From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>, CAI Qian <caiqian@redhat.com>,
linux-xfs <linux-xfs@vger.kernel.org>,
xfs@oss.sgi.com
Subject: Re: xfs_file_splice_read: possible circular locking dependency detected
Date: Thu, 8 Sep 2016 19:22:22 -0700 [thread overview]
Message-ID: <CA+55aFzohsUXj_3BeFNr2t50Wm=G+7toRDEz=Tk7VJqP3n1hXQ@mail.gmail.com> (raw)
In-Reply-To: <20160909015324.GD30056@dastard>
On Thu, Sep 8, 2016 at 6:53 PM, Dave Chinner <david@fromorbit.com> wrote:
>
> So if we race iwth a truncate, the pages in spd.pages[] that are
> beyond the new EOF may or may not have been removed from the page
> cache.
So I'm not sure why we'd need to care?
The thing is, if the splicer and the hole puncher aren't synchronized,
then there is by definition no "before/after" point.
The splice data may be "stale" in the sense that we look at the page
after the hole punch has happened and the page no longer has a
->mapping associated with it, but it is equally valid to treat that as
just a case of "the read happened before the hole punch".
Put another way: it's not wrong to use the ostensibly "stale" data, it
just means that the splice acts as if the IO had happened before the
data became stale.
The whole point of "splice" is for the pipe to act as a in-kernel
buffer. So a splice does not *synchronize* the two end-points, quite
the reverse: it is meant to act as a "read + write" with the pipe
itself being the buffer in between (and because it's a in-kernel
buffer rather than a user space buffer like a real read()+write() pair
would be, it means that we then *can* do things like zero-copy, but
realistically it really aims for "one-copy" rather than "two-copy".
So if the splice buffer contains stale values, then that's exactly
similar to a user space application having done a "read()" of old
data, then the file is truncated (or hole punched), and then the
application does a "write()" on that data. The target clearly sees
*different* data than is on the filesystem at that point, but since
"complete synchronization" has never been a guarantee of splice() in
the first place, that's just not a downside.
If an application expects to have "splice()" give some kind of data
consistency guarantees wrt people writing to the file (or with
truncate or hole punching), then the application would have to
implement that serialization itself. Splice in itself does not do
serialization, it does data copying.
Linus
next prev parent reply other threads:[~2016-09-09 2:22 UTC|newest]
Thread overview: 152+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <723420070.1340881.1472835555274.JavaMail.zimbra@redhat.com>
[not found] ` <1832555471.1341372.1472835736236.JavaMail.zimbra@redhat.com>
2016-09-03 0:39 ` xfs_file_splice_read: possible circular locking dependency detected Dave Chinner
2016-09-03 0:57 ` Linus Torvalds
2016-09-03 1:45 ` Al Viro
2016-09-06 23:59 ` Dave Chinner
2016-09-08 20:35 ` Al Viro
2016-09-06 21:53 ` CAI Qian
2016-09-06 23:34 ` Dave Chinner
2016-09-08 15:29 ` CAI Qian
