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

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Date:	Mon, 08 Jun 2009 23:31:16 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	viro@...IV.linux.org.uk, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, hugh@...itas.com, tj@...nel.org,
	adobriyan@...il.com, torvalds@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, gregkh@...e.de, npiggin@...e.de,
	akpm@...ux-foundation.org, hch@...radead.org
Subject: Re: [PATCH 0/23] File descriptor hot-unplug support v2

Miklos Szeredi <miklos@...redi.hu> 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

Eric
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