is actually installed and @uninstalled is invoked before
> + * the breakpoint is actually uninstalled.  As a result the @pre_ and
> + * @post_handler() routines may be invoked when not expected, but this way you
> + * will know that during the time interval from @installed to @uninstalled, all
> + * events are faithfully reported.  (It is not possible to do any better than
> + * this in general, because on SMP systems there is no way to set a debug
> + * register simultaneously on all CPUs.)  The same isn't always true with
> + * user-space breakpoints, but the differences should not be visible to a
> + * user process.
> + *
> + * If you need to know whether your kernel-space breakpoint was installed
> + * immediately upon registration, you can check the return value from
> + * register_kernel_hw_breakpoint().  If the value is not > 0, you can
> + * give up and unregister the breakpoint right away.
> + *
> + * @node and @status are intended for internal use.  However @status
> + * may be read to determine whether or not the breakpoint is currently
> + * installed.  (The value is not reliable unless local interrupts are
> + * disabled.)
> + *
> + * This sample code sets a breakpoint on pid_max and registers a callback
> + * function for writes to that variable.  Note that it is not portable
> + * as written, because not all architectures support HW_BREAKPOINT_LEN_4.
> + *
> + * ----------------------------------------------------------------------
> + *
> + * #include <asm/hw_breakpoint.h>
> + *
> + * static void my_pre_handler(struct hw_breakpoint *bp, struct pt_regs *regs)
> + * {
> + * 	printk(KERN_DEBUG "Inside pre_handler of breakpoint exception\n");
> + * 	dump_stack();
> + *  	.......<more debugging output>........
> + * }
> + *
> + * static void my_post_handler(struct hw_breakpoint *bp, struct pt_regs *regs)
> + * {
> + * 	printk(KERN_DEBUG "Inside post_handler of breakpoint exception\n");
> + * 	dump_stack();
> + *  	.......<more debugging output>........
> + * }
> + *
> + * static struct hw_breakpoint my_bp;
> + *
> + * static int init_module(void)
> + * {
> + *	..........<do anything>............
> + *	int bkpt_type = HW_BREAKPOINT_WRITE;
> + *
> + *	if (pre_handler_supported(bkpt_type)
> + *		my_bp.pre_handler = my_pre_handler;
> + *	if (post_handler_supported(bkpt_type)
> + *		my_bp.post_handler = my_post_handler;
> + *	my_bp.priority = HW_BREAKPOINT_PRIO_NORMAL;
> + *	rc = register_kernel_hw_breakpoint(&my_bp, &pid_max,
> + *			HW_BREAKPOINT_LEN_4, bkpt_type);
> + *	..........<do anything>............
> + * }
> + *
> + * static void cleanup_module(void)
> + * {
> + *	..........<do anything>............
> + *	unregister_kernel_hw_breakpoint(&my_bp);
> + *	..........<do anything>............
> + * }
> + *
> + * ----------------------------------------------------------------------
> + *
> + */
> +struct hw_breakpoint {
> +	struct list_head	node;
> +	void		(*installed)(struct hw_breakpoint *);
> +	void		(*uninstalled)(struct hw_breakpoint *);
> +	void		(*triggered)(struct hw_breakpoint *,
> +							struct pt_regs *);
> +	struct arch_hw_breakpoint	info;
> +	u8		priority;
> +	u8		status;
> +};
> +
> +/*
> + * Inline accessor routines to retrieve the arch-specific parts of
> + * a breakpoint structure:
> + */
> +static const void *hw_breakpoint_get_kaddress(struct hw_breakpoint *bp);
> +static const void __user *hw_breakpoint_get_uaddress(struct hw_breakpoint *bp);
> +static unsigned hw_breakpoint_get_len(struct hw_breakpoint *bp);
> +static unsigned hw_breakpoint_get_type(struct hw_breakpoint *bp);
> +//TODO: Prasad
> +static inline unsigned long hw_breakpoint_lookup_name(const char *name);
> +
> +/*
> + * len and type values are defined in include/asm/hw_breakpoint.h.
