Re: [PATCH RFC v2 rcu 2/8] srcu: Create an srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe()

Paul E. McKenney
2022-10-02T23:47:02Z

On Sun, Oct 02, 2022 at 11:47:10PM +0200, Frederic Weisbecker wrote:
> On Sun, Oct 02, 2022 at 09:09:57AM -0700, Paul E. McKenney wrote:
> > On Sun, Oct 02, 2022 at 05:55:16PM +0200, Frederic Weisbecker wrote:
> > > On Thu, Sep 29, 2022 at 11:07:25AM -0700, Paul E. McKenney wrote:
> > > > @@ -1090,7 +1121,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > > > int ss_state;
> > > >
> > > > check_init_srcu_struct(ssp);
> > > > - idx = srcu_read_lock(ssp);
> > > > + idx = __srcu_read_lock_nmisafe(ssp);
> > >
> > > Why do we need to force the atomic based version here (even if CONFIG_NEED_SRCU_NMI_SAFE=y)?
> >
> > In kernels built with CONFIG_NEED_SRCU_NMI_SAFE=n, we of course need it.
> > As you say, in kernels built with CONFIG_NEED_SRCU_NMI_SAFE=y, we don't.
> > But it doesn't hurt to always use __srcu_read_lock_nmisafe() here, and
> > this is nowhere near a fastpath, so there is little benefit to using
> > __srcu_read_lock() when it is safe to do so.
> >
> > In addition, note that it is possible that a given srcu_struct structure's
> > first grace period is executed before its first reader. In that
> > case, we have no way of knowing which of __srcu_read_lock_nmisafe()
> > or __srcu_read_lock() to choose.
> >
> > So this code always does it the slow(ish) safe way.
>
> But then srcu_read_lock_nmisafe() would work as well, right?

Almost.

The problem is that without the leading "__", this would convince SRCU
that this is an NMI-safe srcu_struct. Which it might not be. Worse yet,
if this srcu_struct had already done an srcu_read_lock(), it would splat.

> > > > ss_state = smp_load_acquire(&ssp->srcu_size_state);
> > > > if (ss_state < SRCU_SIZE_WAIT_CALL)
> > > > sdp = per_cpu_ptr(ssp->sda, 0);
> > > > @@ -1123,7 +1154,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > > > srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> > > > else if (needexp)
> > > > srcu_funnel_exp_start(ssp, sdp_mynode, s);
> > > > - srcu_read_unlock(ssp, idx);
> > > > + __srcu_read_unlock_nmisafe(ssp, idx);
> > > > return s;
> > > > }
> > > >
> > > > @@ -1427,13 +1458,13 @@ void srcu_barrier(struct srcu_struct *ssp)
> > > > /* Initial count prevents reaching zero until all CBs are posted. */
> > > > atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
> > > >
> > > > - idx = srcu_read_lock(ssp);
> > > > + idx = __srcu_read_lock_nmisafe(ssp);
> > >
> > > And same here?
> >
> > Yes, same here. ;-)
>
> Now bonus question: why do SRCU grace period starting/tracking
> need to be in an SRCU read side critical section? :o)

Because I am lazy and like to keep things simple? ;-)

More seriously, take a look at srcu_gp_start_if_needed() and the functions
it calls and ask yourself what bad things could happen if they were
preempted for an arbitrarily long period of time.

Thanx, Paul