linux-kernel - Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write

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Date:	Wed, 11 May 2016 10:28:53 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <Waiman.Long@....com>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for
 down_write_killable

On Wed 11-05-16 09:23:57, Michal Hocko wrote:
> On Tue 10-05-16 14:38:06, Peter Zijlstra wrote:
[...]
> > However, at the time I was thinking that if we have:
> > 
> > 	reader (owner)
> > 	writer (pending)
> > 	reader (blocked on writer)
> > 
> > and writer would get cancelled, the up_read() would do a wakeup and kick
> > the blocked reader.
> > 
> > But yes, immediately kicking further pending waiters might be better.
> 
> OK, that makes sense. We shouldn't be waiting for the first reader to
> do up_read.

I am still trying to wrap my head around the wake up logic (I managed to
forget all the juicy details). I still do not see the correctness
issue with the current code so the following should be "just an
optimization". But as I've said I might be missing something here.

Does the following look correct/reasonable? This is absolutely untested
and more for a discussion:
---
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index df4dcb883b50..726620c97366 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -516,7 +516,27 @@ EXPORT_SYMBOL(rwsem_down_write_failed);
 __visible struct rw_semaphore * __sched
 rwsem_down_write_failed_killable(struct rw_semaphore *sem)
 {
-	return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
+	struct rw_semaphore * ret;
+
+	ret = __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
+	if (IS_ERR(ret)) {
+		long count;
+
+		raw_spin_lock_irq(&sem->wait_lock);
+		count = READ_ONCE(sem->count);
+
+		/*
+		 * If there were already threads queued before us and there are
+		 * no active writers, the lock must be read owned; so we try to
+		 * wake any read locks that were queued ahead of us so they
+		 * do not have to wait for up_read to wake up.
+		 */
+		if (count > RWSEM_WAITING_BIAS)
+			sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
+		raw_spin_unlock_irq(&sem->wait_lock);
+	}
+
+	return ret;
 }
 EXPORT_SYMBOL(rwsem_down_write_failed_killable);
 
-- 
Michal Hocko
SUSE Labs