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Date: Mon, 11 Aug 2008 14:21:50 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
David Miller <davem@...emloft.net>,
Roland McGrath <roland@...hat.com>,
Ulrich Drepper <drepper@...hat.com>,
Rusty Russell <rusty@...tcorp.com.au>,
Jeremy Fitzhardinge <jeremy@...p.org>,
Gregory Haskins <ghaskins@...ell.com>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 0/5] ftrace: to kill a daemon
* Steven Rostedt (rostedt@...dmis.org) wrote:
>
> [ updated patch ]
>
> On Fri, 8 Aug 2008, Steven Rostedt wrote:
> >
> >
> > The do_something_silly had an mcount pointer to it. I put in printks in
> > the ftrace enabled code to see where this was preempted. It was preempted
> > several times before and after the nops, but never at either nop.
> >
> > Maybe I didn't run it enough (almost 2 hours), but perhaps it is very
> > unlikely to be preempted at a nop if there's something coming up next.
> >
> > Yes a string of nops may be preempted, but perhaps only two nops followed
> > by an actual command might be skipped quickly.
> >
> > I'll write some hacks to look at where it is preempted in the scheduler
> > itself, and see if I see it preempting at the second nop ever.
>
> I put that code in the scheduler and it does indeed preempt in the nops!
> It also uncovered a nasty bug I had in my code:
>
> [...]
>
> > Index: linux-tip.git/arch/x86/kernel/ftrace.c
> > ===================================================================
> > --- linux-tip.git.orig/arch/x86/kernel/ftrace.c 2008-06-26 14:58:54.000000000 -0400
> > +++ linux-tip.git/arch/x86/kernel/ftrace.c 2008-08-08 17:48:04.000000000 -0400
> > @@ -127,6 +127,46 @@ notrace int ftrace_mcount_set(unsigned l
> > return 0;
> > }
> >
> > +static const unsigned char *second_nop;
> > +
> > +void arch_ftrace_pre_enable(void)
> > +{
> > + struct task_struct *g, *p;
> > + unsigned long **ip;
> > +
>
> [...]
>
> > + ip = task_thread_info(p)->ip;
> > + if (!ip)
> > + continue;
> > + if (memcmp(*ip, second_nop, x86_nop5_part2) == 0)
> > + /* Match, move the ip forward */
> > + *ip += x86_nop5_part2;
>
> Notice the bug? Hint, *ip is of type unsigned long *.
>
> Here's the updated fix:
>
Hi Steven,
I'm actually a bit worried about this scheduler-centric approach. The
problem is that any trap that could be generated in the middle of the
3/2 nops (think of performance counters if the APIC is set as a trap
gate) which would stack an interruptible trap handler on top of those
instructions would lead to having a return IP pointing in the wrong
spot, but because the scheduler would interrupt the trap handler and not
the trapped code, it would not detect this.
I am not sure wheter we do actually have the possibility to have these
interruptible trap handlers considering the way the kernel sets up the
gates, but I think the "let's figure out where the IP stopped" approach
is a bit complex and fragile.
Trying to find a fast atomic 5-bytes nop, involving the
microarchitecture guys, seems like a better approach to me.
Mathieu
> Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> ---
> arch/x86/kernel/alternative.c | 29 ++++++++++++++++++++-------
> arch/x86/kernel/asm-offsets_32.c | 1
> arch/x86/kernel/asm-offsets_64.c | 1
> arch/x86/kernel/entry_32.S | 4 +++
> arch/x86/kernel/entry_64.S | 4 +++
> arch/x86/kernel/ftrace.c | 41 +++++++++++++++++++++++++++++++++++++++
> include/asm-x86/ftrace.h | 5 ++++
> include/asm-x86/thread_info.h | 4 +++
> kernel/trace/ftrace.c | 12 +++++++++++
> 9 files changed, 94 insertions(+), 7 deletions(-)
>
> Index: linux-tip.git/arch/x86/kernel/alternative.c
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/alternative.c 2008-06-05 11:52:24.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/alternative.c 2008-08-08 16:20:23.000000000 -0400
> @@ -140,13 +140,26 @@ static const unsigned char *const p6_nop
> };
> #endif
>
> +/*
> + * Some versions of x86 CPUs have a two part NOP5. This
> + * can break ftrace if a process is preempted between
> + * the two. ftrace needs to know what the second nop
> + * is to handle this case.
