On 7/15/2026 4:50 PM, Paul E. McKenney wrote:
> On Thu, Jun 25, 2026 at 08:42:56PM -0400, Joel Fernandes wrote:
>> The deferred-QS irq-work handler previously cleared defer_qs_pending
>> only when the handler ran inside an active rcu_read_lock() critical
>> section (rcu_preempt_depth() > 0). Paul McKenney pointed out a common
>> multi-segment compound pattern where the handler fires between
>> segments and segment N+1's arming attempt is silently suppressed by
>> the rcu_read_unlock_special() pending-gate:
>>
>> rcu_read_lock(); // segment 1 starts
>> // may be preempted/boosted here
>> local_irq_disable();
>> rcu_read_unlock(); // segment 1 ends; arms defer_qs_pending
>> preempt_disable();
>> local_irq_enable(); // handler MAY fire here: depth==0, but
>> // but preempt is disabled, so it cant
>> // nudge.
>>
>> rcu_read_lock(); // segment 2 starts
>> preempt_enable();
>> local_irq_disable();
>> rcu_read_unlock(); // arming attempt suppressed incorrectly -- (1)
>> local_irq_enable();
>>
>> Waiting for the next __note_gp_changes() clear is too slow for the
>> compound case, we need the deferred QS report sooner.
>>
>> Therefore, make the irq_work handler clear defer_qs_pending whenever
>> rcu_in_compounded_section() is true so that (1) can do the arming.
>>
>> In addition, introduce rcu_preempt_deferred_qs_try_report(), a small
>> helper that reports the deferred QS (and releases any RCU priority
>> boost) directly, but only from a clean, non-reader/compound context.
>> When the handler lands in such a clean context it now reports the QS
>> directly instead of merely nudging the scheduler: this makes the
>> irq_work robust under preempt=none / voluntary, where a
>> set_need_resched() nudge would not enter __schedule() at IRQ exit and
>> the QS would otherwise wait for the next tick. When still compounded,
>> the handler falls back to clearing defer_qs_pending as before. The
>> bounded-delay rescue hrtimer added in a later patch reuses this same
>> helper.
>
> The patch generally looks like an unambiguous improvement, but just to
> make sure that I understand...
>
> None of the code below is even built if CONFIG_PREEMPT_NONE=y. So is
> the text above referring to kernels built with CONFIG_PREEMPT_DYNAMIC=y
> and booted with either preempt=none or preempt=voluntary? If so, we
> need to explicitly state that.
I mentioned "preempt=none/voluntary" above, but you want me to also mention
CONFIG_PREEMPT_DYNAMIC=y in the commit message? If yes, I can add that.
However, just to note, this patch is relevant also for fully-preemptible
modes. The preempt=none side-effect of this patch is just a bonus (since we
can't get the aid of the scheduler in those modes).
Let me know if that makes sense, alternatively I can also just remove the
paragraph in the commit messages starting with "In addition,..".
Thanks.
>
> Thanx, Paul
>
>> Signed-off-by: Joel Fernandes <[email protected]>
>> ---
>> kernel/rcu/tree_plugin.h | 46 ++++++++++++++++++++++++++++------------
>> 1 file changed, 33 insertions(+), 13 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index 7555022bf38a..7768a40677a4 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -702,7 +702,32 @@ notrace void rcu_preempt_deferred_qs(struct task_struct
>> *t)
>> }
>>
>> /*
>> - * Minimal handler to give the scheduler a chance to re-evaluate.
>> + * Report a deferred quiescent state but only from a safe context.
>> + *
>> + * Both callers (the irq_work handler and the bounded-delay rescue hrtimer)
>> + * run in hardirq context, so preempt_count() always has the HARDIRQ bit
>> set;
>> + * the compound-section check below deliberately inspects only the
>> + * PREEMPT_MASK | SOFTIRQ_MASK bits, which reflect the INTERRUPTED caller's
>> + * state, not ours.
>> + */
>> +static bool rcu_preempt_deferred_qs_try_report(struct task_struct *t)
>> +{
>> + unsigned long flags;
>> +
>> + if (rcu_preempt_depth() > 0 ||
>> + (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)))
>> + return false;
>> +
>> + if (rcu_preempt_need_deferred_qs(t)) {
>> + local_irq_save(flags);
>> + rcu_preempt_deferred_qs_irqrestore(t, flags);
>> + }
>> + return true;
>> +}
>> +
>> +/*
>> + * Minimal handler to give the scheduler a chance to re-evaluate, and to
>> + * report the deferred QS directly when the handler lands in a clean
>> context.
>> */
>> static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
>> {
>> @@ -712,19 +737,14 @@ static void rcu_preempt_deferred_qs_handler(struct
>> irq_work *iwp)
>> rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
>>
>> /*
>> - * If the IRQ work handler happens to run in the middle of RCU read-side
>> - * critical section, it could be ineffective in getting the scheduler's
>> - * attention to report a deferred quiescent state (the whole point of
>> the
>> - * IRQ work). For this reason, requeue the IRQ work.
>> - *
>> - * Basically, we want to avoid following situation:
>> - * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING)
>> - * 2. CPU enters new rcu_read_lock()
>> - * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0
>> - * 4. rcu_read_unlock() does not re-queue work (state still PENDING)
>> - * 5. Deferred QS reporting does not happen.
>> + * If the handler fired in a clean context, report the deferred QS
>> + * directly. This makes the irq_work robust under preempt=none /
>> + * voluntary, where the set_need_resched() nudge would not enter
>> + * __schedule() at IRQ exit. Otherwise we are still inside a reader /
>> + * compound section: just clear defer_qs_pending so the next
>> + * rcu_read_unlock() can rearm.
>> */
>> - if (rcu_preempt_depth() > 0)
>> + if (!rcu_preempt_deferred_qs_try_report(current))
>> rcu_defer_qs_clear(rdp);
>> }
>>
>> --
>> 2.34.1
>>
