Re: [PATCH 18/23] vfs: Teach epoll to use file_hotplug_lock

From: ebiederm@xmission.com (Eric W. Biederman)
To: Davide Libenzi <davidel@xmailserver.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-pci@vger.kernel.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org, Hugh Dickins <hugh@veritas.com>,
	Tejun Heo <tj@kernel.org>, Alexey Dobriyan <adobriyan@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@suse.de>,
	Nick Piggin <npiggin@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	"Eric W. Biederman" <ebiederm@aristanetworks.com>
Subject: Re: [PATCH 18/23] vfs: Teach epoll to use file_hotplug_lock
Date: Wed, 03 Jun 2009 13:53:48 -0700	[thread overview]
Message-ID: <m1tz2xox7n.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <alpine.DEB.1.10.0906030754550.17143@makko.or.mcafeemobile.com> (Davide Libenzi's message of "Wed\, 3 Jun 2009 07\:57\:40 -0700 \(PDT\)")

Davide Libenzi <davidel@xmailserver.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