[PATCH RT 07/14] sched,preempt: Fix preempt_count manipulations

From: Steven Rostedt
Date: Tue Jul 12 2016 - 12:56:20 EST

3.14.72-rt76-rc1 stable review patch.
If anyone has any objections, please let me know.


From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Vikram reported that his ARM64 compiler managed to 'optimize' away the
preempt_count manipulations in code like:


Irrespective of that fact that that is horrible code that should be
fixed for many reasons, it does highlight a deficiency in the generic
preempt_count manipulators. As it is never right to combine/elide
preempt_count manipulations like this.

Therefore sprinkle some volatile in the two generic accessors to
ensure the compiler is aware of the fact that the preempt_count is
observed outside of the regular program-order view and thus cannot be
optimized away like this.

x86; the only arch not using the generic code is not affected as we
do all this in asm in order to use the segment base per-cpu stuff.

Cc: stable@xxxxxxxxxxxxxxx
Cc: stable-rt@xxxxxxxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: a787870924db ("sched, arch: Create asm/preempt.h")
Reported-by: Vikram Mulukutla <markivx@xxxxxxxxxxxxxx>
Tested-by: Vikram Mulukutla <markivx@xxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
include/asm-generic/preempt.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
index 54352f4dde1a..65759d8b0b46 100644
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -7,10 +7,10 @@

static __always_inline int preempt_count(void)
- return current_thread_info()->preempt_count;
+ return READ_ONCE(current_thread_info()->preempt_count);

-static __always_inline int *preempt_count_ptr(void)
+static __always_inline volatile int *preempt_count_ptr(void)
return &current_thread_info()->preempt_count;