linux-kernel - Re: [RFC, PATCH 1/2] qrcu: "quick" srcu implementation

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Message-ID: <20061130033757.GA4110@oleg>
Date:	Thu, 30 Nov 2006 06:37:57 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, Jens Axboe <jens.axboe@...cle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Josh Triplett <josh@...edesktop.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCH 1/2] qrcu: "quick" srcu implementation

On 11/30, Oleg Nesterov wrote:
>
> On 11/29, Paul E. McKenney wrote:
> >
> > Hmmm...  Now I am wondering if the memory barriers inherent in the
> > __wait_event() suffice for this last barrier...  :-/  Thoughts?
> > 
> > > +	smp_mb();
> 
> Fastpath skips __wait_event(), and it is possible that the reader does
> lock/unlock between the first 'mb()' and 'if (atomic_read() == 1)'.

In fact, a slow path needs (I think) it too. We can have an unrelated
wakeup, and then the reader does unlock() before we check !atomic_read()
in the __wait_event()'s loop. The reader removes us from ->wq, in that
case finish_wait() does nothing.

Oleg.

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