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From: Andrew Morton <akpm@osdl.org>
To: suparna@in.ibm.com
Cc: daniel@osdl.org, janetmor@us.ibm.com, pbadari@us.ibm.com,
	linux-aio@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch
Date: Thu, 1 Jan 2004 20:36:12 -0800	[thread overview]
Message-ID: <20040101203612.15debff2.akpm@osdl.org> (raw)
In-Reply-To: <20040102042036.GA3311@in.ibm.com>

Suparna Bhattacharya <suparna@in.ibm.com> wrote:
>
> On Wed, Dec 31, 2003 at 03:42:39PM -0800, Andrew Morton wrote:
> > Daniel McNeil <daniel@osdl.org> wrote:
> > >
> > > The other potential race in filemap_fdatawait() was that it
> > > removed the page from the locked list while waiting for a writeback
> > > and if there was a 2nd filemap_fdatawait() running on another cpu,
> > > it would not wait for the page being written since it would never see
> > > it on the list.
> > 
> > That would only happen if one thread or the other was not running under
> > i_sem.   The only path I see doing that is in generic_file_direct_IO()?
> 
> Yes, and we should simply fix generic_file_direct_IO to avoid doing so.
> We anyway issue filemap_fdatawait later with i_sem held.
> 
> The race that we need to worry about is between background writeouts
> (which don't take i_sem) and filemap_fdatawrite/filemap_fdatawait - i.e
> the first one discussed.

Well Daniel has raised a second race here, betwen filemap_fdatawait() and
filemap_fdatwait().  The background writeback code does not execute
filemap_datawait() anyway, so no prob.

Yes, extending i_sem coverage in the case O_DIRECT reads should suit.  In
the (vastly) common case mapping->nrpages is zero anyway, so we shouldn't
even enter that code.

> > > +		/*
> > > +		 * If the page is locked, it might be in process of being 
> > > +		 * setup for writeback but without PG_writeback set 
> > > +		 * and with PG_dirty cleared.
> > > +		 * (PG_dirty is cleared BEFORE PG_writeback is set)
> > > +		 * So, wait for the PG_locked to clear, then start over.
> > > +		 */
> > > +		if (PageLocked(page)) {
> > > +			page_cache_get(page);
> > > +			spin_unlock(&mapping->page_lock);
> > > +			wait_on_page_locked(page);
> > > +			page_cache_release(page);
> > > +			goto restart;
> > > +		}
> > 
> > Why is this a problem which needs addressing here?  If some other thread is
> > in the process of starting I/O against this page then the page must have
> > been clean when this thread ran filemap_fdatawrite()?
> 
> This is the same race that we have been discussing (background writer
> pulled this page off io_pages, put it on locked pages but hasn't set 
> PG_writeback as yet). To me it seemed that Daniel's solution was just an 
> alternative to what you proposed - i.e. adding lock_page() to filemap_fdatawait.
> I have to think a little about the fix -- AFAICS but we are all talking
> about the same (real) problem here.

Yup.  Realish, anyway.  Unless we can demonstrate that this is the cause of
the O_DIRECT data-exposure problems, this race isn't really very
interesting.  It should be plugged though I guess.


  reply	other threads:[~2004-01-02  4:35 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <3FCD4B66.8090905@us.ibm.com>
2003-12-06  1:29 ` [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix Daniel McNeil
2003-12-08 18:23   ` Daniel McNeil
2003-12-12  0:51     ` Daniel McNeil
2003-12-17  1:25       ` [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch Daniel McNeil
2003-12-17  2:03         ` Andrew Morton
2003-12-17 19:25           ` Daniel McNeil
2003-12-17 20:17             ` Janet Morgan
2003-12-31  9:18           ` Suparna Bhattacharya
2003-12-31  9:35             ` Andrew Morton
2003-12-31  9:55               ` Suparna Bhattacharya
2003-12-31  9:59                 ` Andrew Morton
2003-12-31 10:09                   ` Suparna Bhattacharya
2003-12-31 10:10                     ` Andrew Morton
2003-12-31 10:48                       ` Suparna Bhattacharya
2003-12-31 10:53                         ` Andrew Morton
2003-12-31 10:54                           ` Andrew Morton
2003-12-31 11:17                             ` Andrew Morton
2003-12-31 22:34                               ` [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch Daniel McNeil
2003-12-31 22:41                                 ` [PATCH linux-2.6.1-rc1-mm1] aiodio_fallback_bio_count.patch Daniel McNeil
2003-12-31 23:46                                   ` Andrew Morton
2004-01-02  5:14                                     ` Suparna Bhattacharya
2004-01-02  7:46                                       ` Andrew Morton
2004-01-05  3:55                                         ` Suparna Bhattacharya
2004-01-05  5:06                                           ` Andrew Morton
2004-01-05  5:28                                             ` Suparna Bhattacharya
2004-01-05  5:28                                               ` Andrew Morton
2004-01-05  6:06                                                 ` Suparna Bhattacharya
2004-01-05  6:14                                                 ` Lincoln Dale
2003-12-31 22:47                                 ` [PATCH linux-2.6.1-rc1-mm1] dio_isize.patch Daniel McNeil
2003-12-31 23:42                                 ` [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch Andrew Morton
2004-01-02  4:20                                   ` Suparna Bhattacharya
2004-01-02  4:36                                     ` Andrew Morton [this message]
2004-01-02  5:50                               ` [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch Suparna Bhattacharya
2004-01-02  7:31                                 ` Andrew Morton
2004-01-05 13:49                                 ` Marcelo Tosatti
2004-01-05 20:27                                   ` Andrew Morton
2004-03-29 15:44                                 ` Marcelo Tosatti
2004-01-11 23:14                               ` Janet Morgan
2004-01-11 23:44                                 ` Andrew Morton
2004-01-12 18:00                                   ` filemap_fdatawait.patch Daniel McNeil
2004-01-12 19:39                                   ` [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch Janet Morgan
2004-01-12 19:46                                     ` Daniel McNeil
2004-01-13  4:12                                 ` Janet Morgan
2003-12-30  4:53       ` [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix Suparna Bhattacharya
2003-12-31  0:29         ` Daniel McNeil
2003-12-31  6:09           ` Suparna Bhattacharya
2004-01-08 23:55             ` Daniel McNeil
2004-01-09  3:55               ` Suparna Bhattacharya
2004-02-05  1:39                 ` [PATCH 2.6.2-rc3-mm1] DIO read race fix Daniel McNeil
2004-02-05  1:54                   ` Badari Pulavarty
2004-02-05  2:07                   ` Andrew Morton
2004-02-05  2:54                     ` Janet Morgan
2004-02-05  3:19                       ` Andrew Morton
2004-02-05  3:43                         ` Suparna Bhattacharya
2004-02-05  5:33                   ` Andrew Morton
2004-02-05 17:52                     ` Daniel McNeil
2004-02-05 18:53                     ` Badari Pulavarty
2004-03-29 15:41           ` [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix Suparna Bhattacharya

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