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

[Mukesh Ojha <quic_mojha@quicinc.com>]

Hi @Hilf,

Thanks for looking into this issue.

On 10/11/2022 4:16 PM, Hillf Danton wrote:
> On 10/10/22 06:24 Mukesh Ojha <quic_mojha@quicinc.com>
>> Hi Waiman,
>>
>> 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?
>>>
>>
>> Looks like, There is still a window for a race.
>>
>> There is a chance when a reader who came first added it's BIAS and
>> goes to slowpath and before it gets added to wait list it got
>> preempted by RT task which  goes to slowpath as well and being the
>> first waiter gets its hand-off bit set and not able to get the lock
>> due to following condition in rwsem_try_write_lock()

[]

>>
>>   630                 if (count & RWSEM_LOCK_MASK) {  ==> reader has
>> sets its bias
>> ..
>> ...
>>
>>   634
>>   635                         new |= RWSEM_FLAG_HANDOFF;
>>   636                 } else {
>>   637                         new |= RWSEM_WRITER_LOCKED;
>>
>>
>> ---------------------->----------------------->-------------------------
>>
>> First reader (1)          writer(2) RT task             Lock holder(3)
>>
>> It sets
>> RWSEM_READER_BIAS.
>> while it is going to
>> slowpath(as the lock
>> was held by (3)) and
>> before it got added
>> to the waiters list
>> it got preempted
>> by (2).
>>                         RT task also takes
>>                          the slowpath and add              release the
>>                          itself into waiting list          rwsem lock
>>              and since it is the first         clear the
>>                          it is the next one to get         owner.
>>                          the lock but it can not
>>                          get the lock as (count &
>>                          RWSEM_LOCK_MASK) is set
>>                          as (1) has added it but
>>                          not able to remove its
>>              adjustment.

[]

>>
> Hey Mukesh,
> 
> Can you test the diff if it makes sense to you?
> 
> It simply prevents the first waiter from spinning any longer after detecting
> it barely makes any progress to spin without lock owner.
> 
> Hillf
> 
> --- mainline/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -611,26 +611,15 @@ static inline bool rwsem_try_write_lock(
>   	long count, new;
>   
>   	lockdep_assert_held(&sem->wait_lock);
> +	waiter->handoff_set = false;
>   
>   	count = atomic_long_read(&sem->count);
>   	do {
>   		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>   
>   		if (has_handoff) {
> -			/*
> -			 * Honor handoff bit and yield only when the first
> -			 * waiter is the one that set it. Otherwisee, we
> -			 * still try to acquire the rwsem.
> -			 */
> -			if (first->handoff_set && (waiter != first))
> +			if (waiter != first)
>   				return false;

you mean, you want to check and change waiter->handoff_set on every run 
rwsem_try_write_lock().

But does it break optimistic spinning ? @waiman ?

-Mukesh

> -
> -			/*
> -			 * First waiter can inherit a previously set handoff
> -			 * bit and spin on rwsem if lock acquisition fails.
> -			 */
> -			if (waiter == first)
> -				waiter->handoff_set = true;
>   		}
>   
>   		new = count;