linux-kernel - Re: [PATCH] anon_vmas: Convert the rwsem to an rwlock

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Message-ID: <CA+55aFyMXxH-PGewqxse=VJzMdHWkyN4iKMh-VgQ1X2yJFnAgw@mail.gmail.com>
Date:	Sat, 28 Sep 2013 12:43:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Waiman Long <Waiman.Long@...com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] anon_vmas: Convert the rwsem to an rwlock_t

On Sat, Sep 28, 2013 at 12:37 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> -               down_write_nest_lock(&anon_vma->root->rwsem, &mm->mmap_sem);
> +               down_write_nest_lock(&anon_vma->root->rwlock, &mm->mmap_sem);

That's just completely bogus, and cannot work.

Maybe just a "write_lock(&anon_vma->root->rwlock)" (which is just
anon_vma_unlock_write(anon_vma)). But I think we might have a lockdep
issue. I'm not quite sure what's up with the nesting there.

> -       if (rwsem_is_locked(&anon_vma->root->rwsem)) {
> +       if (write_can_lock(&anon_vma->root->rwlock)) {
>                 anon_vma_lock_write(anon_vma);
>                 anon_vma_unlock_write(anon_vma);
>         }

That's the wrong way around. It should be

        if (!write_can_lock(&anon_vma->root->rwlock)) {

so some more testing definitely needed.

          Linus
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