Patch "percpu: make this_cpu_generic_read() atomic w.r.t. interrupts" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
percpu-make-this_cpu_generic_read-atomic-w.r.t.-interrupts.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mark Rutland <[email protected]>
Date: Tue, 26 Sep 2017 12:41:52 +0100
Subject: percpu: make this_cpu_generic_read() atomic w.r.t. interrupts

From: Mark Rutland <[email protected]>

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.

---
 include/asm-generic/percpu.h |   24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

@@ -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;						\

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/percpu-make-this_cpu_generic_read-atomic-w.r.t.-interrupts.patch