4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Mark Rutland <mark.rutl...@arm.com>
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.rutl...@arm.com>
Cc: Arnd Bergmann <a...@arndb.de>
Cc: Christoph Lameter <c...@linux.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Pranith Kumar <bobby.pr...@gmail.com>
Cc: Tejun Heo <t...@kernel.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: linux-a...@vger.kernel.org
Signed-off-by: Tejun Heo <t...@kernel.org>
[Mark: backport to v4.4.y]
Signed-off-by: Mark Rutland <mark.rutl...@arm.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
include/asm-generic/percpu.h | 24 ++++++++++++++++++++++--
1 file changed, 22 insertions(+), 2 deletions(-)
--- a/include/asm-generic/percpu.h
+++ b/include/asm-generic/percpu.h
@@ -105,15 +105,35 @@ do {
\
(__ret); \
})
-#define this_cpu_generic_read(pcp) \
+#define __this_cpu_generic_read_nopreempt(pcp) \
({ \
typeof(pcp) __ret; \
preempt_disable(); \
- __ret = *this_cpu_ptr(&(pcp)); \
+ __ret = READ_ONCE(*raw_cpu_ptr(&(pcp))); \
preempt_enable(); \
__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; \
+})
+
#define this_cpu_generic_to_op(pcp, val, op) \
do { \
unsigned long __flags; \
