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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160512121204.GQ3192@twins.programming.kicks-ass.net>
Date:	Thu, 12 May 2016 14:12:04 +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 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.