> + * Available values vary according to the architecture.  On i386 the
> + * possibilities are:
> + *
> + *	HW_BREAKPOINT_LEN_1
> + *	HW_BREAKPOINT_LEN_2
> + *	HW_BREAKPOINT_LEN_4
> + *	HW_BREAKPOINT_LEN_EXECUTE
> + *	HW_BREAKPOINT_RW
> + *	HW_BREAKPOINT_READ
> + *	HW_BREAKPOINT_EXECUTE
> + *
> + * On other architectures HW_BREAKPOINT_LEN_8 may be available, and the
> + * 1-, 2-, and 4-byte lengths may be unavailable.  There also may be
> + * HW_BREAKPOINT_WRITE.  You can use #ifdef to check at compile time.
> + */
> +
> +/* Standard HW breakpoint priority levels (higher value = higher priority) */
> +#define HW_BREAKPOINT_PRIO_NORMAL	25
> +#define HW_BREAKPOINT_PRIO_PTRACE	50
> +#define HW_BREAKPOINT_PRIO_HIGH		75
> +
> +/* HW breakpoint status values (0 = not registered) */
> +#define HW_BREAKPOINT_REGISTERED	1
> +#define HW_BREAKPOINT_INSTALLED		2
> +
> +static DEFINE_MUTEX(hw_breakpoint_mutex);	/* Protects everything */
> +
> +/*
> + * The following two routines are meant to be called only from within
> + * the ptrace or utrace subsystems.  The tsk argument will usually be a
> + * process being debugged by the current task, although it is also legal
> + * for tsk to be the current task.  In any case it must be guaranteed
> + * that tsk will not start running in user mode while its breakpoints are
> + * being modified.
> + */
> +int register_user_hw_breakpoint(struct task_struct *tsk,
> +		struct hw_breakpoint *bp,
> +		const void __user *address, unsigned len, unsigned type);
> +void unregister_user_hw_breakpoint(struct task_struct *tsk,
> +		struct hw_breakpoint *bp);
> +
> +/*
> + * Kernel breakpoints are not associated with any particular thread.
> + */
> +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp);
> +void unregister_kernel_hw_breakpoint(struct hw_breakpoint *bp);
> +
> +struct thread_hw_breakpoint *alloc_thread_hw_breakpoint(
> +						struct task_struct *tsk);
> +
> +#endif	/* __KERNEL__ */
> +#endif	/* _ASM_GENERIC_HW_BREAKPOINT_H */
> Index: linux-HBKPT-kgdb-2.6.28/kernel/hw_breakpoint.c
> ===================================================================
> --- /dev/null
> +++ linux-HBKPT-kgdb-2.6.28/kernel/hw_breakpoint.c
> @@ -0,0 +1,722 @@
> +/*
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + *
> + * Copyright (C) 2007 Alan Stern
> + */
> +
> +/*
> + * HW_breakpoint: a unified kernel/user-space hardware breakpoint facility,
> + * using the CPU's debug registers.
> + *
> + * This file contains the arch-independent routines.  It is not meant
> + * to be compiled as a standalone source file; rather it should be
> + * #include'd by the arch-specific implementation.
> + */
> +
> +
> +/*
> + * Install the debug register values for a new thread.
> + */
> +void switch_to_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> +	struct thread_hw_breakpoint *thbi = tsk->thread.hw_breakpoint_info;
> +	struct cpu_hw_breakpoint *chbi;
> +	struct kernel_bp_data *thr_kbpdata;
> +
> +	/* This routine is on the hot path; it gets called for every
> +	 * context switch into a task with active breakpoints.  We
> +	 * must make sure that the common case executes as quickly as
> +	 * possible.
> +	 */
> +	chbi = &per_cpu(cpu_info, get_cpu());
> +	chbi->bp_task = tsk;
> +
> +	/* Use RCU to synchronize with external updates */
> +	rcu_read_lock();
> +
> +	/* Other CPUs might be making updates to the list of kernel
> +	 * breakpoints at this time.  If they are, they will modify
> +	 * the other entry in kbpdata[] -- the one not pointed to
> +	 * by chbi->cur_kbpdata.  So the update itself won't affect
> +	 * us directly.
> +	 *
> +	 * However when the update is finished, an IPI will arrive
> +	 * telling this CPU to change chbi->cur_kbpdata.  We need
> +	 * to use a single consistent kbpdata[] entry, the present one.
