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

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Date:   Thu, 24 Nov 2016 16:33:28 -0800
From:   Nikolaus Rath <Nikolaus@...h.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     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 Nov 24 2016, Miklos Szeredi <miklos@...redi.hu> wrote:
> 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.

I trust that this is a good plan, but from the description I can't quite
tell how the filesystem would make the distinction between umount/abort
based on this. Would FUSE_DESTROY be send only for unmount, but not for
abort?


> 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.

Already done at https://github.com/libfuse/libfuse/issues/122.


Thanks!
-Nikolaus

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