x86 rwsem: avoid taking slow path when stealing write lock (a31a369b) · Commits · 戴 / test

modify __down_write[_nested] and __down_write_trylock to grab the write lock whenever the active count is 0, even if there are queued waiters (they must be writers pending wakeup, since the active count is 0). Note that this is an optimization only; architectures without this optimization will still work fine: - __down_write() would take the slow path which would take the wait_lock and then try stealing the lock (as in the spinlocked rwsem implementation) - __down_write_trylock() would fail, but callers must be ready to deal with that - since there are some writers pending wakeup, they could have raced with us and obtained the lock before we steal it. Signed-off-by:

Michel Lespinasse <walken@google.com> Reviewed-by:

Peter Hurley <peter@hurleysoftware.com> Acked-by:

Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/rwsem.h

+21 −7

Original line number	Diff line number	Diff line
		@@ -105,8 +105,8 @@ static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
		asm volatile("# beginning down_write\n\t"
		LOCK_PREFIX " xadd %1,(%2)\n\t"
		/* adds 0xffff0001, returns the old value */
		" test %1,%1\n\t"
		/* was the count 0 before? */
		" test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
		/* was the active mask 0 before? */
		" jz 1f\n"
		" call call_rwsem_down_write_failed\n"
		"1:\n"
		@@ -126,11 +126,25 @@ static inline void __down_write(struct rw_semaphore *sem)
		*/
		static inline int __down_write_trylock(struct rw_semaphore *sem)
		{
		long ret = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
		RWSEM_ACTIVE_WRITE_BIAS);
		if (ret == RWSEM_UNLOCKED_VALUE)
		return 1;
		return 0;
		long result, tmp;
		asm volatile("# beginning __down_write_trylock\n\t"
		" mov %0,%1\n\t"
		"1:\n\t"
		" test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
		/* was the active mask 0 before? */
		" jnz 2f\n\t"
		" mov %1,%2\n\t"
		" add %3,%2\n\t"
		LOCK_PREFIX " cmpxchg %2,%0\n\t"
		" jnz 1b\n\t"
		"2:\n\t"
		" sete %b1\n\t"
		" movzbl %b1, %k1\n\t"
		"# ending __down_write_trylock\n\t"
		: "+m" (sem->count), "=&a" (result), "=&r" (tmp)
		: "er" (RWSEM_ACTIVE_WRITE_BIAS)
		: "memory", "cc");
		return result;
		}

		/*

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