> +	 * So we'll copy the pointer to a local variable, thr_kbpdata,
> +	 * and we must prevent the compiler from aliasing the two
> +	 * pointers.  Only a compiler barrier is required, not a full
> +	 * memory barrier, because everything takes place on a single CPU.
> +	 */
> + restart:
> +	thr_kbpdata = chbi->cur_kbpdata;
> +	barrier();

Couldn't the above two lines instead be:

	thr_kbpdata = ACCESS_ONCE(chbi->cur_kbpdata);

This would prevent the pointer aliasing, but would make it very clear
exactly how the compiler was to be restricted.

> +	/* Normally we can keep the same debug register settings as the
> +	 * last time this task ran.  But if the kernel breakpoints have
> +	 * changed or any user breakpoints have been registered or
> +	 * unregistered, we need to handle the updates and possibly
> +	 * send out some notifications.
> +	 */
> +	if (unlikely(thbi->gennum != thr_kbpdata->gennum)) {
> +		struct hw_breakpoint *bp;
> +		int i;
> +		int num;
> +
> +		thbi->gennum = thr_kbpdata->gennum;
> +		arch_update_thbi(thbi, thr_kbpdata);
> +		num = thr_kbpdata->num_kbps;
> +
> +		/* This code can be invoked while a debugger is actively
> +		 * updating the thread's breakpoint list. We use RCU to
> +		 * protect our access to the list pointers. */
> +		thbi->num_installed = 0;
> +		i = HB_NUM;
> +		list_for_each_entry_rcu(bp, &thbi->thread_bps, node) {
> +
> +			/* If this register is allocated for kernel bps,
> +			 * don't install.  Otherwise do. */
> +			if (--i < num) {
> +				if (bp->status == HW_BREAKPOINT_INSTALLED) {
> +					if (bp->uninstalled)
> +						(bp->uninstalled)(bp);
> +					bp->status = HW_BREAKPOINT_REGISTERED;
> +				}
> +			} else {
> +				++thbi->num_installed;
> +				if (bp->status != HW_BREAKPOINT_INSTALLED) {
> +					bp->status = HW_BREAKPOINT_INSTALLED;
> +					if (bp->installed)
> +						(bp->installed)(bp);
> +				}
> +			}
> +		}
> +	}
> +
> +	/* Set the debug register */
> +	arch_install_thbi(thbi);
> +
> +	/* Were there any kernel breakpoint changes while we were running? */
> +	if (unlikely(chbi->cur_kbpdata != thr_kbpdata)) {
> +
> +		/* Some debug registers now be assigned to kernel bps and
> +		 * we might have messed them up.  Reload all the kernel bps
> +		 * and then reload the thread bps.
> +		 */
> +		arch_install_chbi(chbi);
> +		goto restart;
> +	}
> +
> +	rcu_read_unlock();
> +	put_cpu_no_resched();
> +}
> +
> +/*
> + * Install the debug register values for just the kernel, no thread.
> + */
> +static void switch_to_none_hw_breakpoint(void)
> +{
> +	struct cpu_hw_breakpoint *chbi;
> +
> +	chbi = &per_cpu(cpu_info, get_cpu());
> +	chbi->bp_task = NULL;
> +
> +	/* This routine gets called from only two places.  In one
> +	 * the caller holds the hw_breakpoint_mutex; in the other
> +	 * interrupts are disabled.  In either case, no kernel
> +	 * breakpoint updates can arrive while the routine runs.
> +	 * So we don't need to use RCU.
> +	 */
> +	arch_install_none(chbi);
> +	put_cpu_no_resched();
> +}
> +
> +/*
> + * Update the debug registers on this CPU.
> + */
> +static void update_this_cpu(void *unused)
> +{
> +	struct cpu_hw_breakpoint *chbi;
> +	struct task_struct *tsk = current;
> +
> +	chbi = &per_cpu(cpu_info, get_cpu());
> +
> +	/* Install both the kernel and the user breakpoints */
> +	arch_install_chbi(chbi);
> +	if (test_tsk_thread_flag(tsk, TIF_DEBUG))
> +		switch_to_thread_hw_breakpoint(tsk);
> +
> +	put_cpu_no_resched();
> +}
> +
> +/*
> + * Tell all CPUs to update their debug registers.
> + *
> + * The caller must hold hw_breakpoint_mutex.
