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    SubjectRe: [PATCH] locking/rwlocks: do not starve writers
    On Fri, Jun 17, 2022 at 7:42 PM Waiman Long <longman@redhat.com> wrote:
    >
    > On 6/17/22 11:24, Eric Dumazet wrote:
    > > On Fri, Jun 17, 2022 at 5:00 PM Waiman Long <longman@redhat.com> wrote:
    > >> On 6/17/22 10:57, Shakeel Butt wrote:
    > >>> On Fri, Jun 17, 2022 at 7:43 AM Waiman Long <longman@redhat.com> 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.
    > >>
    > > If only tasklist_lock is problematic and needs the unfair variant,
    > > then changing a few read_lock() for tasklist_lock will be less
    > > invasive than ~1000 read_lock() elsewhere....
    >
    > After a second thought, I think the right way is to introduce a fair
    > variant, if needed. If an arch isn't using qrwlock, the native rwlock
    > implementation will be unfair. In that sense, unfair rwlock is the
    > default. We will only need to change the relevant network read_lock()
    > calls to use the fair variant which will still be unfair if qrwlock
    > isn't used. We are not going to touch other read_lock call that don't
    > care about fair or unfair.
    >

    Hmm... backporting this kind of invasive change to stable kernels will
    be a daunting task.

    Were rwlocks always unfair, and we have been lucky ?

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