linux-kernel - Re: [PATCH 18/23] vfs: Teach epoll to use file_hotplug

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Message-ID: <m1tz2xox7n.fsf@fess.ebiederm.org>
Date:	Wed, 03 Jun 2009 13:53:48 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>,
	Tejun Heo <tj@...nel.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 18/23] vfs: Teach epoll to use file_hotplug_lock

Davide Libenzi <davidel@...ilserver.org> writes:

> On Tue, 2 Jun 2009, Eric W. Biederman wrote:
>
>> I am not clear what problem you have.
>> 
>> Is it the sprinkling the code that takes and removes the lock?  Just
>> the VFS needs to be involved with that.  It is a slightly larger
>> surface area than doing the work inside the file operations as we
>> sometimes call the same method from 3-4 different places but it is
>> definitely a bounded problem.
>> 
>> Is it putting in the handful lines per subsystem to actually use this
>> functionality?  At that level something generic that is maintained
>> outside of the subsystem is better than the mess we have with 4-5
>> different implementations in the subsystems that need it, each having
>> a different assortment of bugs.
>
> Come on, only in the open fast path, there are at least two spin 
> lock/unlock and two atomic ops. Without even starting to count all the 
> extra branches and software added.
> Is this stuff *really* needed, or we can faitly happily live w/out?

????

What code are you talking about?

To the open path a few memory writes and a smp_wmb.  No atomics and no
spin lock/unlocks.

Are you complaining because I retain the file_list?

Eric
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