Re: [PATCH] locking/rwlocks: do not starve writers

[Waiman Long]

On 6/17/22 08:07, Peter Zijlstra wrote:
> On Fri, Jun 17, 2022 at 02:10:39AM -0700, Eric Dumazet wrote:
> > --- a/kernel/locking/qrwlock.c
> > +++ b/kernel/locking/qrwlock.c
> > @@ -23,16 +23,6 @@ void queued_read_lock_slowpath(struct qrwlock *lock)
> >   	/*
> >   	 * Readers come here when they cannot get the lock without waiting
> >   	 */
> > -	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));
> > -		return;
> > -	}
> >   	atomic_sub(_QR_BIAS, &lock->cnts);
> >   
> >   	trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
> This is known to break tasklist_lock.
We certainly can't break the current usage of tasklist_lock.

I am aware of this problem with networking code and is thinking about 
either relaxing the check to exclude softirq or provide a 
read_lock_unfair() variant for networking use. I think tasklist_lock 
isn't taken from softirq context, but I may be wrong. Providing a 
read_lock_unfair() will require quite a bit of work in the supporting 
infrastructure as well.

Cheers,
Longman