linux-kernel - Re: [PATCH v4 72/73] xfs: Convert mru cache to XArray

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Message-ID: <20171207160634.il3vt5d6a4v5qesi@thunk.org>
Date:   Thu, 7 Dec 2017 11:06:34 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jens Axboe <axboe@...nel.dk>,
        Rehas Sachdeva <aquannie@...il.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 72/73] xfs: Convert mru cache to XArray

On Wed, Dec 06, 2017 at 06:06:48AM -0800, Matthew Wilcox wrote:
> > Unfortunately for you, I don't find arguments along the lines of
> > "lockdep will save us" at all convincing.  lockdep already throws
> > too many false positives to be useful as a tool that reliably and
> > accurately points out rare, exciting, complex, intricate locking
> > problems.
> 
> But it does reliably and accurately point out "dude, you forgot to take
> the lock".  It's caught a number of real problems in my own testing that
> you never got to see.

The problem is that if it has too many false positives --- and it's
gotten *way* worse with the completion callback "feature", people will
just stop using Lockdep as being too annyoing and a waste of developer
time when trying to figure what is a legitimate locking bug versus
lockdep getting confused.

<Rant>I can't even disable the new Lockdep feature which is throwing
lots of new false positives --- it's just all or nothing.</Rant>

Dave has just said he's already stopped using Lockdep, as a result.

     	      	   			      - Ted