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Date: Thu, 12 May 2016 15:58:16 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
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        Davidlohr Bueso <dave@stgolabs.net>, Waiman Long <Waiman.Long@hpe.com>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for
 down_write_killable
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On Thu, May 12, 2016 at 02:19:07PM +0200, Michal Hocko wrote:
> On Thu 12-05-16 14:12:04, Peter Zijlstra wrote:
> > On Wed, May 11, 2016 at 08:03:46PM +0200, Michal Hocko wrote:
> > > I still cannot say I would understand why the pending
> > > RWSEM_WAITING_BIAS matters but I would probably need to look at the code
> > > again with a clean head, __rwsem_wake is quite tricky...
> > 
> > Ah, you're asking why an unconditional __rwsem_wake(ANY) isn't enough?
> > 
> > Because; if at that point there's nobody waiting, we're left with an
> > empty list and WAITER_BIAS set. This in turn will make all fast paths
> > fail.
> > 
> > Look at rwsem_down_read_failed() for instance; if we enter that we'll
> > unconditionally queue ourself, with nobody left to come wake us.
> 
> This is still not clear to me because rwsem_down_read_failed will call
> __rwsem_do_wake if the count is RWSEM_WAITING_BIAS so we shouldn't go to
> sleep and get the lock. So you are right that we would force everybody
> to the slow path which is not great but shouldn't cause incorrect
> behavior. I guess I must be missing something obvious here...


Ah me too; I missed the obvious: we do the __rwsem_do_wake() after we
add ourselves to the list, which means we'll also wake ourselves.

I'll have more thinking..