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

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Message-ID: <20190718110928.GT3463@hirez.programming.kicks-ass.net>
Date:   Thu, 18 Jul 2019 13:09:28 +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


It's simpler like so:

On Thu, Jul 18, 2019 at 01:04:46PM +0200, Peter Zijlstra wrote:
> X = 0;
> 
> 	rwsem_down_read()
> 	  for (;;) {
> 	    set_current_state(TASK_UNINTERRUPTIBLE);
>
>								X = 1;
>                                                               rwsem_up_write();
> 								  rwsem_mark_wake()
> 								    atomic_long_add(adjustment, &sem->count);
> 								    smp_store_release(&waiter->task, NULL);
> 
> 	    if (!waiter.task)
> 	      break;
> 
> 	    ...
> 	  }
> 
> 	r = X;
>