> + */
> +static void update_all_cpus(void)
> +{
> +	/* We don't need to use any sort of memory barrier.  The IPI
> +	 * carried out by on_each_cpu() includes its own barriers.
> +	 */
> +	on_each_cpu(update_this_cpu, NULL, 0);
> +	synchronize_rcu();

Don't we need the rcu_read_lock() / rcu_read_unlock() pair from
load_debug_registers() to move down into update_this_cpu() in order
for this to be guaranteed to work?  As the code reads now, the
update_this_cpu() calls running on other CPUs are not running under
RCU protection, right?

> +}
> +
> +/*
> + * Load the debug registers during startup of a CPU.
> + */
> +void load_debug_registers(void)
> +{
> +	unsigned long flags;
> +
> +	/* Prevent IPIs for new kernel breakpoint updates */
> +	local_irq_save(flags);
> +
> +	rcu_read_lock();
> +	update_this_cpu(NULL);
> +	rcu_read_unlock();
> +
> +	local_irq_restore(flags);
> +}
> +
> +/*
> + * Take the 4 highest-priority breakpoints in a thread and accumulate
> + * their priorities in tprio.  Highest-priority entry is in tprio[3].
> + */
> +static void accum_thread_tprio(struct thread_hw_breakpoint *thbi)
> +{
> +	int i;
> +
> +	for (i = HB_NUM - 1; i >= 0 && thbi->bps[i]; --i)
> +		tprio[i] = max(tprio[i], thbi->bps[i]->priority);
> +}
> +
> +/*
> + * Recalculate the value of the tprio array, the maximum priority levels
> + * requested by user breakpoints in all threads.
> + *
> + * Each thread has a list of registered breakpoints, kept in order of
> + * decreasing priority.  We'll set tprio[0] to the maximum priority of
> + * the first entries in all the lists, tprio[1] to the maximum priority
> + * of the second entries in all the lists, etc.  In the end, we'll know
> + * that no thread requires breakpoints with priorities higher than the
> + * values in tprio.
> + *
> + * The caller must hold hw_breakpoint_mutex.
> + */
> +static void recalc_tprio(void)
> +{
> +	struct thread_hw_breakpoint *thbi;
> +
> +	memset(tprio, 0, sizeof tprio);
> +
> +	/* Loop through all threads having registered breakpoints
> +	 * and accumulate the maximum priority levels in tprio.
> +	 */
> +	list_for_each_entry(thbi, &thread_list, node)
> +		accum_thread_tprio(thbi);
> +}
> +
> +/*
> + * Decide how many debug registers will be allocated to kernel breakpoints
> + * and consequently, how many remain available for user breakpoints.
> + *
> + * The priorities of the entries in the list of registered kernel bps
> + * are compared against the priorities stored in tprio[].  The 4 highest
> + * winners overall get to be installed in a debug register; num_kpbs
> + * keeps track of how many of those winners come from the kernel list.
> + *
> + * If num_kbps changes, or if a kernel bp changes its installation status,
> + * then call update_all_cpus() so that the debug registers will be set
> + * correctly on every CPU.  If neither condition holds then the set of
> + * kernel bps hasn't changed, and nothing more needs to be done.
> + *
> + * The caller must hold hw_breakpoint_mutex.
> + */
> +static void balance_kernel_vs_user(void)
> +{
> +	int k, u;
> +	int changed = 0;
> +	struct hw_breakpoint *bp;
> +	struct kernel_bp_data *new_kbpdata;
> +
> +	/* Determine how many debug registers are available for kernel
> +	 * breakpoints as opposed to user breakpoints, based on the
> +	 * priorities.  Ties are resolved in favor of user bps.
> +	 */
> +	k = 0;			/* Next kernel bp to allocate */
> +	u = HB_NUM - 1;		/* Next user bp to allocate */
> +	bp = list_entry(kernel_bps.next, struct hw_breakpoint, node);
> +	while (k <= u) {
> +		if (&bp->node == &kernel_bps || tprio[u] >= bp->priority)
> +			--u;		/* User bps win a slot */
> +		else {
> +			++k;		/* Kernel bp wins a slot */
> +			if (bp->status != HW_BREAKPOINT_INSTALLED)
> +				changed = 1;
> +			bp = list_entry(bp->node.next, struct hw_breakpoint,
> +					node);
> +		}
> +	}
> +	if (k != cur_kbpdata->num_kbps)
> +		changed = 1;
> +
> +	/* Notify the remaining kernel breakpoints that they are about
> +	 * to be uninstalled.
