linux-kernel - Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock ASAP

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Date:   Sun, 9 Oct 2016 08:17:48 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <Waiman.Long@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        Jason Low <jason.low2@...com>,
        Jonathan Corbet <corbet@....net>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock
 ASAP

On Fri, Oct 07, 2016 at 08:47:51AM +1100, Dave Chinner wrote:
> Except that it's DAX, and in 4.7-rc1 that used shared locking at the
> XFS level and never took exclusive locks.
> 
> *However*, the DAX IO path locking in XFS  has changed in 4.9-rc1 to
> match the buffered IO single writer POSIX semantics - the test is a
> bad test based on the fact it exercised a path that is under heavy
> development and so can't be used as a regression test across
> multiple kernels.

That being said - I wonder if we should allow the shared lock on DAX
files IFF the user is specifying O_DIRECT in the open mode..