linux-kernel - Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

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Date:	Fri, 5 Jun 2015 02:45:58 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	der.herr@...r.at
Subject: Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

On Tue, May 26, 2015 at 02:57:53PM -0700, Linus Torvalds wrote:

> Because that is another example of a complete failure of a locking
> primitive that was just too specialized to be worth it.

<notices stale include in fs/file_table.c and removes it>

FWIW, I hadn't really looked into stop_machine uses, but fs/locks.c one
is really not all that great - there we have a large trashcan of a list
(every file_lock on the system) and the only use of that list is /proc/locks
output generation.  Sure, additions take this CPU's spinlock.  And removals
take pretty much a random one - losing the timeslice and regaining it on
a different CPU is quite likely with the uses there.

Why do we need a global lock there, anyway?  Why not hold only one for
the chain currently being traversed?  Sure, we'll need to get and drop
them in ->next() that way; so what?
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