> +	 */
> +	list_for_each_entry_from(bp, &kernel_bps, node) {
> +		if (bp->status == HW_BREAKPOINT_INSTALLED) {
> +			if (bp->uninstalled)
> +				(bp->uninstalled)(bp);
> +			bp->status = HW_BREAKPOINT_REGISTERED;
> +			changed = 1;
> +		}
> +	}
> +
> +	if (changed) {
> +		cur_kbpindex ^= 1;
> +		new_kbpdata = &kbpdata[cur_kbpindex];
> +		new_kbpdata->gennum = cur_kbpdata->gennum + 1;
> +		new_kbpdata->num_kbps = k;
> +		arch_new_kbpdata(new_kbpdata);
> +		u = 0;
> +		list_for_each_entry(bp, &kernel_bps, node) {
> +			if (u >= k)
> +				break;
> +			new_kbpdata->bps[u] = bp;
> +			++u;
> +		}
> +		rcu_assign_pointer(cur_kbpdata, new_kbpdata);
> +
> +		/* Tell all the CPUs to update their debug registers */
> +		update_all_cpus();
> +
> +		/* Notify the breakpoints that just got installed */
> +		for (u = 0; u < k; ++u) {
> +			bp = new_kbpdata->bps[u];
> +			if (bp->status != HW_BREAKPOINT_INSTALLED) {
> +				bp->status = HW_BREAKPOINT_INSTALLED;
> +				if (bp->installed)
> +					(bp->installed)(bp);
> +			}
> +		}
> +	}
> +}
> +
> +/*
> + * Return the pointer to a thread's hw_breakpoint info area,
> + * and try to allocate one if it doesn't exist.
> + *
> + * The caller must hold hw_breakpoint_mutex.
> + */
> +struct thread_hw_breakpoint *alloc_thread_hw_breakpoint(
> +		struct task_struct *tsk)
> +{
> +	if (!tsk->thread.hw_breakpoint_info && !(tsk->flags & PF_EXITING)) {
> +		struct thread_hw_breakpoint *thbi;
> +
> +		thbi = kzalloc(sizeof(struct thread_hw_breakpoint),
> +				GFP_KERNEL);
> +		if (thbi) {
> +			INIT_LIST_HEAD(&thbi->node);
> +			INIT_LIST_HEAD(&thbi->thread_bps);
> +
> +			/* Force an update the next time tsk runs */
> +			thbi->gennum = cur_kbpdata->gennum - 2;
> +			tsk->thread.hw_breakpoint_info = thbi;
> +		}
> +	}
> +	return tsk->thread.hw_breakpoint_info;
> +}
> +
> +/*
> + * Erase all the hardware breakpoint info associated with a thread.
> + *
> + * If tsk != current then tsk must not be usable (for example, a
> + * child being cleaned up from a failed fork).
> + */
> +void flush_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> +	struct thread_hw_breakpoint *thbi = tsk->thread.hw_breakpoint_info;
> +	struct hw_breakpoint *bp;
> +
> +	if (!thbi)
> +		return;
> +	mutex_lock(&hw_breakpoint_mutex);
> +
> +	/* Let the breakpoints know they are being uninstalled */
> +	list_for_each_entry(bp, &thbi->thread_bps, node) {
> +		if (bp->status == HW_BREAKPOINT_INSTALLED && bp->uninstalled)
> +			(bp->uninstalled)(bp);
> +		bp->status = 0;
> +	}
> +
> +	/* Remove tsk from the list of all threads with registered bps */
> +	list_del(&thbi->node);
> +
> +	/* The thread no longer has any breakpoints associated with it */
> +	clear_tsk_thread_flag(tsk, TIF_DEBUG);
> +	tsk->thread.hw_breakpoint_info = NULL;
> +	kfree(thbi);
> +
> +	/* Recalculate and rebalance the kernel-vs-user priorities */
> +	recalc_tprio();
> +	balance_kernel_vs_user();
> +
> +	/* Actually uninstall the breakpoints if necessary */
> +	if (tsk == current)
> +		switch_to_none_hw_breakpoint();
> +	mutex_unlock(&hw_breakpoint_mutex);
> +}
> +
> +/*
> + * Copy the hardware breakpoint info from a thread to its cloned child.
