Re: [PATCH v2 1/2] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath

[Waiman Long <longman@redhat.com>]

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?

Cheers,
Longman