Re: [PATCH v2] rcu: Latch normal synchronize_rcu() path on flood

[Joel Fernandes <joelagnelf@nvidia.com>]

On Mon, 02 Mar 2026 11:04:04 +0100, Uladzislau Rezki (Sony) wrote:

>  * The latch is cleared only when the pending requests are fully
>    drained(nr == 0);

> +static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> +{
> +	long nr;
> +
> +	llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
> +	nr = atomic_long_inc_return(&rcu_sr_normal_count);
> +
> +	/* Latch: only when flooded and if unlatched. */
> +	if (nr >= RCU_SR_NORMAL_LATCH_THR)
> +		(void)atomic_cmpxchg(&rcu_sr_normal_latched, 0, 1);
> +}

I think there is a stuck-latch race here. Once llist_add() places the
entry in srs_next, the GP kthread can pick it up and fire
rcu_sr_normal_complete() before the latching cmpxchg runs. If the last
in-flight completion drains count to zero in that window, the unlatch
cmpxchg(latched, 1, 0) fails (latched is still 0 at that moment), and
then the latching cmpxchg(latched, 0, 1) fires anyway — with count=0:

  CPU 0 (add_req, count just hit 64)       GP kthread
  ----------------------------------       ----------
  llist_add()    <-- entry now in srs_next
  inc_return()   --> nr = 64
  [preempted]
                                            rcu_sr_normal_complete() x64:
                                              dec_return -> count: 64..1..0
                                              count==0:
                                              cmpxchg(latched, 1, 0)
                                                --> FAILS (latched still 0)
  [resumes]
  cmpxchg(latched, 0, 1) --> latched = 1

  Final state: count=0, latched=1  -->  STUCK LATCH

All subsequent synchronize_rcu() callers see latched==1 and take the
fallback path (not counted). With no new SR-normal callers,
rcu_sr_normal_complete() is never reached again, so the unlatch
cmpxchg(latched, 1, 0) never fires. The latch is permanently stuck.

This requires preemption for a full GP duration between llist_add() and
the cmpxchg, which is probably more likely on PREEMPT_RT or heavily loaded
systems.

The fix: move the cmpxchg *before* llist_add(), so the entry is not
visible to the GP kthread until after the latch is already set.

That should fix it, thoughts?

thanks,

-- 
Joel Fernandes