linux-kernel - Re: kernel BUG at mm/truncate.c:475!

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Message-ID: <1292317895.6803.1329.camel@twins>
Date:	Tue, 14 Dec 2010 10:11:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Michael Leun <lkml20101129@...ton.leun.net>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: kernel BUG at mm/truncate.c:475!

On Mon, 2010-12-13 at 23:31 -0800, Hugh Dickins wrote:
> > > +   clear_bit_unlock(AS_UNMAPPING, &mapping->flags);
> > > +   smp_mb__after_clear_bit();
> > > +   wake_up_bit(&mapping->flags, AS_UNMAPPING);
> > > +
> > 
> > I do think this was premature optimisation.  The open-coded lock is
> > hidden from lockdep so we won't find out if this introduces potential
> > deadlocks.  It would be better to add a new mutex at least temporarily,
> > then look at replacing it with a MiklosLock later on, when the code is
> > bedded in.
> > 
> > At which time, replacing mutexes with MiklosLocks becomes part of a
> > general "shrink the address_space" exercise in which there's no reason
> > to exclusively concentrate on that new mutex!
> 
> Yes, I very much agree with you there: valiant effort by Miklos to
> avoid bloat, but we're better off using a known primitive for now.

Also, bit-spinlocks _suck_.. They're not fair, they're expensive and
like already noted they're hidden from lockdep.

Ideally we should be removing bit-spinlocks from the kernel, not add
more.
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