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>

2 files changed, 5 insertions, 4 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 73f7fe8fd4d..a6be725cb04 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 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 4d33224c055..4c683587055 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);
 }