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Date: Fri, 6 Feb 2009 14:40:30 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH v2 1/7] ring-buffer: add NMI protection for spinlocks
* Steven Rostedt (rostedt@...dmis.org) wrote:
> From: Steven Rostedt <srostedt@...hat.com>
>
> Impact: prevent deadlock in NMI
>
> The ring buffers are not yet totally lockless with writing to
> the buffer. When a writer crosses a page, it grabs a per cpu spinlock
> to protect against a reader. The spinlocks taken by a writer are not
> to protect against other writers, since a writer can only write to
> its own per cpu buffer. The spinlocks protect against readers that
> can touch any cpu buffer. The writers are made to be reentrant
> with the spinlocks disabling interrupts.
>
> The problem arises when an NMI writes to the buffer, and that write
> crosses a page boundary. If it grabs a spinlock, it can be racing
> with another writer (since disabling interrupts does not protect
> against NMIs) or with a reader on the same CPU. Luckily, most of the
> users are not reentrant and protects against this issue. But if a
> user of the ring buffer becomes reentrant (which is what the ring
> buffers do allow), if the NMI also writes to the ring buffer then
> we risk the chance of a deadlock.
>
> This patch moves the ftrace_nmi_enter called by nmi_enter() to the
> ring buffer code. It replaces the current ftrace_nmi_enter that is
> used by arch specific code to arch_ftrace_nmi_enter and updates
> the Kconfig to handle it.
>
> When an NMI is called, it will set a per cpu variable in the ring buffer
> code and will clear it when the NMI exits. If a write to the ring buffer
> crosses page boundaries inside an NMI, a trylock is used on the spin
> lock instead. If the spinlock fails to be acquired, then the entry
> is discarded.
>
> This bug appeared in the ftrace work in the RT tree, where event tracing
> is reentrant. This workaround solved the deadlocks that appeared there.
>
> Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/kernel/ftrace.c | 8 +++---
> include/linux/ftrace_irq.h | 10 ++++++++-
> kernel/trace/Kconfig | 8 +++++++
> kernel/trace/ring_buffer.c | 48 ++++++++++++++++++++++++++++++++++++++++++-
> 5 files changed, 68 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4277640..3da7121 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -34,6 +34,7 @@ config X86
> select HAVE_FUNCTION_TRACER
> select HAVE_FUNCTION_GRAPH_TRACER
> select HAVE_FUNCTION_TRACE_MCOUNT_TEST
> + select HAVE_FTRACE_NMI_ENTER if DYNAMIC_FTRACE || FUNCTION_GRAPH_TRACER
> select HAVE_KVM
> select HAVE_ARCH_KGDB
> select HAVE_ARCH_TRACEHOOK
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 4d33224..4c68358 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -113,7 +113,7 @@ static void ftrace_mod_code(void)
> MCOUNT_INSN_SIZE);
> }
>
> -void ftrace_nmi_enter(void)
> +void arch_ftrace_nmi_enter(void)
> {
> atomic_inc(&in_nmi);
> /* Must have in_nmi seen before reading write flag */
> @@ -124,7 +124,7 @@ void ftrace_nmi_enter(void)
> }
> }
>
> -void ftrace_nmi_exit(void)
> +void arch_ftrace_nmi_exit(void)
> {
> /* Finish all executions before clearing in_nmi */
> smp_wmb();
> @@ -376,12 +376,12 @@ int ftrace_disable_ftrace_graph_caller(void)
> */
> static atomic_t in_nmi;
>
> -void ftrace_nmi_enter(void)
> +void arch_ftrace_nmi_enter(void)
> {
> atomic_inc(&in_nmi);
> }
>
> -void ftrace_nmi_exit(void)
> +void arch_ftrace_nmi_exit(void)
> {
> atomic_dec(&in_nmi);
> }
> diff --git a/include/linux/ftrace_irq.h b/include/linux/ftrace_irq.h
> index 366a054..29de677 100644
> --- a/include/linux/ftrace_irq.h
> +++ b/include/linux/ftrace_irq.h
> @@ -2,7 +2,15 @@
> #define _LINUX_FTRACE_IRQ_H
>
>
> -#if defined(CONFIG_DYNAMIC_FTRACE) || defined(CONFIG_FUNCTION_GRAPH_TRACER)
