linux-kernel - Re: Linux 3.16-rc6

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Message-ID: <20140724183643.GM3935@laptop>
Date:	Thu, 24 Jul 2014 20:36:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Borislav Petkov <bp@...en8.de>, Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>
Subject: Re: Linux 3.16-rc6

On Thu, Jul 24, 2014 at 11:18:16AM -0700, Linus Torvalds wrote:
> On Thu, Jul 24, 2014 at 5:58 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > So going by the nifty picture rostedt made:
> >
> > [   61.454336]        CPU0                    CPU1
> > [   61.454336]        ----                    ----
> > [   61.454336]   lock(&(&p->alloc_lock)->rlock);
> > [   61.454336]                                local_irq_disable();
> > [   61.454336]                                lock(tasklist_lock);
> > [   61.454336]                                lock(&(&p->alloc_lock)->rlock);
> > [   61.454336]   <Interrupt>
> > [   61.454336]     lock(tasklist_lock);
> 
> So this *should* be fine. It always has been in the past, and it was
> certainly the *intention* that it should continue to work with
> qrwlock, even in the presense of pending writers on other cpu's.
> 
> The qrwlock rules are that a read-lock in an interrupt is still going
> to be unfair and succeed if there are other readers.

Ah, indeed. Should have checked :/

> So it sounds to me like the new lockdep rules in tip/master are too
> strict and are throwing a false positive.

Right. Waiman can you have a look?
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