2016-09-08 17:56 ` Al Viro
2016-09-08 18:12 ` Linus Torvalds
2016-09-08 18:18 ` Linus Torvalds
2016-09-08 20:44 ` Al Viro
2016-09-08 20:57 ` Al Viro
2016-09-08 21:23 ` Al Viro
2016-09-08 21:38 ` Dave Chinner
2016-09-08 23:55 ` Al Viro
2016-09-09 1:53 ` Dave Chinner
2016-09-09 2:22 ` Linus Torvalds [this message]
2016-09-09 2:26 ` Linus Torvalds
2016-09-09 2:34 ` Al Viro
2016-09-09 2:50 ` Linus Torvalds
2016-09-09 22:19 ` Al Viro
2016-09-10 2:06 ` Linus Torvalds
2016-09-14 3:16 ` Al Viro
2016-09-14 3:39 ` Nicholas Piggin
2016-09-14 4:01 ` Linus Torvalds
2016-09-18 5:33 ` Al Viro
2016-09-19 3:08 ` Nicholas Piggin
2016-09-19 6:11 ` Al Viro
2016-09-19 7:26 ` Nicholas Piggin
2016-09-14 3:49 ` Linus Torvalds
2016-09-14 4:26 ` Al Viro
2016-09-17 8:20 ` Al Viro
2016-09-17 19:00 ` Al Viro
2016-09-17 20:15 ` Linus Torvalds
2016-09-18 19:31 ` skb_splice_bits() and large chunks in pipe (was " Al Viro
2016-09-18 20:12 ` Linus Torvalds
2016-09-18 22:31 ` Al Viro
2016-09-19 0:18 ` Linus Torvalds
2016-09-19 0:22 ` Al Viro
2016-09-19 0:22 ` Al Viro
2016-09-20 9:51 ` Herbert Xu
2016-09-23 19:00 ` [RFC][CFT] splice_read reworked Al Viro
2016-09-23 19:01 ` [PATCH 01/11] fix memory leaks in tracing_buffers_splice_read() Al Viro
2016-09-23 19:02 ` [PATCH 02/11] splice_to_pipe(): don't open-code wakeup_pipe_readers() Al Viro
2016-09-23 19:02 ` [PATCH 03/11] splice: switch get_iovec_page_array() to iov_iter Al Viro
2016-09-23 19:02 ` Al Viro
2016-09-23 19:03 ` [PATCH 04/11] splice: lift pipe_lock out of splice_to_pipe() Al Viro
2016-09-23 19:45 ` Linus Torvalds
2016-09-23 20:10 ` Al Viro
2016-09-23 20:36 ` Linus Torvalds
2016-09-24 3:59 ` Al Viro
2016-09-24 17:29 ` Al Viro
2016-09-27 15:38 ` Nicholas Piggin
2016-09-27 15:53 ` Chuck Lever
2016-09-27 15:53 ` Chuck Lever
2016-09-24 3:59 ` [PATCH 04/12] " Al Viro
2016-09-26 13:35 ` Miklos Szeredi
2016-09-26 13:35 ` Miklos Szeredi
2016-09-27 4:14 ` Al Viro
2016-09-27 4:14 ` Al Viro
2016-12-17 19:54 ` Andreas Schwab
2016-12-18 19:28 ` Linus Torvalds
2016-12-18 19:57 ` Andreas Schwab
2016-12-18 20:12 ` Al Viro
2016-12-18 20:30 ` Al Viro
2016-12-18 22:10 ` Linus Torvalds
2016-12-18 22:18 ` Al Viro
2016-12-18 22:22 ` Linus Torvalds
2016-12-18 22:49 ` Andreas Schwab
2016-12-21 18:56 ` Andreas Schwab
2016-12-21 19:12 ` Linus Torvalds
2016-09-24 4:00 ` [PATCH 06/12] new helper: add_to_pipe() Al Viro
2016-09-26 13:49 ` Miklos Szeredi
2016-09-24 4:01 ` [PATCH 10/12] new iov_iter flavour: pipe-backed Al Viro
2016-09-29 20:53 ` Miklos Szeredi
2016-09-29 22:50 ` Al Viro
2016-09-29 22:50 ` Al Viro
2016-09-30 7:30 ` Miklos Szeredi
2016-10-03 3:34 ` [RFC] O_DIRECT vs EFAULT (was Re: [PATCH 10/12] new iov_iter flavour: pipe-backed) Al Viro
2016-10-03 17:07 ` Linus Torvalds
2016-10-03 18:54 ` Al Viro
2016-09-24 4:01 ` [PATCH 11/12] switch generic_file_splice_read() to use of ->read_iter() Al Viro
2016-09-24 4:02 ` [PATCH 12/12] switch default_file_splice_read() to use of pipe-backed iov_iter Al Viro
2016-09-23 19:03 ` [PATCH 05/11] skb_splice_bits(): get rid of callback Al Viro
