linux-kernel - Re: [PATCH-tip v7 09/20] locking/rwsem: Always release wait

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Date:   Fri, 3 May 2019 15:37:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH-tip v7 09/20] locking/rwsem: Always release wait_lock
 before waking up tasks

On Sun, Apr 28, 2019 at 05:25:46PM -0400, Waiman Long wrote:

> +			/*
> +			 * This waiter may have become first in the wait
> +			 * list after re-acquring the wait_lock. The
> +			 * rwsem_first_waiter() test in the main while
> +			 * loop below will correctly detect that. We do
> +			 * need to reload count to perform proper trylock
> +			 * and avoid missed wakeup.
> +			 */
> +			count = atomic_long_read(&sem->count);
> +		}
>  	} else {
>  		count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count);
>  	}

I've been eyeing that count usage for the past few patches, and this
here makes me think we should get rid of it.

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -400,13 +400,14 @@ static void __rwsem_mark_wake(struct rw_
  * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
  * bit is set or the lock is acquired with handoff bit cleared.
  */
-static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem,
+static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 					enum writer_wait_state wstate)
 {
-	long new;
+	long count, new;
 
 	lockdep_assert_held(&sem->wait_lock);
 
+	count = atomic_long_read(&sem->count);
 	do {
 		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
 
@@ -760,25 +761,16 @@ rwsem_down_write_slowpath(struct rw_sema
 			wake_up_q(&wake_q);
 			wake_q_init(&wake_q);	/* Used again, reinit */
 			raw_spin_lock_irq(&sem->wait_lock);
-			/*
-			 * This waiter may have become first in the wait
-			 * list after re-acquring the wait_lock. The
-			 * rwsem_first_waiter() test in the main while
-			 * loop below will correctly detect that. We do
-			 * need to reload count to perform proper trylock
-			 * and avoid missed wakeup.
-			 */
-			count = atomic_long_read(&sem->count);
 		}
 	} else {
-		count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count);
+		atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
 	}
 
 wait:
 	/* wait until we successfully acquire the lock */
 	set_current_state(state);
 	for (;;) {
-		if (rwsem_try_write_lock(count, sem, wstate))
+		if (rwsem_try_write_lock(sem, wstate))
 			break;
 
 		raw_spin_unlock_irq(&sem->wait_lock);
@@ -819,7 +811,6 @@ rwsem_down_write_slowpath(struct rw_sema
 		}
 
 		raw_spin_lock_irq(&sem->wait_lock);
-		count = atomic_long_read(&sem->count);
 	}
 	__set_current_state(TASK_RUNNING);
 	list_del(&waiter.list);