linux-kernel - Re: [PATCH 1/3] lglock: remove online variants of lock

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Message-ID: <20120508045044.GJ22082@ZenIV.linux.org.uk>
Date:	Tue, 8 May 2012 05:50:44 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Nick Piggin <npiggin@...nel.dk>, Andi Kleen <ak@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/3] lglock: remove online variants of lock

On Tue, May 08, 2012 at 01:29:45PM +0930, Rusty Russell wrote:
> Optimizing the slow paths adds a lot of complexity.  If you need to
> grab every lock often, you have other problems.

Applied, but I'm not too happy about the situation with vfsmount_lock ;-/
On kernels built for a lot of possible CPUs the loss of ..._online()
versions will get painful.  OTOH, we can always put the map + single
spinlock + single notifier into lglock.c and reproduce the old logics.
I'll do a patch along those lines and put it on a separate branch;
then we'll be able to test and compare.

Contention from cpu map spinlock becoming shared between different
lglocks (all two of them) is not an issue - we never use the
files_lock one anyway (there we can't use _online variants).
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