From: "NeilBrown" <neilb@suse.de>
To: "Al Viro" <viro@zeniv.linux.org.uk>
Cc: "Chuck Lever" <chuck.lever@oracle.com>,
"Christian Brauner" <brauner@kernel.org>,
"Jens Axboe" <axboe@kernel.dk>, "Oleg Nesterov" <oleg@redhat.com>,
"Jeff Layton" <jlayton@kernel.org>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-nfs@vger.kernel.org
Subject: Re: [PATCH 1/3] nfsd: use __fput_sync() to avoid delayed closing of files.
Date: Wed, 13 Dec 2023 11:28:04 +1100 [thread overview]
Message-ID: <170242728484.12910.12134295135043081177@noble.neil.brown.name> (raw)
In-Reply-To: <20231211232135.GF1674809@ZenIV>
On Tue, 12 Dec 2023, Al Viro wrote:
> On Mon, Dec 11, 2023 at 11:13:30PM +0000, Al Viro wrote:
>
> > dentry_kill() means ->d_release(), ->d_iput() and anything final iput()
> > could do. Including e.g. anything that might be done by afs_silly_iput(),
> > with its "send REMOVE to server, wait for completion". No, that's not
> > a deadlock per se, but it can stall you a bit more than you would
> > probably consider tolerable... Sure, you could argue that AFS ought to
> > make that thing asynchronous, but...
> >
> > Anyway, it won't be "safe to use in most contexts". ->mmap_lock alone
> > is enough for that, and that's just the one I remember to have given
> > us a lot of headache. And that's without bringing the "nfsd won't
> > touch those files" cases - make it generally accessible and you get
> > to audit all locks that might be taken when we close a socket, etc.
>
> PS: put it that way - I can buy "nfsd is doing that only to regular
> files and not on an arbitrary filesystem, at that; having the thread
> wait on that sucker is not going to cause too much trouble"; I do *not*
> buy turning it into a thing usable outside of a very narrow set of
> circumstances.
>
Can you say more about "not on an arbitrary filesystem" ?
I guess you means that procfs and/or sysfs might be problematic as may
similar virtual filesystems (nfsd maybe).
Could we encode some of this in the comment for __fput_sync ??
/**
* __fput_sync : drop reference to a file synchronously
* @f: file to drop
*
* Drop a reference on a file and do most cleanup work before returning.
*
* Due the the wide use of files in the design of Linux, dropping the
* final reference to a file can result in dropping the final reference
* to any of a large variety of other objects. Dropping those final
* references can result in nearly arbitrary work. It should be assumed
* that, unless prior checks or actions confirm otherwise, calling
* __fput_sync() might:
* - allocate memory
* - perform synchronous IO
* - wait for a remote service (for networked filesystems)
* - take ->i_rwsem and other related VFS and filesystem locks
* - take ->s_umount (if file is on a MNT_INTERNAL filesystem)
* - take locks in a device driver if the file is CHR, BLK or SOCK
*
* If the caller cannot be confident that none of these will cause a
* problem, it should use fput() instead.
*
* Note that the final unmount of a lazy-unmounted non-MNT_INTERNAL
* filesystem will always be handled asynchronously. Individual drivers
* might also leave some clean up to asynchronous threads.
*/
Thanks,
NeilBrown
next prev parent reply other threads:[~2023-12-13 0:28 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-08 3:27 [PATCH 0/3] nfsd: fully close all files in the nfsd threads NeilBrown
2023-12-08 3:27 ` [PATCH 1/3] nfsd: use __fput_sync() to avoid delayed closing of files NeilBrown
2023-12-08 15:01 ` Chuck Lever
2023-12-10 22:47 ` NeilBrown
2023-12-11 19:01 ` Chuck Lever
2023-12-11 22:04 ` NeilBrown
2023-12-12 16:17 ` Chuck Lever
2023-12-11 19:11 ` Al Viro
2023-12-11 22:23 ` NeilBrown
2023-12-11 23:13 ` Al Viro
2023-12-11 23:21 ` Al Viro
2023-12-13 0:28 ` NeilBrown [this message]
2023-12-15 18:27 ` David Laight
2023-12-15 19:35 ` Chuck Lever III
2023-12-15 22:36 ` NeilBrown
2023-12-15 18:28 ` David Laight
2023-12-16 1:50 ` Dave Chinner
2023-12-08 3:27 ` [PATCH 2/3] nfsd: Don't leave work of closing files to a work queue NeilBrown
2023-12-08 3:27 ` [PATCH 3/3] VFS: don't export flush_delayed_fput() NeilBrown
2023-12-08 11:40 ` [PATCH 0/3] nfsd: fully close all files in the nfsd threads Jeff Layton
2023-12-08 14:33 ` Jens Axboe
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