2016-09-23 19:03 ` Al Viro
2016-09-23 19:04 ` [PATCH 06/11] new helper: add_to_pipe() Al Viro
2016-09-23 19:04 ` [PATCH 07/11] fuse_dev_splice_read(): switch to add_to_pipe() Al Viro
2016-09-23 19:06 ` [PATCH 08/11] cifs: don't use memcpy() to copy struct iov_iter Al Viro
2016-09-23 19:08 ` [PATCH 09/11] fuse_ioctl_copy_user(): don't open-code copy_page_{to,from}_iter() Al Viro
2016-09-26 9:31 ` Miklos Szeredi
2016-09-23 19:09 ` [PATCH 10/11] new iov_iter flavour: pipe-backed Al Viro
2016-09-23 19:10 ` [PATCH 11/11] switch generic_file_splice_read() to use of ->read_iter() Al Viro
2016-09-30 13:32 ` [RFC][CFT] splice_read reworked CAI Qian
2016-09-30 17:42 ` CAI Qian
2016-09-30 18:33 ` CAI Qian
2016-09-30 18:33 ` CAI Qian
2016-10-03 1:37 ` Al Viro
2016-10-03 17:49 ` CAI Qian
2016-10-04 17:39 ` local DoS - systemd hang or timeout (WAS: Re: [RFC][CFT] splice_read reworked) CAI Qian
2016-10-04 21:42 ` tj
2016-10-05 14:09 ` CAI Qian
2016-10-05 15:30 ` tj
2016-10-05 15:54 ` CAI Qian
2016-10-05 18:57 ` CAI Qian
2016-10-05 20:05 ` Al Viro
2016-10-06 12:20 ` CAI Qian
2016-10-06 12:25 ` CAI Qian
2016-10-06 16:11 ` CAI Qian
2016-10-06 17:00 ` Linus Torvalds
2016-10-06 18:12 ` CAI Qian
2016-10-07 9:57 ` Dave Chinner
2016-10-07 15:25 ` Linus Torvalds
2016-10-07 7:08 ` Jan Kara
2016-10-07 14:43 ` CAI Qian
2016-10-07 15:27 ` CAI Qian
2016-10-07 18:56 ` CAI Qian
2016-10-09 21:54 ` Dave Chinner
2016-10-10 14:10 ` CAI Qian
2016-10-10 20:14 ` CAI Qian
2016-10-10 21:57 ` Dave Chinner
2016-10-12 19:50 ` [bisected] " CAI Qian
2016-10-12 20:59 ` Dave Chinner
2016-10-13 16:25 ` CAI Qian
2016-10-13 20:49 ` Dave Chinner
2016-10-13 20:56 ` CAI Qian
2016-10-09 21:51 ` Dave Chinner
2016-10-21 15:38 ` [4.9-rc1+] overlayfs lockdep CAI Qian
2016-10-24 12:57 ` Miklos Szeredi
2016-10-07 9:27 ` local DoS - systemd hang or timeout (WAS: Re: [RFC][CFT] splice_read reworked) Dave Chinner
2016-10-27 12:52 ` local DoS - systemd hang or timeout with cgroup traces CAI Qian
2016-10-03 1:42 ` [RFC][CFT] splice_read reworked Al Viro
2016-10-03 14:06 ` CAI Qian
2016-10-03 15:20 ` CAI Qian
2016-10-03 21:12 ` Dave Chinner
2016-10-04 13:57 ` CAI Qian
2016-10-03 20:32 ` CAI Qian
2016-10-03 20:35 ` Al Viro
2016-10-04 13:29 ` CAI Qian
2016-10-04 14:28 ` Al Viro
2016-10-04 16:21 ` CAI Qian
2016-10-04 20:12 ` Al Viro
2016-10-05 14:30 ` CAI Qian
2016-10-05 16:07 ` Al Viro
2016-09-09 2:31 ` xfs_file_splice_read: possible circular locking dependency detected Al Viro
2016-09-09 2:39 ` Linus Torvalds
2016-09-09 2:26 ` Al Viro
2016-09-09 2:19 ` Al Viro
2016-09-08 18:01 ` Linus Torvalds
2016-09-08 20:39 ` CAI Qian
2016-09-08 21:19 ` Dave Chinner
2016-09-08 21:30 ` Al Viro
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CA+55aFzohsUXj_3BeFNr2t50Wm=G+7toRDEz=Tk7VJqP3n1hXQ@mail.gmail.com' \
--to=torvalds@linux-foundation.org \
--cc=caiqian@redhat.com \
--cc=david@fromorbit.com \
--cc=linux-xfs@vger.kernel.org \
--cc=viro@zeniv.linux.org.uk \
--cc=xfs@oss.sgi.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.