Patch "bitops: Do not default to __clear_bit() for __clear_bit_unlock()" has been added to the 4.5-stable tree

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

bitops: Do not default to __clear_bit() for __clear_bit_unlock()

to the 4.5-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:
bitops-do-not-default-to-__clear_bit-for-__clear_bit_unlock.patch
and it can be found in the queue-4.5 subdirectory.

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

From: Peter Zijlstra <[email protected]>
Date: Wed, 9 Mar 2016 12:40:54 +0100
Subject: bitops: Do not default to __clear_bit() for __clear_bit_unlock()

From: Peter Zijlstra <[email protected]>

commit f75d48644c56a31731d17fa693c8175328957e1d upstream.

__clear_bit_unlock() is a special little snowflake. While it carries the non-atomic '__' prefix, it is specifically documented to pair with test_and_set_bit() and therefore should be 'somewhat' atomic.

Therefore the generic implementation of __clear_bit_unlock() cannot use the fully non-atomic __clear_bit() as a default.

If an arch is able to do better; is must provide an implementation of __clear_bit_unlock() itself.

Specifically, this came up as a result of hackbench livelock'ing in slab_lock() on ARC with SMP + SLUB + !LLSC.

The issue was incorrect pairing of atomic ops.

slab_lock() -> bit_spin_lock() -> test_and_set_bit() slab_unlock() -> __bit_spin_unlock() -> __clear_bit()

The non serializing __clear_bit() was getting "lost"

 80543b8e:	ld_s       r2,[r13,0] <--- (A) Finds PG_locked is set
 80543b90:	or         r3,r2,1    <--- (B) other core unlocks right here
 80543b94:	st_s       r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock)

Fixes ARC STAR 9000817404 (and probably more).

---
 include/asm-generic/bitops/lock.h |   14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

@@ -29,16 +29,16 @@ do {					\
  * @nr: the bit to set
  * @addr: the address to start counting from
  *
- * This operation is like clear_bit_unlock, however it is not atomic.
- * It does provide release barrier semantics so it can be used to unlock
- * a bit lock, however it would only be used if no other CPU can modify
- * any bits in the memory until the lock is released (a good example is
- * if the bit lock itself protects access to the other bits in the word).
+ * A weaker form of clear_bit_unlock() as used by __bit_lock_unlock(). If all
+ * the bits in the word are protected by this lock some archs can use weaker
+ * ops to safely unlock.
+ *
+ * See for example x86's implementation.
  */
 #define __clear_bit_unlock(nr, addr)	\
 do {					\
-	smp_mb();			\
-	__clear_bit(nr, addr);		\
+	smp_mb__before_atomic();	\
+	clear_bit(nr, addr);		\
 } while (0)
 
 #endif /* _ASM_GENERIC_BITOPS_LOCK_H_ */

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

queue-4.5/edac-sb_edac-fix-computation-of-channel-address.patch queue-4.5/sched-fair-avoid-using-decay_load_missed-with-a-negative-value.patch queue-4.5/x86-iopl-64-properly-context-switch-iopl-on-xen-pv.patch queue-4.5/sched-cputime-fix-steal_account_process_tick-to-always-return-jiffies.patch queue-4.5/perf-tools-fix-checking-asprintf-return-value.patch queue-4.5/bitops-do-not-default-to-__clear_bit-for-__clear_bit_unlock.patch queue-4.5/x86-iopl-fix-iopl-capability-check-on-xen-pv.patch queue-4.5/perf-x86-intel-add-definition-for-pt-pmi-bit.patch queue-4.5/x86-microcode-intel-make-early-loader-look-for-builtin-microcode-too.patch queue-4.5/perf-core-fix-perf_sched_count-derailment.patch queue-4.5/x86-microcode-untangle-from-blk_dev_initrd.patch queue-4.5/x86-irq-cure-live-lock-in-fixup_irqs.patch queue-4.5/x86-entry-compat-keep-ts_compat-set-during-signal-delivery.patch