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From:   Peter Oskolkov <posk@google.com>
Date:   Mon, 31 Aug 2020 16:08:41 -0700
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Subject: Re: [PATCH 1/2 v5] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     paulmck <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Chris Kennelly <ckennelly@google.com>,
        Peter Oskolkov <posk@posk.io>
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On Mon, Aug 31, 2020 at 8:35 AM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>

Thanks for the review!

>
> ----- On Aug 26, 2020, at 7:02 PM, Peter Oskolkov posk@google.com wrote:
> [...]
> >
> > static void ipi_mb(void *info)
> > {
> > +#ifdef CONFIG_RSEQ
> > +     int *flags = info;
> > +
> > +     if (flags && (*flags == MEMBARRIER_FLAG_RSEQ))
> > +             rseq_preempt(current);
> > +#endif
>
> Please lift this into a new ipi_rseq(), which will be defined as an empty function
> if RSEQ is not defined.

Done.

>
>
> >       smp_mb();       /* IPIs should be serializing but paranoid. */
> > }
> >
> > @@ -129,19 +143,26 @@ static int membarrier_global_expedited(void)
> >       return 0;
> > }
> >
> > -static int membarrier_private_expedited(int flags)
> > +static int membarrier_private_expedited(int flags, int cpu_id)
> > {
> >       int cpu;
> >       cpumask_var_t tmpmask;
> >       struct mm_struct *mm = current->mm;
> >
> > -     if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
> > +     if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
>
> I'm not sure why we need to change the behavior from a mask on flags to
> an equality, which means this behaves more like a list of items rather
> than flags.
>
> It's one thing to disallow combining things like SYNC_CORE and RSEQ in the
> ABI, but I wonder why we need to change the flags behavior to an equality
> for the internal flags.

I do not feel too strongly about this, but using "flags & XXX" implies
that flags is a bitmask that can have more than one bit set. I was actually
confused initially by this and was trying to figure out where / how more than
one bit can be set, and where / how this is handled. By explicitly using "=="
the code indicates that (at the moment) this is not a bitmask.

I can revert the change back to "&" if you think it is better than having "==".

>
> >               if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> >                       return -EINVAL;
> >               if (!(atomic_read(&mm->membarrier_state) &
> >                     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
> >                       return -EPERM;
> > +     } else if (flags == MEMBARRIER_FLAG_RSEQ) {
> > +             if (!IS_ENABLED(CONFIG_RSEQ))
> > +                     return -EINVAL;
> > +             if (!(atomic_read(&mm->membarrier_state) &
> > +                   MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY))
> > +                     return -EPERM;
> >       } else {
> > +             BUG_ON(flags != 0);
> >               if (!(atomic_read(&mm->membarrier_state) &
> >                     MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
> >                       return -EPERM;
> > @@ -174,6 +195,8 @@ static int membarrier_private_expedited(int flags)
> >                */
> >               if (cpu == raw_smp_processor_id())
> >                       continue;
> > +             if (cpu_id >= 0 && cpu != cpu_id)
> > +                     continue;
>
> When the cpu is specified, it seems rather inefficient to iterate on all
> cpus to skip all but the one we are looking for. I suspect we don't want
> to go through the loop in that case.

Done. The code is a bit more complicated now, but definitely more
efficient.

>
> >               p = rcu_dereference(cpu_rq(cpu)->curr);
> >               if (p && p->mm == mm)
> >                       __cpumask_set_cpu(cpu, tmpmask);
> > @@ -181,7 +204,7 @@ static int membarrier_private_expedited(int flags)
> >       rcu_read_unlock();
> >
> >       preempt_disable();
> > -     smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> > +     smp_call_function_many(tmpmask, ipi_mb, &flags, 1);
> >       preempt_enable();
> >
> >       free_cpumask_var(tmpmask);
> > @@ -283,11 +306,18 @@ static int membarrier_register_private_expedited(int
> > flags)
> >           set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED,
> >           ret;
> >
> > -     if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
> > +     if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
>
> Same comment about changing this internal flags behavior from mask to equality.

Same reply :)

I can revert the change, but it will look weird, imho - the code does not
treat flags as a bitmask, and changing it to actually work with flags a bitmask
will make it more complicated without a real use case at the moment.

>
> >               if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> >                       return -EINVAL;
> >               ready_state =
> >                       MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY;
> > +     } else if (flags == MEMBARRIER_FLAG_RSEQ) {
> > +             if (!IS_ENABLED(CONFIG_RSEQ))
> > +                     return -EINVAL;
> > +             ready_state =
> > +                     MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY;
> > +     } else {
> > +             BUG_ON(flags != 0);
> >       }
> >
> >       /*
> > @@ -299,6 +329,8 @@ static int membarrier_register_private_expedited(int flags)
> >               return 0;
> >       if (flags & MEMBARRIER_FLAG_SYNC_CORE)
> >               set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE;
> > +     if (flags & MEMBARRIER_FLAG_RSEQ)
> > +             set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ;
>
> This one still behaves like a mask, so there is a discrepancy between registration
> and action functions.

Yes, and I vaguely remember you saying that commands being distinct bits
is for "discoverability", not for any "ORing" of commands at the moment.

[...]

> > +SYSCALL_DEFINE3(membarrier, int, cmd, int, flags, int, cpu_id)
> > {
> > -     if (unlikely(flags))
> > +     if (unlikely(flags) && cmd != MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
>
> I would prefer that we deal with flags and cpu_id entirely here rather than
> half here, half below, with e.g.:
>
> switch (cmd) {
> case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
>     if (unlikely(flags && flags != MEMBARRIER_CMD_FLAG_CPU))
>         return -EINVAL;
>     break;
> default:
>     if (unlikely(flags))
>         return -EINVAL;
> }
>
> if (!(flags & MEMBARRIER_CMD_FLAG_CPU))
>     cpu_id = -1;

Done.

[...]