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

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Date:	Wed, 3 Jun 2009 07:57:40 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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@...well.aristanetworks.com>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 18/23] vfs: Teach epoll to use file_hotplug_lock

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?


- Davide


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