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Tue, 10 Mar 2026 09:28:23 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 10 Mar 2026 17:28:22 +0100 To: Frederic Weisbecker Cc: Uladzislau Rezki , Joel Fernandes , "Paul E.McKenney" , Vishal Chourasia , Shrikanth Hegde , Neeraj upadhyay , RCU , LKML , Samir M Subject: Re: [PATCH v2] rcu: Latch normal synchronize_rcu() path on flood Message-ID: References: <20260302100404.2624503-1-urezki@gmail.com> <14e954e4-cfa6-4069-a25f-ccb444d17535@nvidia.com> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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: > > > > > > 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