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
