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Date: Fri, 5 Jun 2015 02:45:58 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>, Peter Zijlstra <peterz@infradead.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Subject: Re: [RFC][PATCH 0/5] Optimize percpu-rwsem
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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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