Commit e88d62cd4b2f0b1ae55e9008e79c2794b1fc914d upstream.
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.
For native word types, we can avoid tearing using READ_ONCE(), but this
won't work in all cases (e.g. 64-bit types on most 32-bit platforms).
This patch reworks this_cpu_generic_read() to use READ_ONCE() where
possible, otherwise falling back to disabling interrupts.
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: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
[Mark: backport to v3.16.y]
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
---
include/linux/percpu.h | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 8419053d0f2e..37ff713a7c7f 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -276,12 +276,33 @@ do { \
* used.
*/
-#define _this_cpu_generic_read(pcp) \
-({ typeof(pcp) ret__; \
+#define __this_cpu_generic_read_nopreempt(pcp) \
+({ \
+ typeof(pcp) __ret; \
preempt_disable(); \
- ret__ = *this_cpu_ptr(&(pcp)); \
+ __ret = ACCESS_ONCE(*raw_cpu_ptr(&(pcp))); \
preempt_enable(); \
- ret__; \
+ __ret; \
+})
+
+#define __this_cpu_generic_read_noirq(pcp) \
+({ \
+ typeof(pcp) __ret; \
+ unsigned long __flags; \
+ raw_local_irq_save(__flags); \
+ __ret = *raw_cpu_ptr(&(pcp)); \
+ raw_local_irq_restore(__flags); \
+ __ret; \
+})
+
+#define _this_cpu_generic_read(pcp) \
+({ \
+ typeof(pcp) __ret; \
+ if (__native_word(pcp)) \
+ __ret = __this_cpu_generic_read_nopreempt(pcp); \
+ else \
+ __ret = __this_cpu_generic_read_noirq(pcp); \
+ __ret; \
})
#ifndef this_cpu_read
--
2.11.0