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Date: Fri, 26 Jun 2015 18:09:46 +0000 (UTC)
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Paul Turner <pjt@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Andrew Hunter <ahh@...gle.com>,
Andi Kleen <andi@...stfloor.org>,
Lai Jiangshan <laijs@...fujitsu.com>,
linux-api <linux-api@...r.kernel.org>,
linux-kernel@...r.kernel.org, rostedt <rostedt@...dmis.org>,
Josh Triplett <josh@...htriplett.org>,
Ingo Molnar <mingo@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...capital.net>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Chris Lameter <cl@...ux.com>
Subject: Re: [RFC PATCH 2/3] restartable sequences: x86 ABI
----- On Jun 24, 2015, at 6:26 PM, Paul Turner pjt@...gle.com wrote:
> Implements the x86 (i386 & x86-64) ABIs for interrupting and restarting
> execution within restartable sequence sections.
>
> With respect to the x86-specific ABI:
> On 32-bit: Upon restart, the interrupted rip is placed in %ecx
> On 64-bit (or x32): Upon restart, the interrupted rip is placed in %r10
>
> While potentially surprising at first glance, this choice is strongly motivated
> by the fact that the available scratch registers under the i386 function call
> ABI overlap with those used as argument registers under x86_64.
>
> Given that sequences are already personality specific and that we always want
> the arguments to be available for sequence restart, it's much more natural to
> ultimately differentiate the ABI in these two cases.
>
> Signed-off-by: Paul Turner <pjt@...gle.com>
> ---
> arch/x86/include/asm/restartable_sequences.h | 50 +++++++++++++++++++
> arch/x86/kernel/Makefile | 2 +
> arch/x86/kernel/restartable_sequences.c | 69 ++++++++++++++++++++++++++
> arch/x86/kernel/signal.c | 12 +++++
> kernel/restartable_sequences.c | 11 +++-
> 5 files changed, 141 insertions(+), 3 deletions(-)
> create mode 100644 arch/x86/include/asm/restartable_sequences.h
> create mode 100644 arch/x86/kernel/restartable_sequences.c
>
> diff --git a/arch/x86/include/asm/restartable_sequences.h
> b/arch/x86/include/asm/restartable_sequences.h
> new file mode 100644
> index 0000000..0ceb024
> --- /dev/null
> +++ b/arch/x86/include/asm/restartable_sequences.h
> @@ -0,0 +1,50 @@
> +#ifndef _ASM_X86_RESTARTABLE_SEQUENCES_H
> +#define _ASM_X86_RESTARTABLE_SEQUENCES_H
> +
> +#include <asm/processor.h>
> +#include <asm/ptrace.h>
> +#include <linux/sched.h>
> +
> +#ifdef CONFIG_RESTARTABLE_SEQUENCES
> +
> +static inline bool arch_rseq_in_crit_section(struct task_struct *p,
> + struct pt_regs *regs)
> +{
> + struct task_struct *leader = p->group_leader;
> + struct restartable_sequence_state *rseq_state = &leader->rseq_state;
> +
> + unsigned long ip = (unsigned long)regs->ip;
> + if (unlikely(ip < (unsigned long)rseq_state->crit_end &&
> + ip >= (unsigned long)rseq_state->crit_start))
> + return true;
> +
> + return false;
> +}
> +
> +static inline bool arch_rseq_needs_notify_resume(struct task_struct *p)
> +{
> +#ifdef CONFIG_PREEMPT
> + /*
> + * Under CONFIG_PREEMPT it's possible for regs to be incoherent in the
> + * case that we took an interrupt during syscall entry. Avoid this by
> + * always deferring to our notify-resume handler.
> + */
> + return true;
I'm a bit puzzled about this. If I look at perf_get_regs_user() in the perf
code, task_pt_regs() seems to return the user-space pt_regs for a task with
a current->mm set (iow, not a kernel thread), even if an interrupt nests on
top of a system call. The only corner-case is NMIs, where an NMI may interrupt
in the middle of setting up the task pt_regs, but scheduling should never happen
there, right ?
Since it's impossible for kernel threads to have a rseq critical section,
we should be able to assume that every time task_pt_regs() returns a
non-userspace (user_mode(regs) != 0) pt_regs implies that scheduling applies
to a kernel thread. Therefore, following this line of thoughts,
arch_rseq_in_crit_section() should work for CONFIG_PREEMPT kernels too.
So what I am missing here ?
Thanks,
Mathieu
> +#else
> + return arch_rseq_in_crit_section(p, task_pt_regs(p));
> +#endif
> +}
> +
> +void arch_rseq_handle_notify_resume(struct pt_regs *regs);
> +void arch_rseq_check_critical_section(struct task_struct *p,
> + struct pt_regs *regs);
> +
> +#else /* !CONFIG_RESTARTABLE_SEQUENCES */
> +
> +static inline void arch_rseq_handle_notify_resume(struct pt_regs *regs) {}
> +static inline void arch_rseq_check_critical_section(struct task_struct *p,
> + struct pt_regs *regs) {}
> +
> +#endif
> +
> +#endif /* _ASM_X86_RESTARTABLE_SEQUENCES_H */
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index febaf18..bd7827d 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -113,6 +113,8 @@ obj-$(CONFIG_TRACING) += tracepoint.o
> obj-$(CONFIG_IOSF_MBI) += iosf_mbi.o
> obj-$(CONFIG_PMC_ATOM) += pmc_atom.o
>
> +obj-$(CONFIG_RESTARTABLE_SEQUENCES) += restartable_sequences.o
> +
> ###
> # 64 bit specific files
> ifeq ($(CONFIG_X86_64),y)
> diff --git a/arch/x86/kernel/restartable_sequences.c
> b/arch/x86/kernel/restartable_sequences.c
> new file mode 100644
> index 0000000..3b38013
> --- /dev/null
> +++ b/arch/x86/kernel/restartable_sequences.c
> @@ -0,0 +1,69 @@
> +/*
> + * Restartable Sequences: x86 ABI.
