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

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Message-ID: <20150526185727.GA3763@redhat.com>
Date:	Tue, 26 May 2015 20:57:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

On 05/26, Linus Torvalds wrote:
>
> On Tue, May 26, 2015 at 4:43 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > This is a derived work of the cpu hotplug lock rework I did in 2013 which never
> > really went anywhere because Linus didn't like it.
> >
> > This applies those same optimizations to the percpu-rwsem. Seeing how we did
> > all the work it seemed a waste to not use it at all.
>
> So I *still* don't like it.
>
> We literally have one single percpu-rwsem IN THE WHOLE KERNEL TREE.
>
> One.

Well. IIRC Tejun is going to turn signal_struct->group_rwsem into
percpu-rwsem.

And it can have more users. Say, __sb_start_write/etc  does something
similar, and last time I checked this code it looked buggy to me.

> And that single use that you are optimizing for isn't even explained.
> It's not really even clear that that thing is needed: fork() already
> takes the mmap_sem for writing, and I'm not at all convinced that the
> uprobes code couldn't just use mmap_sem for every mm it traverses,

I don't think we can do this... At least I do not see how.

register_for_each_vma() would need to lock all ->mmap_sem's in system
to avoid the race with fork().

Oleg.

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