diff options
Diffstat (limited to 'kernel/arch/arc/include/asm/cmpxchg.h')
| -rw-r--r-- | kernel/arch/arc/include/asm/cmpxchg.h | 22 |
1 files changed, 11 insertions, 11 deletions
diff --git a/kernel/arch/arc/include/asm/cmpxchg.h b/kernel/arch/arc/include/asm/cmpxchg.h index 44fd531f4..af7a2db13 100644 --- a/kernel/arch/arc/include/asm/cmpxchg.h +++ b/kernel/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) |