Re: [PATCH 0/23] File descriptor hot-unplug support v2

From: Eric W. Biederman
Date: Tue Jun 09 2009 - 02:31:33 EST

Miklos Szeredi <miklos@xxxxxxxxxx> writes:

> On Mon, 8 Jun 2009, Al Viro wrote:
>> On Mon, Jun 08, 2009 at 06:44:41PM +0200, Miklos Szeredi wrote:
>> > I'm still not getting what the problem is. AFAICS file operations are
>> > either
>> >
>> > a) non-interruptible but finish within a short time or
>> > b) may block indefinitely but are interruptible (or at least killable).
>> >
>> > Anything else is already problematic, resulting in processes "stuck in
>> > D state".
>> Welcome to reality...
>> * bread() is non-interruptible
>> * so's copy_from_user()/copy_to_user()
> And why should revoke(2) care? Just wait for the damn thing to
> finish. Why exactly do these need to be interruptible?

Agreed. I expect the data size is going to be a page or less. Which
is at most 64K on some weird architectures. I think that counts as a
short time waiting for disk I/O. Baring thrashing.

> Okay, if we want revoke or umount -f to be instantaneous then all that
> needs to be taken care of. But does it *need* to be?

Good question. I wonder what umount -f needs when we yank out a usb drive.

> My idea of revoke is something like below:
> - make sure no new operations are started on the file
> - check state of tasks for ongoing operations, if interruptible send signal

Figuring out who to send a signal to is tricky. Still it should be doable
in the common case.

> - wait for all pending operations to finish
> - kill file

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