Re: [PATCH v3 1/5] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath

[Waiman Long <longman@redhat.com>]

On 10/24/22 09:17, Peter Zijlstra wrote:
> On Mon, Oct 17, 2022 at 05:13:52PM -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.
> I'm struggling to connect the Changelog to the actual patch. I see the
> problem, but I don't see how the below helps or is even related to the
> described problem.

Sorry if the description isn't clear, I will rephrase it to make it 
clearer. The basic idea is that a non-first waiter can mistakenly 
believe that it can spin on the lock. However, when 
rwsem_try_write_lock() is called, it can never acquire the lock and move 
on because it is not the first waiter:

                        if (first->handoff_set && (waiter != first))
                                 return false;

If that waiter happen to be an RT task, it can block the real first 
waiter to acquire the lock if it happen to run the same CPU.

Cheers,
Longman