ring-buffer: add NMI protection for spinlocks

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@redhat.com>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 73f7fe8fd4d1c52d0e6851745067035fc008821e..a6be725cb04923e71f1a4605225b82921063d514 100644 (file)
--- 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 if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64)
        select HAVE_ARCH_KGDB if !X86_VOYAGER
        select HAVE_ARCH_TRACEHOOK

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 4d33224c055f71bcd40f089eb5f439fdd8370492..4c683587055b54ac35b030d19e5eca8aa73ddcd9 100644 (file)
--- 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 366a054d0b05d25bc65f27cacc4b7a4660beb62e..29de6779a96380f5a3a64bbed27c6c4b8ec03efe 100644 (file)
--- 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 28f2644484d9bd70fc0ef16f2ca85d4e38f03741..25131a5d5e4f374cd16b91a5c08d66cc683d70fd 100644 (file)
--- 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 b36d7374ceefd816b0d7b47f5e77b6bbc5292334..a60a6a852f4285a15191f5b9bbfa34f661ade832 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4,6 +4,7 @@
  * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
  */
 #include <linux/ring_buffer.h>
+#include <linux/ftrace_irq.h>
 #include <linux/spinlock.h>
 #include <linux/debugfs.h>
 #include <linux/uaccess.h>
@@ -18,6 +19,35 @@
 
 #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
@@ -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;
+               } 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;
 }