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

[Waiman Long]

On 6/17/22 10:57, Shakeel Butt wrote:
> On Fri, Jun 17, 2022 at 7:43 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> > 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.
> read_lock_unfair() for networking use or tasklist_lock use?

I mean to say read_lock_fair(), but it could also be the other way 
around. Thanks for spotting that.

Cheers,
Longman