linux-kernel - Re: [PATCH] Fix data race in mark_rt_mutex

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Date:   Thu, 26 Jan 2023 10:42:07 +0100
From:   Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     paulmck@...nel.org, Arjan van de Ven <arjan@...ux.intel.com>,
        mingo@...hat.com, will@...nel.org, boqun.feng@...il.com,
        akpm@...l.org, tglx@...utronix.de, joel@...lfernandes.org,
        stern@...land.harvard.edu, diogo.behrens@...wei.com,
        jonas.oberhauser@...wei.com, linux-kernel@...r.kernel.org,
        Hernan Ponce de Leon <hernanl.leon@...wei.com>,
        stable@...r.kernel.org,
        Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Subject: Re: [PATCH] Fix data race in mark_rt_mutex_waiters

On 1/24/2023 5:04 PM, Waiman Long wrote:
> 
> On 1/24/23 10:52, Peter Zijlstra wrote:
>> On Tue, Jan 24, 2023 at 10:42:24AM -0500, Waiman Long wrote:
>>
>>> I would suggest to do it as suggested by PeterZ. Instead of set_bit(),
>>> however, it is probably better to use atomic_long_or() like
>>>
>>> atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t 
>>> *)&lock->owner)
>> That function doesn't exist, atomic_long_or() is implicitly relaxed for
>> not returning a value.
>>
> You are right. atomic_long_or() doesn't have variants like some others.
> 
> Cheers,
> Longman
> 

When you say "replace the whole of that function", do you mean "barrier 
included"? I argue in the other email that I think this should not 
affect correctness (at least not obviously), but removing the barrier is 
doing more than just fixing the data race as this patch suggests.

Hernan