> + */
> +int x86_nop5_part2;
> +
> #ifdef CONFIG_X86_64
>
> extern char __vsyscall_0;
> const unsigned char *const *find_nop_table(void)
> {
> - return boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> - boot_cpu_data.x86 < 6 ? k8_nops : p6_nops;
> + if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> + boot_cpu_data.x86 < 6) {
> + x86_nop5_part2 = 2; /* K8_NOP2 */
> + return k8_nops;
> + } else
> + /* keep k86_nop5_part2 NULL */
> + return p6_nops;
> }
>
> #else /* CONFIG_X86_64 */
> @@ -154,12 +167,13 @@ const unsigned char *const *find_nop_tab
> static const struct nop {
> int cpuid;
> const unsigned char *const *noptable;
> + int nop5_part2;
> } noptypes[] = {
> - { X86_FEATURE_K8, k8_nops },
> - { X86_FEATURE_K7, k7_nops },
> - { X86_FEATURE_P4, p6_nops },
> - { X86_FEATURE_P3, p6_nops },
> - { -1, NULL }
> + { X86_FEATURE_K8, k8_nops, 2},
> + { X86_FEATURE_K7, k7_nops, 1 },
> + { X86_FEATURE_P4, p6_nops, 0 },
> + { X86_FEATURE_P3, p6_nops, 0 },
> + { -1, NULL, 0 }
> };
>
> const unsigned char *const *find_nop_table(void)
> @@ -170,6 +184,7 @@ const unsigned char *const *find_nop_tab
> for (i = 0; noptypes[i].cpuid >= 0; i++) {
> if (boot_cpu_has(noptypes[i].cpuid)) {
> noptable = noptypes[i].noptable;
> + x86_nop5_part2 = noptypes[i].nop5_part2;
> break;
> }
> }
> Index: linux-tip.git/arch/x86/kernel/asm-offsets_32.c
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/asm-offsets_32.c 2008-07-27 10:43:26.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/asm-offsets_32.c 2008-08-08 15:46:55.000000000 -0400
> @@ -59,6 +59,7 @@ void foo(void)
> OFFSET(TI_restart_block, thread_info, restart_block);
> OFFSET(TI_sysenter_return, thread_info, sysenter_return);
> OFFSET(TI_cpu, thread_info, cpu);
> + OFFSET(TI_ip, thread_info, ip);
> BLANK();
>
> OFFSET(GDS_size, desc_ptr, size);
> Index: linux-tip.git/arch/x86/kernel/asm-offsets_64.c
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/asm-offsets_64.c 2008-07-27 10:43:26.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/asm-offsets_64.c 2008-08-08 15:52:34.000000000 -0400
> @@ -41,6 +41,7 @@ int main(void)
> ENTRY(addr_limit);
> ENTRY(preempt_count);
> ENTRY(status);
> + ENTRY(ip);
> #ifdef CONFIG_IA32_EMULATION
> ENTRY(sysenter_return);
> #endif
> Index: linux-tip.git/arch/x86/kernel/entry_32.S
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/entry_32.S 2008-07-27 10:43:26.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/entry_32.S 2008-08-08 17:13:27.000000000 -0400
> @@ -304,7 +304,11 @@ need_resched:
> jz restore_all
> testl $X86_EFLAGS_IF,PT_EFLAGS(%esp) # interrupts off (exception path) ?
> jz restore_all
> + lea PT_EIP(%esp), %eax
> + movl %eax, TI_ip(%ebp)
> call preempt_schedule_irq
> + GET_THREAD_INFO(%ebp)
> + movl $0, TI_ip(%ebp)
> jmp need_resched
> END(resume_kernel)
> #endif
> Index: linux-tip.git/arch/x86/kernel/entry_64.S
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/entry_64.S 2008-07-27 10:43:26.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/entry_64.S 2008-08-08 17:12:47.000000000 -0400
> @@ -837,7 +837,11 @@ ENTRY(retint_kernel)
> jnc retint_restore_args
> bt $9,EFLAGS-ARGOFFSET(%rsp) /* interrupts off? */
> jnc retint_restore_args
> + leaq RIP-ARGOFFSET(%rsp), %rax
> + movq %rax, TI_ip(%rcx)
> call preempt_schedule_irq
> + GET_THREAD_INFO(%rcx)
> + movq $0, TI_ip(%rcx)
> jmp exit_intr
> #endif
>
> Index: linux-tip.git/arch/x86/kernel/ftrace.c
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/ftrace.c 2008-06-26 14:58:54.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/ftrace.c 2008-08-08 20:22:19.000000000 -0400
> @@ -127,6 +127,44 @@ notrace int ftrace_mcount_set(unsigned l
> return 0;
> }
>
> +static const unsigned char *second_nop;
> +
> +void arch_ftrace_pre_enable(void)
> +{
> + struct task_struct *g, *p;
> + void **ip;
> +
> + if (!second_nop)
> + return;
> +
> + /*
> + * x86 has a two part nop to handle 5 byte instructions.
