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

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Date:   Fri, 30 Sep 2022 10:17:32 +0530
From:   Mukesh Ojha <quic_mojha@...cinc.com>
To:     Waiman Long <longman@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <john.p.donnelly@...cle.com>,
        Hillf Danton <hdanton@...a.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in
 down_write() slowpath

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@...hat.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
>