> +#ifdef CONFIG_FTRACE_NMI_ENTER
> +extern void arch_ftrace_nmi_enter(void);
> +extern void arch_ftrace_nmi_exit(void);
> +#else
> +static inline void arch_ftrace_nmi_enter(void) { }
> +static inline void arch_ftrace_nmi_exit(void) { }
> +#endif
> +
> +#ifdef CONFIG_RING_BUFFER
> extern void ftrace_nmi_enter(void);
> extern void ftrace_nmi_exit(void);
> #else
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 28f2644..25131a5 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -9,6 +9,9 @@ config USER_STACKTRACE_SUPPORT
> config NOP_TRACER
> bool
>
> +config HAVE_FTRACE_NMI_ENTER
> + bool
> +
> config HAVE_FUNCTION_TRACER
> bool
>
> @@ -37,6 +40,11 @@ config TRACER_MAX_TRACE
> config RING_BUFFER
> bool
>
> +config FTRACE_NMI_ENTER
> + bool
> + depends on HAVE_FTRACE_NMI_ENTER
> + default y
> +
> config TRACING
> bool
> select DEBUG_FS
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index b36d737..a60a6a8 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -4,6 +4,7 @@
> * Copyright (C) 2008 Steven Rostedt <srostedt@...hat.com>
> */
> #include <linux/ring_buffer.h>
> +#include <linux/ftrace_irq.h>
> #include <linux/spinlock.h>
> #include <linux/debugfs.h>
> #include <linux/uaccess.h>
> @@ -19,6 +20,35 @@
> #include "trace.h"
>
> /*
> + * Since the write to the buffer is still not fully lockless,
> + * we must be careful with NMIs. The locks in the writers
> + * are taken when a write crosses to a new page. The locks
> + * protect against races with the readers (this will soon
> + * be fixed with a lockless solution).
> + *
> + * Because we can not protect against NMIs, and we want to
> + * keep traces reentrant, we need to manage what happens
> + * when we are in an NMI.
> + */
> +static DEFINE_PER_CPU(int, rb_in_nmi);
> +
> +void ftrace_nmi_enter(void)
> +{
> + __get_cpu_var(rb_in_nmi)++;
> + /* call arch specific handler too */
> + arch_ftrace_nmi_enter();
> +}
> +
> +void ftrace_nmi_exit(void)
> +{
> + arch_ftrace_nmi_exit();
> + __get_cpu_var(rb_in_nmi)--;
> + /* NMIs are not recursive */
> + WARN_ON_ONCE(__get_cpu_var(rb_in_nmi));
> +}
> +
> +
> +/*
> * A fast way to enable or disable all ring buffers is to
> * call tracing_on or tracing_off. Turning off the ring buffers
> * prevents all ring buffers from being recorded to.
> @@ -982,6 +1012,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> struct ring_buffer *buffer = cpu_buffer->buffer;
> struct ring_buffer_event *event;
> unsigned long flags;
> + bool lock_taken = false;
>
> commit_page = cpu_buffer->commit_page;
> /* we just need to protect against interrupts */
> @@ -995,7 +1026,19 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> struct buffer_page *next_page = tail_page;
>
> local_irq_save(flags);
> - __raw_spin_lock(&cpu_buffer->lock);
> + /*
> + * NMIs can happen after we take the lock.
> + * If we are in an NMI, only take the lock
> + * if it is not already taken. Otherwise
> + * simply fail.
> + */
> + if (unlikely(__get_cpu_var(rb_in_nmi))) {
> + if (!__raw_spin_trylock(&cpu_buffer->lock))
> + goto out_unlock;
Hi Steven,
You're not silently discarding an event, aren't you ? ;)
Please keep at least a count of "events_lost" so it can be reported
later by a printk().
Mathieu
> + } else
> + __raw_spin_lock(&cpu_buffer->lock);
> +
> + lock_taken = true;
>
> rb_inc_page(cpu_buffer, &next_page);
>
> @@ -1097,7 +1140,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> if (tail <= BUF_PAGE_SIZE)
> local_set(&tail_page->write, tail);
>
> - __raw_spin_unlock(&cpu_buffer->lock);
> + if (likely(lock_taken))
> + __raw_spin_unlock(&cpu_buffer->lock);
> local_irq_restore(flags);
> return NULL;
> }
> --
> 1.5.6.5
>
> --
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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