> + * If a task was preempted after the first nop, and has
> + * not ran the second nop, if we modify the code, we can
> + * crash the system. Thus, we will look at all the tasks
> + * and if any of them was preempted and will run the
> + * second nop next, we simply move their ip pointer past
> + * the second nop.
> + */
> +
> + /*
> + * Don't need to grab the task list lock, we are running
> + * in kstop_machine
> + */
> + do_each_thread(g, p) {
> + /*
> + * In entry.S we save the ip when a task is preempted
> + * and reset it when it is back running.
> + */
> + ip = (void **)task_thread_info(p)->ip;
> + if (!ip)
> + continue;
> + if (memcmp(*ip, second_nop, x86_nop5_part2) == 0)
> + /* Match, move the ip forward */
> + *ip += x86_nop5_part2;
> + } while_each_thread(g, p);
> +}
> +
> int __init ftrace_dyn_arch_init(void *data)
> {
> const unsigned char *const *noptable = find_nop_table();
> @@ -137,5 +175,8 @@ int __init ftrace_dyn_arch_init(void *da
>
> ftrace_nop = (unsigned long *)noptable[MCOUNT_INSN_SIZE];
>
> + if (x86_nop5_part2)
> + second_nop = noptable[x86_nop5_part2];
> +
> return 0;
> }
> Index: linux-tip.git/include/asm-x86/ftrace.h
> ===================================================================
> --- linux-tip.git.orig/include/asm-x86/ftrace.h 2008-08-08 13:00:51.000000000 -0400
> +++ linux-tip.git/include/asm-x86/ftrace.h 2008-08-08 16:41:09.000000000 -0400
> @@ -17,6 +17,11 @@ static inline unsigned long ftrace_call_
> */
> return addr - 1;
> }
> +
> +extern int x86_nop5_part2;
> +extern void arch_ftrace_pre_enable(void);
> +#define ftrace_pre_enable arch_ftrace_pre_enable
> +
> #endif
>
> #endif /* CONFIG_FTRACE */
> Index: linux-tip.git/include/asm-x86/thread_info.h
> ===================================================================
> --- linux-tip.git.orig/include/asm-x86/thread_info.h 2008-08-07 11:14:43.000000000 -0400
> +++ linux-tip.git/include/asm-x86/thread_info.h 2008-08-08 17:06:15.000000000 -0400
> @@ -29,6 +29,9 @@ struct thread_info {
> __u32 cpu; /* current CPU */
> int preempt_count; /* 0 => preemptable,
> <0 => BUG */
> + unsigned long **ip; /* pointer to ip on stackwhen
> + preempted
> + */
> mm_segment_t addr_limit;
> struct restart_block restart_block;
> void __user *sysenter_return;
> @@ -47,6 +50,7 @@ struct thread_info {
> .flags = 0, \
> .cpu = 0, \
> .preempt_count = 1, \
> + .ip = NULL, \
> .addr_limit = KERNEL_DS, \
> .restart_block = { \
> .fn = do_no_restart_syscall, \
> Index: linux-tip.git/kernel/trace/ftrace.c
> ===================================================================
> --- linux-tip.git.orig/kernel/trace/ftrace.c 2008-08-08 13:00:52.000000000 -0400
> +++ linux-tip.git/kernel/trace/ftrace.c 2008-08-08 16:18:14.000000000 -0400
> @@ -32,6 +32,10 @@
>
> #include "trace.h"
>
> +#ifndef ftrace_pre_enable
> +# define ftrace_pre_enable() do { } while (0)
> +#endif
> +
> /* ftrace_enabled is a method to turn ftrace on or off */
> int ftrace_enabled __read_mostly;
> static int last_ftrace_enabled;
> @@ -500,6 +504,14 @@ static void ftrace_replace_code(int enab
> else
> new = ftrace_nop_replace();
>
> + /*
> + * Some archs *cough*x86*cough* have more than one nop to cover
> + * the call to mcount. In these cases, special care must be taken
> + * before we start converting nops into calls.
> + */
> + if (enable)
> + ftrace_pre_enable();
> +
> for (pg = ftrace_pages_start; pg; pg = pg->next) {
> for (i = 0; i < pg->index; i++) {
> rec = &pg->records[i];
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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