From: Jan Kara <jack@suse.cz>
To: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>, Jan Kara <jack@suse.cz>,
Stephen Rothwell <sfr@canb.auug.org.au>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Mikulas Patocka <mpatocka@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: linux-next: build failure after merge of the vfs tree
Date: Wed, 4 Jan 2012 19:00:33 +0100 [thread overview]
Message-ID: <20120104180033.GE28907@quack.suse.cz> (raw)
In-Reply-To: <20120104025020.GW23662@dastard>
On Wed 04-01-12 13:50:20, Dave Chinner wrote:
> On Wed, Jan 04, 2012 at 02:17:54AM +0000, Al Viro wrote:
> > I'm still not
> > sure about ->statfs(), BTW - any input on that would be welcome. Can
> > it end up blocked on a frozen fs until said fs is thawed?
>
> I don't see why this should ever happen - ->statfs has to work on
> read-only filesystems so shoul dnot be modifying state, and hence
> should never need to care about the frozen state of the superblock.
Well, I'm also not aware of a filesystem where ->statfs would wait on
frozen filesystem. Just note that e.g. for stat(2) frozen filesystem and
RO filesystem *are* different because of atime updates. So stat(2) can
block on frozen fs because of atime update while on RO filesystem it is
just fine.
> So from a ->statfs POV, a frozen filesystem should look just like a
> read-only filesystem. If frozen filesystems are holding locks that
> ->statfs can block on until the filesystem us thawed, then I'd
> consider that a bug in the filesystem freeze implementation....
In an ideal world yes. Practically, current freeze code has races
(vfs_check_frozen() is a totally racy check) which can leave processes
waiting for frozen fs with filesystem locks held. I believe we need
something like mnt_want_write()/mnt_drop_write() for freezing code in
->page_mkwrite() and ->write_begin/->write_end. I'm now looking into how
to do that in the best way.
> > to convert ustat(2) to "wait for thaw" semantics (should be interruptible,
> > BTW) or document that ->statfs() is not allowed to wait for thawing.
> > It's far too subtle to leave undocumented...
>
> The latter, IMO.
Agreed.
Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR
next prev parent reply other threads:[~2012-01-04 18:00 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20120103124331.f0f0043f8ca464c9ff13f4d3@canb.auug.org.au>
[not found] ` <20120103133942.GC31457@quack.suse.cz>
2012-01-03 14:45 ` linux-next: build failure after merge of the vfs tree Al Viro
2012-01-04 2:17 ` Al Viro
2012-01-04 2:50 ` Dave Chinner
2012-01-04 18:00 ` Jan Kara [this message]
2012-01-04 18:47 ` Christoph Hellwig
2012-01-04 22:26 ` Jan Kara
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