locking/rwbase: Extract __rwbase_write_trylock()

The code in rwbase_write_lock() is a little non-obvious vs the
read+set 'trylock', extract the sequence into a helper function to
clarify the code.

This also provides a single site to fix fast-path ordering.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/YUCq3L+u44NDieEJ@hirez.programming.kicks-ass.net

diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 542b017..48fbbcc 100644 (file)
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -196,6 +196,19 @@ static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
        __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
 }
 
+static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
+{
+       /* Can do without CAS because we're serialized by wait_lock. */
+       lockdep_assert_held(&rwb->rtmutex.wait_lock);
+
+       if (!atomic_read(&rwb->readers)) {
+               atomic_set(&rwb->readers, WRITER_BIAS);
+               return 1;
+       }
+
+       return 0;
+}
+
 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
                                     unsigned int state)
 {
@@ -210,34 +223,30 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
        atomic_sub(READER_BIAS, &rwb->readers);
 
        raw_spin_lock_irqsave(&rtm->wait_lock, flags);
-       /*
-        * set_current_state() for rw_semaphore
-        * current_save_and_set_rtlock_wait_state() for rwlock
-        */
-       rwbase_set_and_save_current_state(state);
+       if (__rwbase_write_trylock(rwb))
+               goto out_unlock;
 
-       /* Block until all readers have left the critical section. */
-       for (; atomic_read(&rwb->readers);) {
+       rwbase_set_and_save_current_state(state);
+       for (;;) {
                /* Optimized out for rwlocks */
                if (rwbase_signal_pending_state(state, current)) {
                        rwbase_restore_current_state();
                        __rwbase_write_unlock(rwb, 0, flags);
                        return -EINTR;
                }
+
+               if (__rwbase_write_trylock(rwb))
+                       break;
+
                raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+               rwbase_schedule();
+               raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 
-               /*
-                * Schedule and wait for the readers to leave the critical
-                * section. The last reader leaving it wakes the waiter.
-                */
-               if (atomic_read(&rwb->readers) != 0)
-                       rwbase_schedule();
                set_current_state(state);
-               raw_spin_lock_irqsave(&rtm->wait_lock, flags);
        }
-
-       atomic_set(&rwb->readers, WRITER_BIAS);
        rwbase_restore_current_state();
+
+out_unlock:
        raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
        return 0;
 }
@@ -253,8 +262,7 @@ static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
        atomic_sub(READER_BIAS, &rwb->readers);
 
        raw_spin_lock_irqsave(&rtm->wait_lock, flags);
-       if (!atomic_read(&rwb->readers)) {
-               atomic_set(&rwb->readers, WRITER_BIAS);
+       if (__rwbase_write_trylock(rwb)) {
                raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
                return 1;
        }