> + *
> + * 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, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
> + *
> + * Copyright (C) 2015, Google, Inc.,
> + * Paul Turner <pjt@...gle.com> and Andrew Hunter <ahh@...gle.com>
> + *
> + */
> +
> +#include <linux/sched.h>
> +#include <linux/uaccess.h>
> +#include <asm/restartable_sequences.h>
> +
> +void arch_rseq_check_critical_section(struct task_struct *p,
> + struct pt_regs *regs)
> +{
> + if (!arch_rseq_in_crit_section(p, regs))
> + return;
> +
> + /* RSEQ only applies to user-mode execution */
> + BUG_ON(!user_mode(regs));
> +
> + /*
> + * The ABI is slightly different for {32,64}-bit threads on x86
> + *
> + * Short version:
> + * x86-64 (or x32): interrupted rip => %r10
> + * i386: interrupted rip => %ecx
> + *
> + * Longer version:
> + * The scratch registers available under the i386 function call ABI
> + * overlap with those used by argument registers under the x86_64 ABI.
> + *
> + * Given that the sequence block is already personality specific in
> + * that it must be entered by 'call' and that we always want the
> + * arguments available for a sequence restart; it's more natural to
> + * differentiate the ABI in these two cases.
> + */
> + if (unlikely(test_tsk_thread_flag(p, TIF_IA32)))
> + regs->cx = regs->ip; /* i386 */
> + else
> + regs->r10 = regs->ip; /* x86-64/x32 */
> +
> + regs->ip = (unsigned long)p->group_leader->rseq_state.crit_restart;
> +}
> +
> +void arch_rseq_handle_notify_resume(struct pt_regs *regs)
> +{
> + struct restartable_sequence_state *rseq_state = ¤t->rseq_state;
> +
> + /* If this update fails our user-state is incoherent. */
> + if (put_user(task_cpu(current), rseq_state->cpu_pointer))
> + force_sig(SIGSEGV, current);
> +
> + arch_rseq_check_critical_section(current, regs);
> +}
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 206996c..987c50b 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -31,6 +31,7 @@
> #include <asm/vdso.h>
> #include <asm/mce.h>
> #include <asm/sighandling.h>
> +#include <asm/restartable_sequences.h>
>
> #ifdef CONFIG_X86_64
> #include <asm/proto.h>
> @@ -617,6 +618,15 @@ setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs)
> sigset_t *set = sigmask_to_save();
> compat_sigset_t *cset = (compat_sigset_t *) set;
>
> + /*
> + * If we are executing in the critical section of a restartable
> + * sequence we need to fix up the user's stack saved ip at this point
> + * so that signal handler return does not allow us to jump back into
> + * the block across a context switch boundary.
> + */
> + if (rseq_active(current))
> + arch_rseq_check_critical_section(current, regs);
> +
> /* Set up the stack frame */
> if (is_ia32_frame()) {
> if (ksig->ka.sa.sa_flags & SA_SIGINFO)
> @@ -755,6 +765,8 @@ do_notify_resume(struct pt_regs *regs, void *unused, __u32
> thread_info_flags)
> if (thread_info_flags & _TIF_NOTIFY_RESUME) {
> clear_thread_flag(TIF_NOTIFY_RESUME);
> tracehook_notify_resume(regs);
> + if (rseq_active(current))
> + arch_rseq_handle_notify_resume(regs);
> }
> if (thread_info_flags & _TIF_USER_RETURN_NOTIFY)
> fire_user_return_notifiers();
> diff --git a/kernel/restartable_sequences.c b/kernel/restartable_sequences.c
> index 72945f2..9102241 100644
> --- a/kernel/restartable_sequences.c
> +++ b/kernel/restartable_sequences.c
> @@ -24,17 +24,22 @@
>
> #ifdef CONFIG_RESTARTABLE_SEQUENCES
>
> +#include <asm/restartable_sequences.h>
> #include <linux/uaccess.h>
> #include <linux/preempt.h>
> #include <linux/syscalls.h>
>
> static void rseq_sched_in_nop(struct preempt_notifier *pn, int cpu) {}
> -static void rseq_sched_out_nop(struct preempt_notifier *pn,
> - struct task_struct *next) {}
> +static void rseq_sched_out(struct preempt_notifier *pn,
> + struct task_struct *next)
> +{
> + if (arch_rseq_needs_notify_resume(current))
> + set_thread_flag(TIF_NOTIFY_RESUME);
> +}
>
> static __read_mostly struct preempt_ops rseq_preempt_ops = {
> .sched_in = rseq_sched_in_nop,
> - .sched_out = rseq_sched_out_nop,
> + .sched_out = rseq_sched_out,
> };
>
> int rseq_register_cpu_pointer_current(int __user *cpu_pointer)
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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