linux-kernel - Re: [RFC][PATCH 0/9] File descriptor hot-unplug support

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Date:	Tue, 14 Apr 2009 00:39:25 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Hugh Dickins <hugh@...itas.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [RFC][PATCH 0/9] File descriptor hot-unplug support

Tejun Heo <tj@...nel.org> writes:

> Hello, Eric.
>
> Eric W. Biederman wrote:
>> A couple of weeks ago I found myself looking at the uio, seeing that
>> it does not support pci hot-unplug, and thinking "Great yet another
>> implementation of hotunplug logic that needs to be added".
>> 
>> I decided to see what it would take to add a generic implementation of
>> the code we have for supporting hot unplugging devices in sysfs, proc,
>> sysctl, tty_io, and now almost in the tun driver.
>> 
>> Not long after I touched the tun driver and made it safe to delete the
>> network device while still holding it's file descriptor open I someone
>> else touch the code adding a different feature and my careful work
>> went up in flames.  Which brought home another point at the best of it
>> this is ultimately complex tricky code that subsystems should not need
>> to worry about.
>
> I like the way it's headed.  I'm trying to add similar 'revoke' or
> 'sever' mechanism at block and char device layers so that low level
> drivers don't have to worry about object lifetimes and so on.  Doing
> it at the file layer makes sense and can probably replace whatever
> mechanism at the chardev.
>
> The biggest obstacle was the extra in-use reference count overhead.  I
> thought it could be solved by implementing generic percpu reference
> count similar to the one used for module reference counting.  Hot path
> overhead could be reduced to local_t cmpxchg (w/o LOCK prefix) on
> per-cpu variable + one branch, which was pretty good.  The problem was
> that space and access overhead for dynamic per-cpu variables wasn't
> too good, so I started working on dynamic percpu allocator.
>
> The dynamic per-cpu allocator is pretty close to completion.  Only
> several archs need to be converted and it's likely to happen during
> next few months.  The plan after that was 1. add per-cpu local_t
> accessors (might replace local_t completely) 2. add generic per-cpu
> reference counter and move module reference counting to it
> 3. implement block/chardev sever (or revoke) support.
>
> I think #3 can be merged with what you're working on.  What do you
> think?

Sounds reasonable.

Do you know of a case where we actually have multiple tasks accessing
a file simultaneously?

I just instrumented up my patch an so far the only case I have found
are multiple processes closing the same file.  Some weird part of
bash forking extra processes.

Eric
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