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SubjectRe: reiserfs locking (v2)
On (07/09/10 05:16), Frederic Weisbecker wrote:
> On Sun, Jul 04, 2010 at 10:15:23AM +0100, Al Viro wrote:
> > On Sat, Jul 03, 2010 at 10:43:23AM +0100, Al Viro wrote:
> > > On Sat, Jul 03, 2010 at 10:24:42AM +0100, Al Viro wrote:
> > >
> > > > Gyah... For the 1001st time: readdir() is far from being the only thing that
> > > > nests mmap_sem inside i_mutex. In particular, write() does the same thing.
> > > >
> > > > So yes, it *is* a real deadlock, TYVM, with no directories involved. Open the
> > > > same file twice, mmap one fd, close it, then have munmap() hitting i_mutex
> > > > in reiserfs_file_release() race with write() through another fd.
> > > >
> > > > Incidentally, reiserfs_file_release() checks in the fastpath look completely
> > > > bogus. Checking i_count? What the hell is that one about? And no, these
> > > > checks won't stop open() coming between them and grabbing i_mutex, so they
> > > > couldn't prevent the deadlock in question anyway.
> > >
> > > ... and unfortunately it's been that way since the the initial merge in 2.4.early.
> > > FWIW, it seems that i_count check was a misguided attempt to check that no other
> > > opened struct file are there, but it's
> > > a) wrong, since way, _way_ back - open() affects d_count, not i_count
> > > b) wrong even with such modification (consider hardlinks)
> > > c) wrong for even more reasons since forever - i_count and d_count could
> > > be bumped by many things at any time
> > > d) hopelessly racy anyway, since another open() could very well have
> > > happened just as we'd finished these checks.
> >
> > OK... See 22093b8f3d387f77 in vfs-2.6.git for-next (should propagate to
> > git.kernel.org shortly). That ought to deal with this crap, assuming I hadn't
> > fucked up somewhere...
>
>
> Looks good. Thanks for fixing this!
>

Seems to work fine with my test app.

Reported-by/Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>


Sergey
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