Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <4e1697d7-9c13-12c9-74bc-dcb97ee2a659@quicinc.com>
Date:   Fri, 30 Sep 2022 10:17:32 +0530
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.13.0
Subject: Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in
 down_write() slowpath
Content-Language: en-US
To:     Waiman Long <longman@redhat.com>
CC:     <linux-kernel@vger.kernel.org>, <john.p.donnelly@oracle.com>,
        Hillf Danton <hdanton@sina.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@redhat.com>
References: <20220929180413.107374-1-longman@redhat.com>
 <6530f135-e8ac-bb6c-4715-1ea8f76cf4c4@redhat.com>
From:   Mukesh Ojha <quic_mojha@quicinc.com>
In-Reply-To: <6530f135-e8ac-bb6c-4715-1ea8f76cf4c4@redhat.com>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

Hi,

On 9/29/2022 11:36 PM, Waiman Long wrote:
> On 9/29/22 14:04, 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.
>>
>> Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more 
>> consistent")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/locking/rwsem.c | 13 ++++++++++---
>>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> Mukesh, can you test if this patch can fix the RT task lockup problem?

I will come back with the result.

Thank you.

> 
> Thanks,
> Longman
>