linux-kernel - Re: [BUG]locking/rwsem: only clean RWSEM_FLAG

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]

Date:   Thu, 11 Nov 2021 16:54:16 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Hillf Danton <hdanton@...a.com>,
        马振华 <mazhenhua@...omi.com>,
        mingo <mingo@...hat.com>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already
 set


On 11/11/21 16:27, Peter Zijlstra wrote:
> On Thu, Nov 11, 2021 at 03:45:30PM -0500, Waiman Long wrote:
>> On 11/11/21 15:35, Peter Zijlstra wrote:
>>> On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
>>>> On 11/11/21 14:20, Peter Zijlstra wrote:
>>>>> On Thu, Nov 11, 2021 at 02:14:48PM -0500, Waiman Long wrote:
>>>>>> As for the PHASE_CHANGE name, we have to be consistent in both rwsem and
>>>>>> mutex. Maybe a follow up patch if you think we should change the
>>>>>> terminology.
>>>>> Well, that's exactly the point, they do radically different things.
>>>>> Having the same name for two different things is confusing.
>>>>>
>>>>> Anyway, let me go read that patch you sent.
>>>> My understanding of handoff is to disable optimistic spinning to let waiters
>>>> in the wait queue have an opportunity to acquire the lock. There are
>>>> difference in details on how to do that in mutex and rwsem, though.
>>> Ah, but the mutex does an actual hand-off, it hands the lock to a
>>> specific waiting task. That is, unlock() sets owner, as opposed to
>>> trylock().
>>>
>>> The rwsem code doesn't, it just forces a phase change. Once a waiter has
>>> been blocked too long, the handoff bit is set, causing new readers to be
>>> blocked. Then we wait for existing readers to complete. At that point,
>>> any next waiter (most likely a writer) should really get the lock (and
>>> in that regards the rwsem code is a bit funny).
>>>
>>> So while both ensure fairness, the means of doing so is quite different.
>>> One hands the lock ownership to a specific waiter, the other arranges
>>> for a quiescent state such that the next waiter can proceed.
>> That is a valid argument. However, the name PHASE_CHANGE sounds weird to me.
>> I am not objecting to changing the term, but probably with a better name
>> NO_OPTSPIN, NO_LOCKSTEALING or something like that to emphasize that fact
>> that optimistic spinning or lock stealing should not be allowed.
> RWSEM_FLAG_QUIESCE ?

I think that is a more acceptable term than PHASE_CHANGE. Will have a 
follow up patch later on. This one is more urgent and I want to get it 
done first.

Cheers,
Longman.