[PATCH 4/4] rwsem: Add ACQUIRE comments

[Peter Zijlstra]

Since we just reviewed read_slowpath for ACQUIRE correctness, add a
few coments to retain our findings.

Acked-by: Will Deacon <w...@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 kernel/locking/rwsem.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1000,6 +1000,7 @@ rwsem_down_read_slowpath(struct rw_semap
        atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
        adjustment = 0;
        if (rwsem_optimistic_spin(sem, false)) {
+               /* rwsem_optimistic_spin() implies ACQUIRE on success */
                /*
                 * Wake up other readers in the wait list if the front
                 * waiter is a reader.
@@ -1014,6 +1015,7 @@ rwsem_down_read_slowpath(struct rw_semap
                }
                return sem;
        } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
+               /* rwsem_reader_phase_trylock() implies ACQUIRE on success */
                return sem;
        }
 
@@ -1067,10 +1069,10 @@ rwsem_down_read_slowpath(struct rw_semap
        wake_up_q(&wake_q);
 
        /* wait to be given the lock */
-       while (true) {
+       for (;;) {
                set_current_state(state);
                if (!smp_load_acquire(&waiter.task)) {
-                       /* Orders against rwsem_mark_wake()'s 
smp_store_release() */
+                       /* Matches rwsem_mark_wake()'s smp_store_release(). */
                        break;
                }
                if (signal_pending_state(state, current)) {
@@ -1078,6 +1080,7 @@ rwsem_down_read_slowpath(struct rw_semap
                        if (waiter.task)
                                goto out_nolock;
                        raw_spin_unlock_irq(&sem->wait_lock);
+                       /* Ordered by sem->wait_lock against rwsem_mark_wake(). 
*/
                        break;
                }
                schedule();
@@ -1087,6 +1090,7 @@ rwsem_down_read_slowpath(struct rw_semap
        __set_current_state(TASK_RUNNING);
        lockevent_inc(rwsem_rlock);
        return sem;
+
 out_nolock:
        list_del(&waiter.list);
        if (list_empty(&sem->wait_list)) {
@@ -1127,8 +1131,10 @@ rwsem_down_write_slowpath(struct rw_sema
 
        /* do optimistic spinning and steal lock if possible */
        if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
-           rwsem_optimistic_spin(sem, true))
+           rwsem_optimistic_spin(sem, true)) {
+               /* rwsem_optimistic_spin() implies ACQUIRE on success */
                return sem;
+       }
 
        /*
         * Disable reader optimistic spinning for this rwsem after
@@ -1188,9 +1194,11 @@ rwsem_down_write_slowpath(struct rw_sema
 wait:
        /* wait until we successfully acquire the lock */
        set_current_state(state);
-       while (true) {
-               if (rwsem_try_write_lock(sem, wstate))
+       for (;;) {
+               if (rwsem_try_write_lock(sem, wstate)) {
+                       /* rwsem_try_write_lock() implies ACQUIRE on success */
                        break;
+               }
 
                raw_spin_unlock_irq(&sem->wait_lock);