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

Steven Rostedt
2016-07-12T16:56:17Z

[-- Attachment #1: 0007-sched-preempt-Fix-preempt_count-manipulations.patch --]
[-- Type: text/plain, Size: 2024 bytes --]

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

------------------

From: Peter Zijlstra <peterz@infradead.org>

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

preempt_enable_no_resched();
put_user();
preempt_disable();

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@vger.kernel.org
Cc: stable-rt@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: a787870924db ("sched, arch: Create asm/preempt.h")
Reported-by: Vikram Mulukutla <markivx@codeaurora.org>
Tested-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
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;
}
--
2.8.1