Patch "ARC: add compiler barrier to LLSC based cmpxchg" has been added to the 3.14-stable tree

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

ARC: add compiler barrier to LLSC based cmpxchg

to the 3.14-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:
arc-add-compiler-barrier-to-llsc-based-cmpxchg.patch
and it can be found in the queue-3.14 subdirectory.

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

From: Vineet Gupta <[email protected]>
Date: Thu, 13 Nov 2014 15:54:01 +0530
Subject: ARC: add compiler barrier to LLSC based cmpxchg

From: Vineet Gupta <[email protected]>

commit d57f727264f1425a94689bafc7e99e502cb135b5 upstream.

When auditing cmpxchg call sites, Chuck noted that gcc was optimizing away some of the desired LDs.

|	do {
|		new = old = *ipi_data_ptr;
|		new |= 1U << msg;
|	} while (cmpxchg(ipi_data_ptr, old, new) != old);

was generating to below

| 8015cef8:	ld         r2,[r4,0]  <-- First LD
| 8015cefc:	bset       r1,r2,r1
|
| 8015cf00:	llock      r3,[r4]  <-- atomic op
| 8015cf04:	brne       r3,r2,8015cf10
| 8015cf08:	scond      r1,[r4]
| 8015cf0c:	bnz        8015cf00
|
| 8015cf10:	brne       r3,r2,8015cf00  <-- Branch doesn't go to orig LD

Although this was fixed by adding a ACCESS_ONCE in this call site, it seems safer (for now at least) to add compiler barrier to LLSC based cmpxchg

---
 arch/arc/include/asm/cmpxchg.h |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

@@ -25,10 +25,11 @@ __cmpxchg(volatile void *ptr, unsigned l
 	"	scond   %3, [%1]	\n"
 	"	bnz     1b		\n"
 	"2:				\n"
-	: "=&r"(prev)
-	: "r"(ptr), "ir"(expected),
-	  "r"(new) /* can't be "ir". scond can't take limm for "b" */
-	: "cc");
+	: "=&r"(prev)	/* Early clobber, to prevent reg reuse */
+	: "r"(ptr),	/* Not "m": llock only supports reg direct addr mode */
+	  "ir"(expected),
+	  "r"(new)	/* can't be "ir". scond can't take LIMM for "b" */
+	: "cc", "memory"); /* so that gcc knows memory is being written here */
 
 	return prev;
 }

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

queue-3.14/arc-add-compiler-barrier-to-llsc-based-cmpxchg.patch