Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

From	Dave Chinner <david@fromorbit.com>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively
Date	2017-09-29 00:10 +0200
Message-ID	<uuPah-6b6-1@gated-at.bofh.it> (permalink)
References	<uuGql-C7-5@gated-at.bofh.it> <uuGqm-C7-31@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Thu, Sep 28, 2017 at 08:39:33AM -0400, Mimi Zohar wrote:
> Don't attempt to take the i_rwsem, if it has already been taken
> exclusively.
> 
> Signed-off-by:  Mimi Zohar <zohar@linux.vnet.ibm.com>

That's bloody awful.

The locking in filesystem IO paths is already complex enough without
adding a new IO path semantic that says "caller has already locked
the i_rwsem in some order and some dependencies that we have no idea
about". Instead of having well defined locking in a small amount of
self contained code, we've now got to search through completely
unfamiliar code to analyse any sort of filesystem lockdep report or
deadlock to determine if that somethign else has screwed up the
filesystem IO path locking.

It also seems to have an undocumented semantic of not updating
access times on the inode, which effectively makes this invisible IO
and means we're assuming that timestamp updates will be done by
correctly callers outside the filesystem IO path. That's almost
certainly going to be a source of bugs in the future.

This seems like a recipe for future disasters to me....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

Thread

[RFC PATCH 0/3] define new read_iter file operation rwf flag Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-28 14:50 +0200
  [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-28 14:50 +0200
    Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Dave Chinner <david@fromorbit.com> - 2017-09-29 00:10 +0200
      Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-29 01:40 +0200
        Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-29 02:20 +0200
          Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-29 02:40 +0200
            Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-29 04:00 +0200
              Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-29 05:30 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively ebiederm@xmission.com (Eric W. Biederman) - 2017-10-01 03:40 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-10-01 14:10 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Linus Torvalds <torvalds@linux-foundation.org> - 2017-10-01 20:50 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Dave Chinner <david@fromorbit.com> - 2017-10-02 00:40 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Linus Torvalds <torvalds@linux-foundation.org> - 2017-10-02 01:20 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Dave Chinner <david@fromorbit.com> - 2017-10-02 06:00 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-10-02 01:50 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively ebiederm@xmission.com (Eric W. Biederman) - 2017-10-02 05:30 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-10-02 14:30 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Dave Chinner <david@fromorbit.com> - 2017-10-02 06:40 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-10-02 14:20 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Jeff Layton <jlayton@redhat.com> - 2017-10-02 14:50 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively ebiederm@xmission.com (Eric W. Biederman) - 2017-10-02 00:10 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Linus Torvalds <torvalds@linux-foundation.org> - 2017-10-02 00:30 +0200
                Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been  taken exclusively Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-10-02 02:00 +0200
  [RFC PATCH 1/3] fs: define new read_iter rwf flag Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-28 14:50 +0200
    Re: [RFC PATCH 1/3] fs: define new read_iter rwf flag Matthew Wilcox <willy@infradead.org> - 2017-09-28 16:00 +0200
      Re: [RFC PATCH 1/3] fs: define new read_iter rwf flag Mimi Zohar <zohar@linux.vnet.ibm.com> - 2017-09-28 16:40 +0200
      Re: [RFC PATCH 1/3] fs: define new read_iter rwf flag Linus Torvalds <torvalds@linux-foundation.org> - 2017-09-28 18:00 +0200

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