linux-kernel - Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write

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Message-ID: <20160512135816.GI3193@twins.programming.kicks-ass.net>
Date:	Thu, 12 May 2016 15:58:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <Waiman.Long@....com>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for
 down_write_killable

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..