linux-kernel - Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT

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Date:	Fri, 13 Sep 2013 10:29:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...ux.ie>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG] completely bonkers use of set_need_resched +
 VM_FAULT_NOPAGE

On Fri, Sep 13, 2013 at 09:46:03AM +0200, Thomas Hellstrom wrote:
> >>if (!bo_tryreserve()) {
> >>     up_read mmap_sem(); // Release the mmap_sem to avoid deadlocks.
> >>     bo_reserve();               // Wait for the BO to become available (interruptible)
> >>     bo_unreserve();           // Where is bo_wait_unreserved() when we need it, Maarten :P
> >>     return VM_FAULT_RETRY; // Go ahead and retry the VMA walk, after regrabbing
> >>}
> 
> Anyway, could you describe what is wrong, with the above solution, because
> it seems perfectly legal to me.

Luckily the rule of law doesn't have anything to do with this stuff --
at least I sincerely hope so.

The thing that's wrong with that pattern is that its still not
deterministic - although its a lot better than the pure trylock. Because
you have to release and re-acquire with the trylock another user might
have gotten in again. Its utterly prone to starvation.

The acquire+release does remove the dead/life-lock scenario from the
FIFO case, since blocking on the acquire will allow the other task to
run (or even get boosted on -rt).

Aside from that there's nothing particularly wrong with it and lockdep
should be happy afaict (but I haven't had my morning juice yet).
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