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authorSteven Rostedt <srostedt@redhat.com>2009-02-05 18:43:07 -0500
committerSteven Rostedt <srostedt@redhat.com>2009-02-07 20:00:17 -0500
commit78d904b46a72fcf15ea6a39672bbef92953876b5 (patch)
tree69f56f7bbd1866491517d902bdf18ab973f0eb5f /kernel/trace/Kconfig
parent1830b52d0de8c60c4f5dfbac134aa8f69d815801 (diff)
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>
Diffstat (limited to 'kernel/trace/Kconfig')
-rw-r--r--kernel/trace/Kconfig8
1 files changed, 8 insertions, 0 deletions
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 28f2644484d..25131a5d5e4 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