linux-ext4 - Re: [PATCH] ext4/jbd2: don't wait (forever) for stale tid caused by wraparound

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Message-ID: <20130321224143.GA5066@quack.suse.cz>
Date:	Thu, 21 Mar 2013 23:41:43 +0100
From:	Jan Kara <jack@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>, Ben Hutchings <ben@...adent.org.uk>,
	George Barnett <gbarnett@...assian.com>,
	linux-ext4@...r.kernel.org,
	Debian kernel maintainers <debian-kernel@...ts.debian.org>
Subject: Re: [PATCH] ext4/jbd2: don't wait (forever) for stale tid caused
 by wraparound

On Thu 21-03-13 17:09:40, Ted Tso wrote:
> On Thu, Mar 21, 2013 at 09:46:38PM +0100, Jan Kara wrote:
> >   Good catch! But shouldn't we rather fix jbd2_log_wait_commit() instead of
> > inventing new function?
> 
> In most of the places where we call jbd2_log_start_commit(), we're
> actually starting the current running transaction.  So the fact that
> we pass in a tid, and we're having to validate that the tid is
> actually a valid one, is a bit of a waste.  So in the long run I think
> it's worth rethinking whether or not jbd2_log_{start,wait}_commit()
> should exist in their current form, or whether we should reorganize
> their functionality (i.e., by having a jbd2_start_running_commit(),
> for example.).  Piling on fixes to jbd2_log_wait_commit() would make
> it get even more complicated, and I think if we separate out the
> various ways in which we use these functions, we can make the code
> simpler and easier to read.
  I don't find jbd2_log_wait_commit() that complex that it couldn't bear
another if :) But given there are really two waiting operations that make
sense:
a) request commit of running transaction and wait for it
b) wait for committing transaction

then I agree there may be a better interface. OTOH I'm somewhat
curious about the new interface because the only race-free way of
identifying a transaction you want to wait for is using its tid.

> In fact, I had started making this rather large set of changes when I
> decided it would be better to save that kind of wholesale refactoring
> for the next merge window.  So the reason why I ended up fixing the
> patch the way I did was to keep things simple.
> 
> Also as I mentioned in the commit description, by using a single
> function I was also able to optimize the locking the locking somewhat.
  Yeah. I'm not as much opposed to the new function doing start commit
& wait but what I dislike is the fact that we have still exposed the
function jbd2_log_wait_commit() which can possibly lockup if tid overflows.
I agree there aren't currently any other callers where this could happen
but in a few years who knows...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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