linux-kernel - Re: [fuse-devel] fuse: feasible to distinguish between umount and abort?

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Message-ID: <CAJfpegsaOBE-4MUZBT=pMSYsTD7kMmstrFfU5D7MYuYw+f15aQ@mail.gmail.com>
Date:   Thu, 24 Nov 2016 10:10:44 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>
Subject: Re: [fuse-devel] fuse: feasible to distinguish between umount and abort?

On Thu, Nov 24, 2016 at 12:11 AM, Nikolaus Rath <Nikolaus@...h.org> wrote:
> Hello,
>
> Currently, both a call to umount(2) and writing "1" to
> /sys/fs/fuse/connections/NNN/abort will put the /dev/fuse fd into the
> same state: reading from it returns ENODEV, and polling on it returns
> POLLERR.
>
> This causes problems for filesystems that want to ensure that the
> mountpoint is free when they exit. If accessing the device fd gives the
> above errors, they have to do an additional check to determine if they
> still need to unmount the mountpoint. This is difficult to do without
> race conditions (think of someone unmounting and immediately re-starting
> a new filesystem instance).
>
> Would it be possible to change the behavior of the /dev/fuse fd so that
> userspace can distinguish between a regular umount and use of the
> /sys/fs/fuse abort)?

Yes.  My proposal would be for the kernel to send FUSE_DESTROY
asynchronously and only return ENODEV once that request was read by
userspace.  Currently FUSE_DESTROY is sent synchronously for fuseblk
mounts, but not for plain fuse mounts.

Please file a bug somewhere.  I don't mind if kernel bugs are also
kept at the github project as long as they can easily be found.

Thanks,
Miklos