[PATCH] percpu: make this_cpu_generic_read() atomic w.r.t. interrupts

[Mark Rutland]

As raw_cpu_generic_read() is a plain read from a raw_cpu_ptr() address,
it's possible (albeit unlikely) that the compiler will split the access
across multiple instructions.

In this_cpu_generic_read() we disable preemption but not interrupts
before calling raw_cpu_generic_read(). Thus, an interrupt could be taken
in the middle of the split load instructions. If a this_cpu_write() or
RMW this_cpu_*() op is made to the same variable in the interrupt
handling path, this_cpu_read() will return a torn value.

Avoid this by using READ_ONCE() to inhibit tearing.

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Pranith Kumar <bobby.prani@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
---
 include/asm-generic/percpu.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
index 0504ef8..79a8a58 100644
--- a/include/asm-generic/percpu.h
+++ b/include/asm-generic/percpu.h
@@ -67,7 +67,7 @@
 
 #define raw_cpu_generic_read(pcp)					\
 ({									\
-	*raw_cpu_ptr(&(pcp));						\
+	READ_ONCE(*raw_cpu_ptr(&(pcp)));				\
 })
 
 #define raw_cpu_generic_to_op(pcp, val, op)				\
-- 
1.9.1