> + */
> +int copy_thread_hw_breakpoint(struct task_struct *tsk,
> +		struct task_struct *child, unsigned long clone_flags)
> +{
> +	/* We will assume that breakpoint settings are not inherited
> +	 * and the child starts out with no debug registers set.
> +	 * But what about CLONE_PTRACE?
> +	 */
> +	clear_tsk_thread_flag(child, TIF_DEBUG);
> +	return 0;
> +}
> +
> +/*
> + * Store the highest-priority thread breakpoint entries in an array.
> + */
> +static void store_thread_bp_array(struct thread_hw_breakpoint *thbi)
> +{
> +	struct hw_breakpoint *bp;
> +	int i;
> +
> +	i = HB_NUM - 1;
> +	list_for_each_entry(bp, &thbi->thread_bps, node) {
> +		thbi->bps[i] = bp;
> +		arch_store_thread_bp_array(thbi, bp, i);
> +		if (--i < 0)
> +			break;
> +	}
> +	while (i >= 0)
> +		thbi->bps[i--] = NULL;
> +
> +	/* Force an update the next time this task runs */
> +	thbi->gennum = cur_kbpdata->gennum - 2;
> +}
> +
> +/*
> + * Insert a new breakpoint in a priority-sorted list.
> + * Return the bp's index in the list.
> + *
> + * Thread invariants:
> + *	tsk_thread_flag(tsk, TIF_DEBUG) set implies
> + *		tsk->thread.hw_breakpoint_info is not NULL.
> + *	tsk_thread_flag(tsk, TIF_DEBUG) set iff thbi->thread_bps is non-empty
> + *		iff thbi->node is on thread_list.
> + */
> +static int insert_bp_in_list(struct hw_breakpoint *bp,
> +		struct thread_hw_breakpoint *thbi, struct task_struct *tsk)
> +{
> +	struct list_head *head;
> +	int pos;
> +	struct hw_breakpoint *temp_bp;
> +
> +	/* tsk and thbi are NULL for kernel bps, non-NULL for user bps */
> +	if (tsk)
> +		head = &thbi->thread_bps;
> +	else
> +		head = &kernel_bps;
> +
> +	/* Equal-priority breakpoints get listed first-come-first-served */
> +	pos = 0;
> +	list_for_each_entry(temp_bp, head, node) {
> +		if (bp->priority > temp_bp->priority)
> +			break;
> +		++pos;
> +	}
> +	bp->status = HW_BREAKPOINT_REGISTERED;
> +	list_add_tail(&bp->node, &temp_bp->node);
> +
> +	if (tsk) {
> +		store_thread_bp_array(thbi);
> +
> +		/* Is this the thread's first registered breakpoint? */
> +		if (list_empty(&thbi->node)) {
> +			set_tsk_thread_flag(tsk, TIF_DEBUG);
> +			list_add(&thbi->node, &thread_list);
> +		}
> +	}
> +	return pos;
> +}
> +
> +/*
> + * Remove a breakpoint from its priority-sorted list.
> + *
> + * See the invariants mentioned above.
> + */
> +static void remove_bp_from_list(struct hw_breakpoint *bp,
> +		struct thread_hw_breakpoint *thbi, struct task_struct *tsk)
> +{
> +	/* Remove bp from the thread's/kernel's list.  If the list is now
> +	 * empty we must clear the TIF_DEBUG flag.  But keep the
> +	 * thread_hw_breakpoint structure, so that the virtualized debug
> +	 * register values will remain valid.
> +	 */
> +	list_del(&bp->node);
> +	if (tsk) {
> +		store_thread_bp_array(thbi);
> +
> +		if (list_empty(&thbi->thread_bps)) {
> +			list_del_init(&thbi->node);
> +			clear_tsk_thread_flag(tsk, TIF_DEBUG);
> +		}
> +	}
> +
> +	/* Tell the breakpoint it is being uninstalled */
> +	if (bp->status == HW_BREAKPOINT_INSTALLED && bp->uninstalled)
> +		(bp->uninstalled)(bp);
> +	bp->status = 0;
> +}
> +
> +/*
> + * Validate the settings in a hw_breakpoint structure.
