rwlock: remove unneeded subtraction

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 <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>

diff --git a/xen/common/rwlock.c b/xen/common/rwlock.c
index dadab372b5e1698cf63ce9cdcf6b54a6f091dd5f..aa15529bbe8ce34db89f3fff484fc4d6d96512f6 100644 (file)
--- 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);
 
     /*