linux-kernel - Re: [patch 0/5] seqlock consolidation

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Message-ID: <20120315204235.GF8943@ZenIV.linux.org.uk>
Date:	Thu, 15 Mar 2012 20:42:35 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [patch 0/5] seqlock consolidation

On Thu, Mar 15, 2012 at 08:17:12PM +0100, Thomas Gleixner wrote:
> > +			hits += replace_path(&fs->root, old_root, new_root);
> > +			hits += replace_path(&fs->pwd, old_root, new_root);
> 
> Wouldn't it be simpler to just do: 
> 
> +			count += replace_path(&fs->root, old_root, new_root);
> +			count += replace_path(&fs->pwd, old_root, new_root);
> 
> > +			write_seqcount_end(&fs->seq);
> > +			while (hits--) {
> >  				count++;
> 
> Instead of that loop ?

This loop also contains path_get_longterm() and we need to do it before
dropping fs->lock.  We are holding a reference to new_root, all right,
but once it's place into ->fs->{root,pwd} of another task and ->fs->lock
is dropped, there's nothing to stop that task of doing chdir() and dropping
its reference.  Which could outweight the single reference we are holding
pretty soon...
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