linux-kernel - Re: [PATCH 1/3] nfsd: use __fput_sync() to avoid delayed closing of files.

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Date:   Mon, 11 Dec 2023 23:21:35 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     NeilBrown <neilb@...e.de>
Cc:     Chuck Lever <chuck.lever@...cle.com>,
        Christian Brauner <brauner@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH 1/3] nfsd: use __fput_sync() to avoid delayed closing of
 files.

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.