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

[Joel Fernandes <joelagnelf@nvidia.com>]

On 3/5/2026 5:59 AM, Uladzislau Rezki wrote:
> 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:
Sure, though you still need the atomic_long_inc_return.

But yes, the approach looks good. :-) Do you think we can have v3 ready for 7.1?
I would like to shoot for that if possible.

thanks,

--
Joel Fernandes


> 
> <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);
>  
> -	/* Latch: only when flooded and if unlatched. */
> -	if (nr >= RCU_SR_NORMAL_LATCH_THR)
> -		(void)atomic_cmpxchg(&rcu_sr_normal_latched, 0, 1);
> +	/* Publish for the GP kthread/worker. */
> +	llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
>  }
>  
>  /*
> @@ -3302,7 +3310,7 @@ static void synchronize_rcu_normal(void)
>  	trace_rcu_sr_normal(rcu_state.name, &rs.head, TPS("request"));
>  
>  	if (READ_ONCE(rcu_normal_wake_from_gp) < 1 ||
> -			atomic_read(&rcu_sr_normal_latched)) {
> +			READ_ONCE(rcu_sr_normal_latched)) {
>  		wait_rcu_gp(call_rcu_hurry);
>  		goto trace_complete_out;
>  	}
> <snip>
> 
> --
> Uladzislau Rezki

-- 
Joel Fernandes