linux-kernel - Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without taking rename

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Message-ID: <522E98B4.1050906@hp.com>
Date:	Mon, 09 Sep 2013 23:57:40 -0400
From:	Waiman Long <waiman.long@...com>
To:	George Spelvin <linux@...izon.com>
CC:	torvalds@...ux-foundation.org, viro@...IV.linux.org.uk,
	aswin@...com, john@...ffel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, scott.norton@...com
Subject: Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without
 taking rename_lock

On 09/09/2013 08:40 PM, George Spelvin wrote:
> I'm really wondering about only trying once before taking the write lock.
> Yes, using the lsbit is a cute hack, but are we using it for its cuteness
> rather than its effectiveness?
>
> Renames happen occasionally.  If that causes all the current pathname
> translations to fall back to the write lock, that is fairly heavy.
> Worse, all of those translations will (unnecessarily) bump the write
> seqcount, triggering *other* translations to fail back to the write-lock
> path.
>
> One patch to fix this would be to have the fallback read algorithm take
> sl->lock but *not* touch sl->seqcount, so it wouldn't break concurrent
> readers.

Actually, a follow-up patch that I am planning to do is to introduce a 
read_seqlock() primitive in seqlock.h that does exactly that. Then the 
write_seqlock() in this patch will be modified to read_seqlock().

-Longman
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