To: Miklos Szeredi <miklos@szeredi.hu>
Cc: viro@ZenIV.linux.org.uk, linux-kernel@vger.kernel.org,
       linux-pci@vger.kernel.org, linux-mm@kvack.org,
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       alan@lxorguk.ukuu.org.uk, gregkh@suse.de, npiggin@suse.de,
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From: ebiederm@xmission.com (Eric W. Biederman)
Date: Mon, 08 Jun 2009 23:31:16 -0700
In-Reply-To: <E1MDuEI-0006BC-1C@pomaz-ex.szeredi.hu> (Miklos Szeredi's message of "Tue\, 09 Jun 2009 07\:50\:38 +0200")
Message-ID: <m1tz2pyl3f.fsf@fess.ebiederm.org>
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Subject: Re: [PATCH 0/23] File descriptor hot-unplug support v2
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Miklos Szeredi <miklos@szeredi.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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