locking/rwsem: Micro-optimize rwsem_try_read_lock_unqueued()

The atomic_long_cmpxchg_acquire() in rwsem_try_read_lock_unqueued() is
replaced by atomic_long_try_cmpxchg_acquire() to simpify the code and
generate slightly better assembly code.

There is no functional change.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/20190404174320.22416-5-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index c213869e1aa75d9e799b2dcc20ed3cd4652b2e28..f6198e1a58f6b1b456978f30784350d8e581d012 100644 (file)
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -259,21 +259,16 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
  */
 static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 {
-       long old, count = atomic_long_read(&sem->count);
+       long count = atomic_long_read(&sem->count);
 
-       while (true) {
-               if (!(count == 0 || count == RWSEM_WAITING_BIAS))
-                       return false;
-
-               old = atomic_long_cmpxchg_acquire(&sem->count, count,
-                                     count + RWSEM_ACTIVE_WRITE_BIAS);
-               if (old == count) {
+       while (!count || count == RWSEM_WAITING_BIAS) {
+               if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
+                                       count + RWSEM_ACTIVE_WRITE_BIAS)) {
                        rwsem_set_owner(sem);
                        return true;
                }
-
-               count = old;
        }
+       return false;
 }
 
 static inline bool owner_on_cpu(struct task_struct *owner)