From: Eiichi Tsukata <devel@etsukata.com>
To: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: andi@firstfloor.org, Chris Mason <clm@fb.com>,
Josef Bacik <josef@toxicpanda.com>,
David Sterba <dsterba@suse.com>, "Theodore Ts'o" <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <yuchao0@huawei.com>,
Miklos Szeredi <miklos@szeredi.hu>,
Bob Peterson <rpeterso@redhat.com>,
Andreas Gruenbacher <agruenba@redhat.com>,
linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net,
linux-fsdevel@vger.kernel.org, cluster-devel@redhat.com,
linux-unionfs@vger.kernel.org
Subject: Re: [PATCH v1 0/4] fs: fix race between llseek SEEK_END and write
Date: Thu, 22 Nov 2018 14:40:50 +0900 [thread overview]
Message-ID: <CANhTXPQzKmsyPO9QrGz6eijjuMFzLN4BhUV=6ABDJysX0xyKfw@mail.gmail.com> (raw)
In-Reply-To: <20181121045440.GM32577@ZenIV.linux.org.uk>
2018年11月21日(水) 13:54 Al Viro <viro@zeniv.linux.org.uk>:
>
> On Wed, Nov 21, 2018 at 11:43:56AM +0900, Eiichi Tsukata wrote:
> > Some file systems (including ext4, xfs, ramfs ...) have the following
> > problem as I've described in the commit message of the 1/4 patch.
> >
> > The commit ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek")
> > removed almost all locks in llseek() including SEEK_END. It based on the
> > idea that write() updates size atomically. But in fact, write() can be
> > divided into two or more parts in generic_perform_write() when pos
> > straddles over the PAGE_SIZE, which results in updating size multiple
> > times in one write(). It means that llseek() can see the size being
> > updated during write().
>
> And? Who has ever promised anything that insane? write(2) can take an arbitrary
> amount of time; another process doing lseek() on independently opened descriptor
> is *not* going to wait for that (e.g. page-in of the buffer being written, which
> just happens to be mmapped from a file on NFS over RFC1149 link).
Thanks.
The lock I added in NFS was nothing but slow down lseek() because a file size is
updated atomically. Even `spin_lock(&inode->i_lock)` is unnecessary.
I'll fix the commit message which only refers to specific local file
systems that use
generic_perform_write() and remove unnecessary locks in some
distributed file systems
(e.g. nfs, cifs, or more) by replacing generic_file_llseek() with
generic_file_llseek_unlocked()
so that `tail` don't have to wait for avian carriers.
next prev parent reply other threads:[~2018-11-22 16:18 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-21 2:43 [PATCH v1 0/4] fs: fix race between llseek SEEK_END and write Eiichi Tsukata
2018-11-21 2:43 ` [PATCH v1 1/4] vfs: " Eiichi Tsukata
2018-11-21 2:43 ` [PATCH v1 2/4] ext4: " Eiichi Tsukata
2018-11-21 2:43 ` [PATCH v1 3/4] f2fs: " Eiichi Tsukata
2018-11-21 9:23 ` Christoph Hellwig
2018-11-22 5:42 ` Eiichi Tsukata
2018-11-21 2:44 ` [PATCH v1 4/4] overlayfs: " Eiichi Tsukata
2018-11-21 4:54 ` [PATCH v1 0/4] fs: " Al Viro
2018-11-22 5:40 ` Eiichi Tsukata [this message]
2018-11-22 7:06 ` Al Viro
2018-11-26 2:54 ` Eiichi Tsukata
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