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Re: working on extent locks for i_mutex

To: Tao Ma <tm@xxxxxx>
Subject: Re: working on extent locks for i_mutex
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 13 Jan 2012 22:52:32 +1100
Cc: Allison Henderson <achender@xxxxxxxxxxxxxxxxxx>, Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>, Lukas Czerner <lczerner@xxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <4F0FD9EB.5050707@xxxxxx>
References: <4F0F9E97.1090403@xxxxxxxxxxxxxxxxxx> <20120113043411.GH2806@dastard> <4F0FD9EB.5050707@xxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Fri, Jan 13, 2012 at 03:14:51PM +0800, Tao Ma wrote:
> On 01/13/2012 12:34 PM, Dave Chinner wrote:
> > On Thu, Jan 12, 2012 at 08:01:43PM -0700, Allison Henderson wrote:
> >> Hi All,
> >>
> >> I know this is an old topic, but I am poking it again because I've
> >> had some work items wrap up, and Im planning on picking up on this
> >> one again.  I am thinking about implementing extent locks to replace
> >> i_mutex.  So I just wanted to touch base with folks and see what
> >> people are working on because I know there were some folks out there
> >> that were thing about doing similar solutions.
> > 
> > What locking API are you looking at? If you are looking at an
> > something like:
> > 
> > read_range_{try}lock(lock, off, len)
> > read_range_unlock(lock, off, len)
> > write_range_{try}lock(lock, off, len)
> > write_range_unlock(lock, off, len)
> > 
> > and implementing with an rbtree or a btree for tracking, then I
> > definitely have a use for it in XFS - replacing the current rwsem
> > that is used for the iolock. Range locks like this are the only
> > thing we need to allow concurrent buffered writes to the same file
> > to maintain the per-write exclusion that posix requires.
> Interesting, so xfs already have these range lock, right? If yes, any
> possibility that the code can be reused in ext4 since we have the same
> thing in mind but don't have any resource to work on it by now.

No, it doesn't have range locks. If has separate locks for IO
exclusion vs metadata modification (i_iolock vs i_ilock). Both are
rwsems, the ilock nests inside and protects the extent list and
other metadata.

What I want to do is replace the i_iolock with a read/write range
lock so that we can do sane cache coherent concurrent IO to separate
ranges of the file. We can't do concurrent modifications to the
extent tree, so we have no need for changing the i_ilock (metadata)
lock to range locks.


> btw, IIRC flock(2) uses a list to indicate the range lock, so if we can
> make these pieces of codes common, at least there are 3 places that can
> benefit from it. ;)

flock is way more complex than simple read/write range locks and has
fixed semantics and lots of scope for difficult to find regressions,
so I wouldn't even bother trying to support them...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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