> + */
> +static int validate_settings(struct hw_breakpoint *bp, struct task_struct *tsk)
> +{
> +	int ret;
> +	unsigned int align;
> +
> +	ret = arch_validate_hwbkpt_settings(bp, &align);
> +	if (ret < 0)
> +		goto err;
> +
> +	/* Check that the low-order bits of the address are appropriate
> +	 * for the alignment implied by len.
> +	 */
> +	if (bp->info.address & align)
> +		return -EINVAL;
> +
> +	/* Check that the virtual address is in the proper range */
> +	if (tsk) {
> +		if (!arch_check_va_in_userspace(bp->info.address, tsk))
> +			return -EFAULT;
> +	} else {
> +		if (!arch_check_va_in_kernelspace(bp->info.address))
> +			return -EFAULT;
> +	}
> + err:
> +	return ret;
> +}
> +
> +/*
> + * Actual implementation of register_user_hw_breakpoint.
> + */
> +int __register_user_hw_breakpoint(struct task_struct *tsk,
> +					struct hw_breakpoint *bp)
> +{
> +	int rc;
> +	struct thread_hw_breakpoint *thbi;
> +	int pos;
> +
> +	bp->status = 0;
> +	rc = validate_settings(bp, tsk);
> +	if (rc)
> +		return rc;
> +
> +	thbi = alloc_thread_hw_breakpoint(tsk);
> +	if (!thbi)
> +		return -ENOMEM;
> +
> +	/* Insert bp in the thread's list */
> +	pos = insert_bp_in_list(bp, thbi, tsk);
> +	arch_register_user_hw_breakpoint(bp, thbi);
> +
> +	/* Update and rebalance the priorities.  We don't need to go through
> +	 * the list of all threads; adding a breakpoint can only cause the
> +	 * priorities for this thread to increase.
> +	 */
> +	accum_thread_tprio(thbi);
> +	balance_kernel_vs_user();
> +
> +	/* Did bp get allocated to a debug register?  We can tell from its
> +	 * position in the list.  The number of registers allocated to
> +	 * kernel breakpoints is num_kbps; all the others are available for
> +	 * user breakpoints.  If bp's position in the priority-ordered list
> +	 * is low enough, it will get a register.
> +	 */
> +	if (pos < HB_NUM - cur_kbpdata->num_kbps) {
> +		rc = 1;
> +
> +		/* Does it need to be installed right now? */
> +		if (tsk == current)
> +			switch_to_thread_hw_breakpoint(tsk);
> +		/* Otherwise it will get installed the next time tsk runs */
> +	}
> +
> +	return rc;
> +}
> +
> +/**
> + * register_user_hw_breakpoint - register a hardware breakpoint for user space
> + * @tsk: the task in whose memory space the breakpoint will be set
> + * @bp: the breakpoint structure to register
> + * @address: location (virtual address) of the breakpoint
> + * @len: encoded extent of the breakpoint address (1, 2, 4, or 8 bytes)
> + * @type: breakpoint type (read-only, write-only, read-write, or execute)
> + *
> + * This routine registers a breakpoint to be associated with @tsk's
> + * memory space and active only while @tsk is running.  It does not
> + * guarantee that the breakpoint will be allocated to a debug register
> + * immediately; there may be other higher-priority breakpoints registered
> + * which require the use of all the debug registers.
> + *
> + * @tsk will normally be a process being debugged by the current process,
> + * but it may also be the current process.
> + *
> + * @address, @len, and @type are checked for validity and stored in
> + * encoded form in @bp.  @bp->triggered and @bp->priority must be set
> + * properly.
> + *
> + * Returns 1 if @bp is allocated to a debug register, 0 if @bp is
> + * registered but not allowed to be installed, otherwise a negative error
> + * code.
> + */
> +int register_user_hw_breakpoint(struct task_struct *tsk,
> +		struct hw_breakpoint *bp,
> +		const void __user *address, unsigned len, unsigned type)
> +{
> +	int rc;
> +
> +	mutex_lock(&hw_breakpoint_mutex);
> +	rc = __register_user_hw_breakpoint(tsk, bp);
> +	mutex_unlock(&hw_breakpoint_mutex);
> +	return rc;
> +}
> +
> +/*
> + * Actual implementation of unregister_user_hw_breakpoint.
