From 26f8eead2b8d19396bb57d00144bb19ae25f8f27 Mon Sep 17 00:00:00 2001 From: =?utf8?q?Roger=20Pau=20Monn=C3=A9?= Date: Fri, 18 Feb 2022 09:01:27 +0100 Subject: [PATCH] rwlock: remove unneeded subtraction MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit There's no need to subtract _QR_BIAS from the lock value for storing in the local cnts variable in the read lock slow path: the users of the value in cnts only care about the writer-related bits and use a mask to get the value. Note that further setting of cnts in rspin_until_writer_unlock already do not subtract _QR_BIAS. Originally _QR_BIAS was subtracted from the result of atomic_add_return_acquire in order to prevent GCC from emitting an unneeded ADD instruction. This being in the lock slow path such optimizations don't seem likely to make any relevant performance difference. Also modern GCC and CLANG versions will already avoid emitting the ADD instruction. Signed-off-by: Roger Pau Monné Reviewed-by: Jan Beulich Reviewed-by: Luca Fancellu --- xen/common/rwlock.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/xen/common/rwlock.c b/xen/common/rwlock.c index dadab372b5..aa15529bbe 100644 --- a/xen/common/rwlock.c +++ b/xen/common/rwlock.c @@ -47,7 +47,7 @@ void queue_read_lock_slowpath(rwlock_t *lock) while ( atomic_read(&lock->cnts) & _QW_WMASK ) cpu_relax(); - cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS; + cnts = atomic_add_return(_QR_BIAS, &lock->cnts); rspin_until_writer_unlock(lock, cnts); /* -- 2.30.2