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

[Uladzislau Rezki <urezki@gmail.com>]

Hello, Frederic!

> Le Thu, Mar 05, 2026 at 11:59:15AM +0100, Uladzislau Rezki a écrit :
> > On Tue, Mar 03, 2026 at 03:45:58PM -0500, Joel Fernandes wrote:
> > > 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?
> > > 
> > Yes and thank you!
> > 
> > We can improve it even more by removing atomic_cmpxchg() in
> > the rcu_sr_normal_add_req() function, because only one context
> > sees the (nr == RCU_SR_NORMAL_LATCH_THR) condition:
> > 
> > <snip>
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 86dc88a70fd0..72b340940e11 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1640,7 +1640,7 @@ static struct workqueue_struct *sync_wq;
> >  
> >  /* Number of in-flight synchronize_rcu() calls queued on srs_next. */
> >  static atomic_long_t rcu_sr_normal_count;
> > -static atomic_t rcu_sr_normal_latched;
> > +static int rcu_sr_normal_latched; /* 0/1 */
> >  
> >  static void rcu_sr_normal_complete(struct llist_node *node)
> >  {
> > @@ -1662,7 +1662,7 @@ static void rcu_sr_normal_complete(struct llist_node *node)
> >  	 * drained and if it has been latched.
> >  	 */
> >  	if (nr == 0)
> > -		(void)atomic_cmpxchg(&rcu_sr_normal_latched, 1, 0);
> > +		(void)cmpxchg(&rcu_sr_normal_latched, 1, 0);
> >  }
> >  
> >  static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > @@ -1808,14 +1808,22 @@ static bool rcu_sr_normal_gp_init(void)
> >  
> >  static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> >  {
> > -	long nr;
> > +	/*
> > +	 * Increment before publish to avoid a complete
> > +	 * vs enqueue race on latch.
> > +	 */
> > +	long nr = atomic_long_inc_return(&rcu_sr_normal_count);
> >  
> > -	llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
> > -	nr = atomic_long_inc_return(&rcu_sr_normal_count);
> > +	/*
> > +	 * Latch on threshold crossing. (nr == RCU_SR_NORMAL_LATCH_THR)
> > +	 * can be true only for one context, avoiding contention on the
> > +	 * write path.
> > +	 */
> > +	if (nr == RCU_SR_NORMAL_LATCH_THR)
> > +		WRITE_ONCE(rcu_sr_normal_latched, 1);
> 
> Isn't it still racy?
> 
> rcu_sr_normal_add_req                                   rcu_sr_normal_complete
> ---------------------                                   ----------------------
>                                                         nr = atomic_long_dec_return(&rcu_sr_normal_count);
>                                                         // nr == 0
>                                                         ======= PREEMPTION =======
> // 64 tasks doing synchronize_rcu()
> rcu_sr_normal_add_req()
>    WRITE_ONCE(rcu_sr_normal_latched, 1);
>                                                         cmpxchg(&rcu_sr_normal_latched, 1, 0);
> 
> 
> Also more generally there is nothing that orders the WRITE_ONCE() with the
> cmpxchg.
>
Yep that i know. This is rather "relaxed" mechanism rather than
a strictly ordered. The race you described can happen but i do not
find it as a problem because as noted it is relaxed policy flag.

But WRITE_ONCE() i can replace by:

if (nr == RCU_SR_NORMAL_LATCH_THR)
    cmpxchg(&rcu_sr_normal_latched, 0, 1);

>
> Is it possible to remove rcu_sr_normal_latched and simply deal with comparisons
> between rcu_sr_normal_count and RCU_SR_NORMAL_LATCH_THR?
> 
It is. But the idea with latch is a bit different then just checking
threshold. The main goal is to detect flood and lath the path until
__all__ users are flushed. I.e. it implements hysteresis to prevent
repeated switches around the threshold.

With your proposal behaviour becomes different.

Thoughts?

--
Uladzislau Rezki