linux-kernel - Re: [PATCH v2] locking/rwsem: add acquire barrier to read

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Message-ID: <20190718110446.GC3419@hirez.programming.kicks-ass.net>
Date:   Thu, 18 Jul 2019 13:04:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jan Stancek <jstancek@...hat.com>
Cc:     Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
        linux-kernel@...r.kernel.org, dbueso@...e.de, mingo@...hat.com,
        jade alglave <jade.alglave@....com>, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath
 exit when queue is empty

On Thu, Jul 18, 2019 at 06:50:52AM -0400, Jan Stancek wrote:
> > In writing this, I also noticed that we don't have any explicit ordering
> > at the end of the reader slowpath when we wait on the queue but get woken
> > immediately:
> > 
> > 	if (!waiter.task)
> > 		break;
> > 
> > Am I missing something?
> 
> I'm assuming this isn't problem, because set_current_state() on line above
> is using smp_store_mb().


X = 0;

								X = 1;
	rwsem_down_read()					rwsem_up_write();

	  for (;;) {
	    set_current_state(TASK_UNINTERRUPTIBLE);

								  rwsem_mark_wake()
								    atomic_long_add(adjustment, &sem->count);
								    smp_store_release(&waiter->task, NULL);

	    if (!waiter.task)
	      break;

	    ...
	  }


	r = X;


can I think result in r==0 just fine, because there's nothing ordering
the load of waiter.task with the store of X.

It is exceedingly unlikely, but not impossible afaict.