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

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Message-ID: <CA+55aFxPyBfsfspXucTdo3PhBsxT4_5kPJN6Ace=-=DcgWODWw@mail.gmail.com>
Date:	Tue, 26 May 2015 14:57:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	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 11:42 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
>
> Instead of dropping percpu-rwsem, I was thinking we could instead look
> for opportunities to convert new users, for instance shinkers, where the
> write lock is also taken just for register and unregister purposes,
> similar to uprobes.

So if there really are useful use cases for this, I don't object to
the patch. It seems to just improve on a currently very low-usage
locking primitive.

And it's not like I conceptually mind the notion of a percpu rwsem, I
just hate seeing specialty locking that isn't really worth it.

Because as it is, with the current single use, I don't think it's even
worth improving on.

I _would_ ask that people who are looking at this also look at our
"lglock" thing. It's pretty much *exactly* the same thing, except for
spinlocks, and that one too has exactly two users (the documentation
states that the only user is stop_machine, but in fact file locking
does too).

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

                       Linus
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