> + */
> +void __unregister_user_hw_breakpoint(struct task_struct *tsk,
> +		struct hw_breakpoint *bp)
> +{
> +	struct thread_hw_breakpoint *thbi = tsk->thread.hw_breakpoint_info;
> +
> +	if (!bp->status)
> +		return;		/* Not registered */
> +
> +	/* Remove bp from the thread's list */
> +	remove_bp_from_list(bp, thbi, tsk);
> +	arch_unregister_user_hw_breakpoint(bp, thbi);
> +
> +	/* Recalculate and rebalance the kernel-vs-user priorities,
> +	 * and actually uninstall bp if necessary.
> +	 */
> +	recalc_tprio();
> +	balance_kernel_vs_user();
> +	if (tsk == current)
> +		switch_to_thread_hw_breakpoint(tsk);
> +}
> +
> +/**
> + * unregister_user_hw_breakpoint - unregister a hardware breakpoint for user space
> + * @tsk: the task in whose memory space the breakpoint is registered
> + * @bp: the breakpoint structure to unregister
> + *
> + * Uninstalls and unregisters @bp.
> + */
> +void unregister_user_hw_breakpoint(struct task_struct *tsk,
> +		struct hw_breakpoint *bp)
> +{
> +	mutex_lock(&hw_breakpoint_mutex);
> +	__unregister_user_hw_breakpoint(tsk, bp);
> +	mutex_unlock(&hw_breakpoint_mutex);
> +}
> +
> +/**
> + * register_kernel_hw_breakpoint - register a hardware breakpoint for kernel space
> + * @bp: the breakpoint structure to register
> + *
> + * This routine registers a breakpoint to be active at all times.  It
> + * does not guarantee that the breakpoint will be allocated to a debug
> + * register immediately; there may be other higher-priority breakpoints
> + * registered which require the use of all the debug registers.
> + *
> + * @bp->triggered and @bp->priority must be set properly.
> + *
> + * Returns 1 if @bp is allocated to a debug register, 0 if @bp is
> + * registered but not allowed to be installed, otherwise a negative error
> + * code.
> + */
> +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +{
> +	int rc;
> +	int pos;
> +
> +	bp->status = 0;
> +	rc = validate_settings(bp, NULL);
> +	if (rc)
> +		return rc;
> +
> +	mutex_lock(&hw_breakpoint_mutex);
> +
> +	/* Insert bp in the kernel's list */
> +	pos = insert_bp_in_list(bp, NULL, NULL);
> +	arch_register_kernel_hw_breakpoint(bp);
> +
> +	/* Rebalance the priorities.  This will install bp if it
> +	 * was allocated a debug register.
> +	 */
> +	balance_kernel_vs_user();
> +
> +	/* Did bp get allocated to a debug register?  We can tell from its
> +	 * position in the list.  The number of registers allocated to
> +	 * kernel breakpoints is num_kbps; all the others are available for
> +	 * user breakpoints.  If bp's position in the priority-ordered list
> +	 * is low enough, it will get a register.
> +	 */
> +	if (pos < cur_kbpdata->num_kbps)
> +		rc = 1;
> +
> +	mutex_unlock(&hw_breakpoint_mutex);
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(register_kernel_hw_breakpoint);
> +
> +/**
> + * unregister_kernel_hw_breakpoint - unregister a hardware breakpoint for kernel space
> + * @bp: the breakpoint structure to unregister
> + *
> + * Uninstalls and unregisters @bp.
> + */
> +void unregister_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +{
> +	if (!bp->status)
> +		return;		/* Not registered */
> +	mutex_lock(&hw_breakpoint_mutex);
> +
> +	/* Remove bp from the kernel's list */
> +	remove_bp_from_list(bp, NULL, NULL);
> +	arch_unregister_kernel_hw_breakpoint(bp);
> +
> +	/* Rebalance the priorities.  This will uninstall bp if it
> +	 * was allocated a debug register.
> +	 */
> +	balance_kernel_vs_user();
> +
> +	mutex_unlock(&hw_breakpoint_mutex);
> +}
> +EXPORT_SYMBOL_GPL(unregister_kernel_hw_breakpoint);
> --
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