Re: [PATCH 2/2] locking/qrwlock: Reduce cacheline contention for rwlocks used in interrupt context

[Waiman Long <longman@redhat.com>]

On 5/11/22 04:30, Peter Zijlstra wrote:
> On Tue, May 10, 2022 at 03:21:34PM -0400, Waiman Long wrote:
>> Even though qrwlock is supposed to be a fair lock, it does allow readers
>> from interrupt context to spin on the lock until it can acquire it making
>> it not as fair. This exception was added due to the requirement to allow
>> recursive read lock in interrupt context. This can also be achieved by
>> just ignoring the writer waiting bit without spinning on the lock.
>>
>> By making this change, we make qrwlock a bit more fair and eliminating
>> the problem of cacheline bouncing for rwlocks that are used heavily in
>> interrupt context, like the networking stack. This should also reduce
>> the chance of lock starvation for those interrupt context rwlocks.
>> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
>> index 2e1600906c9f..d52d13e95600 100644
>> --- a/kernel/locking/qrwlock.c
>> +++ b/kernel/locking/qrwlock.c
>> @@ -18,21 +18,16 @@
>>    * queued_read_lock_slowpath - acquire read lock of a queued rwlock
>>    * @lock: Pointer to queued rwlock structure
>>    */
>> -void queued_read_lock_slowpath(struct qrwlock *lock)
>> +void queued_read_lock_slowpath(struct qrwlock *lock, int cnts)
>>   {
>>   	/*
>> -	 * Readers come here when they cannot get the lock without waiting
>> +	 * Readers come here when they cannot get the lock without waiting.
>> +	 * Readers in interrupt context can steal the lock immediately
>> +	 * if the writer is just waiting (not holding the lock yet).
>>   	 */
>> -	if (unlikely(in_interrupt())) {
>> -		/*
>> -		 * Readers in interrupt context will get the lock immediately
>> -		 * if the writer is just waiting (not holding the lock yet),
>> -		 * so spin with ACQUIRE semantics until the lock is available
>> -		 * without waiting in the queue.
>> -		 */
>> -		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
>> +	if (unlikely(!(cnts & _QW_LOCKED) && in_interrupt()))
>>   		return;
>> -	}
>> +
>>   	atomic_sub(_QR_BIAS, &lock->cnts);
>>   
>>   	trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
> I'm confused; prior to this change:
>
> 	CPU0			CPU1
>
> 	write_lock_irq(&l)
> 				read_lock(&l)
> 				<INRQ>
> 				  read_lock(&l)
> 				  ...
>
> was not deadlock, but now it would AFAICT.

Oh you are right. I missed that scenario in my analysis. My bad.

Please scrap this patch. Patch 1 is just an update to the comment and so 
is still applicable.

Thanks,
Longman