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From:   Peter Oskolkov <posk@google.com>
Date:   Tue, 23 Jun 2020 13:59:22 -0700
Message-ID: <CAPNVh5eNJfe6yBhXeOikdBRyjKJc_p3PJRzK4kX0rRGUswZa0A@mail.gmail.com>
Subject: Re: [RFC PATCH 1/3 v2] futex: introduce FUTEX_SWAP operation
To:     Andrei Vagin <avagin@gmail.com>
Cc:     Peter Oskolkov <posk@posk.io>, Aaron Lu <aaron.lwe@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Andrei Vagin <avagin@google.com>,
        "pjt@google.com" <pjt@google.com>, Ben Segall <bsegall@google.com>
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On Tue, Jun 23, 2020 at 12:45 PM Andrei Vagin <avagin@gmail.com> wrote:
>
> On Tue, Jun 23, 2020 at 11:30:30AM -0700, Peter Oskolkov wrote:
> ...
> > > >  /**
> > > > +static int futex_swap(u32 __user *uaddr, unsigned int flags, u32
> > > > val,
> > > > +               ktime_t *abs_time, u32 __user *uaddr2)
> > > > +{
> > > > + u32 bitset = FUTEX_BITSET_MATCH_ANY;
> > > > + struct task_struct *next = NULL;
> > > > + DEFINE_WAKE_Q(wake_q);
> > > > + int ret;
> > > > +
> > > > + ret = prepare_wake_q(uaddr2, flags, 1, bitset, &wake_q);
> > > > + if (!wake_q_empty(&wake_q)) {
> > > > +         /* Pull the first wakee out of the queue to swap into.
> > > > */
> > > > +         next = container_of(wake_q.first, struct task_struct,
> > > > wake_q);
> > > > +         wake_q.first = wake_q.first->next;
> > > > +         next->wake_q.next = NULL;
> > > > +         /*
> > > > +          * Note that wake_up_q does not touch wake_q.last, so
> > > > we
> > > > +          * do not bother with it here.
> > > > +          */
> > > > +         wake_up_q(&wake_q);
> > >
> > > wake_up_q() doesn't seem to serve any purpose in that the above
> > > assignment of wake_q.first shall make it an empty queue now?
> > > Also, I don't see a need to touch wake_q.first either so I think we
> > > can
> > > get rid of wake_q altogether here.
> >
> > The futex at uaddr2 may have more than one waiter, so we cannot assume
> > that wake_q will be empty when we remove the first element.
>
> The third argument of prepare_wake_q is nr_wake which is one in this
> case, so we can be sure that wake_q will be empty, can't we?

Right, sorry. In an early draft it was possible to wake more than one waiter,
and the code carried over from then/there in this form. I'll remove wake_up_q()
if/when the API will be deemed acceptable by the maintainers.

>
> >
> > >
> > > > + }
> > > > + if (ret < 0)
> > > > +         return ret;
> > > > +
> > > > + return futex_wait(uaddr, flags, val, abs_time, bitset, next);
> > > > +}
> > >