linux-kernel - Re: [PATCH v2 1/2] locking/rwsem: Prevent non-first waiter from spinning in down

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] [day] [month] [year] [list]

Message-ID: <70523e24-755f-f6ae-f665-78e6f5eef575@redhat.com>
Date:   Thu, 13 Oct 2022 11:37:43 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, john.p.donnelly@...cle.com,
        Hillf Danton <hdanton@...a.com>,
        Mukesh Ojha <quic_mojha@...cinc.com>,
        Ting11 Wang 王婷 <wangting11@...omi.com>
Subject: Re: [PATCH v2 1/2] locking/rwsem: Prevent non-first waiter from
 spinning in down_write() slowpath

On 10/13/22 09:33, Waiman Long wrote:
> On 10/13/22 06:02, Peter Zijlstra wrote:
>> On Wed, Oct 12, 2022 at 09:33:32AM -0400, Waiman Long wrote:
>>> A non-first waiter can potentially spin in the for loop of
>>> rwsem_down_write_slowpath() without sleeping but fail to acquire the
>>> lock even if the rwsem is free if the following sequence happens:
>>>
>>>    Non-first waiter       First waiter      Lock holder
>>>    ----------------       ------------      -----------
>>>    Acquire wait_lock
>>>    rwsem_try_write_lock():
>>>      Set handoff bit if RT or
>>>        wait too long
>>>      Set waiter->handoff_set
>>>    Release wait_lock
>>>                           Acquire wait_lock
>>>                           Inherit waiter->handoff_set
>>>                           Release wait_lock
>>>                        Clear owner
>>>                                             Release lock
>>>    if (waiter.handoff_set) {
>>>      rwsem_spin_on_owner(();
>>>      if (OWNER_NULL)
>>>        goto trylock_again;
>>>    }
>>>    trylock_again:
>>>    Acquire wait_lock
>>>    rwsem_try_write_lock():
>>>       if (first->handoff_set && (waiter != first))
>>>           return false;
>>>    Release wait_lock
>>>
>>> It is especially problematic if the non-first waiter is an RT task and
>>> it is running on the same CPU as the first waiter as this can lead to
>>> live lock.
>>>
>> So why not do a better handoff? Specifically, have the owner set owner
>> to first-waiter instead of NULL ? (like the normal mutex code)
>
> I understand your desire to make the rwsem handoff process more like 
> what mutex is currently doing. I certainly think it is doable and will 
> put this in my todo list. However, that needs to be done at unlock and 
> wakeup time. I expect that will require moderate amount of code 
> changes which will make it not that suitable for backporting to the 
> stable releases.
>
> I would like to see these simple fixes get merged first and then we 
> can work on a major revamp of the handoff code. What do you think?
>
I am planning to post additional patches on top to rework the handoff 
code sometimes next week, but I will keep these fix patches for the 
stable releases.

Cheers,
Longman