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

[Waiman Long <longman@redhat.com>]

On 9/30/22 01:06, Mukesh Ojha wrote:
> Hi,
>
> On 9/29/2022 11:34 PM, 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(-)
>>
>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>> index 65f0262f635e..ad676e99e0b3 100644
>> --- a/kernel/locking/rwsem.c
>> +++ b/kernel/locking/rwsem.c
>> @@ -628,6 +628,11 @@ static inline bool rwsem_try_write_lock(struct 
>> rw_semaphore *sem,
>>           new = count;
>>             if (count & RWSEM_LOCK_MASK) {
>> +            /*
>> +             * A waiter (first or not) can set the handoff bit
>> +             * if it is an RT task or wait in the wait queue
>> +             * for too long.
>> +             */
>>               if (has_handoff || (!rt_task(waiter->task) &&
>>                           !time_after(jiffies, waiter->timeout)))
>
> Not related to this issue, however wanted to understand the idea about 
> this.
>
> If RT task comes in any order either come first or later it is setting 
> the RWSEM_FLAG_HANDOFF bit.
> So, here we are giving some priority right a way to RT task however it
> can not get waiter->handoff_set=true since it is not the first 
> waiter.(after this patch), is it not conflicting ?

I have thought about moving the RT task forward in the wait queue, but 
then it will greatly complicate the code to try to do what a PREEMPT_RT 
kernel does using a rt_mutex variant of rwsem. The reason why HANDOFF 
bit is set when an RT task is in the wait queue is speed up the 
progression of the wait queue without the interference of optimistic 
spinner.

>
>
> Why can't we just keep like as below and not set
> new |= RWSEM_FLAG_HANDOFF; and return false from here.
>
> --------------0<------------------------------------
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 65f0262..dbe3e16 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -628,8 +628,8 @@ static inline bool rwsem_try_write_lock(struct
> rw_semaphore *sem,
>                  new = count;
>
>                  if (count & RWSEM_LOCK_MASK) {
> -                       if (has_handoff || (!rt_task(waiter->task) &&
> -                                           !time_after(jiffies,
> waiter->timeout)))
> +                       if (has_handoff || (rt_task(waiter->task) &&
> waiter != first) ||
> +                          (!rt_task(waiter->task) &&
> !time_after(jiffies, waiter->timeout)))
>                                  return false;
>
As I said above, we want to make more forward progress in the wait queue 
if a RT task is waiting there to try to reduce its latency. That is the 
point of that if statement.

Cheers,
Longman