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authorVineet Gupta <vgupta@synopsys.com>2015-08-19 17:23:58 +0530
committerVineet Gupta <vgupta@synopsys.com>2015-08-20 19:05:49 +0530
commit090749502ff20d7d9ec244036fe636b6bf0433b6 (patch)
tree52ce66c6c0a525b4a9eff0ee5917c52f72479ead /arch/arc/include/asm/cmpxchg.h
parent6de6066c0d24a66df465cf87a4041ef7ef35ba6f (diff)
ARC: add/fix some comments in code - no functional change
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Diffstat (limited to 'arch/arc/include/asm/cmpxchg.h')
-rw-r--r--arch/arc/include/asm/cmpxchg.h22
1 files changed, 11 insertions, 11 deletions
diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index 44fd531f4d7b..af7a2db139c9 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -110,18 +110,18 @@ static inline unsigned long __xchg(unsigned long val, volatile void *ptr,
sizeof(*(ptr))))
/*
- * On ARC700, EX insn is inherently atomic, so by default "vanilla" xchg() need
- * not require any locking. However there's a quirk.
- * ARC lacks native CMPXCHG, thus emulated (see above), using external locking -
- * incidently it "reuses" the same atomic_ops_lock used by atomic APIs.
- * Now, llist code uses cmpxchg() and xchg() on same data, so xchg() needs to
- * abide by same serializing rules, thus ends up using atomic_ops_lock as well.
+ * xchg() maps directly to ARC EX instruction which guarantees atomicity.
+ * However in !LLSC config, it also needs to be use @atomic_ops_lock spinlock
+ * due to a subtle reason:
+ * - For !LLSC, cmpxchg() needs to use that lock (see above) and there is lot
+ * of kernel code which calls xchg()/cmpxchg() on same data (see llist.h)
+ * Hence xchg() needs to follow same locking rules.
*
- * This however is only relevant if SMP and/or ARC lacks LLSC
- * if (UP or LLSC)
- * xchg doesn't need serialization
- * else <==> !(UP or LLSC) <==> (!UP and !LLSC) <==> (SMP and !LLSC)
- * xchg needs serialization
+ * Technically the lock is also needed for UP (boils down to irq save/restore)
+ * but we can cheat a bit since cmpxchg() atomic_ops_lock() would cause irqs to
+ * be disabled thus can't possibly be interrpted/preempted/clobbered by xchg()
+ * Other way around, xchg is one instruction anyways, so can't be interrupted
+ * as such
*/
#if !defined(CONFIG_ARC_HAS_LLSC) && defined(CONFIG_SMP)