[PATCH v4 3/8] rcu: clear defer_qs_pending in handler for compounded sections

[Joel Fernandes <joelagnelf@nvidia.com>]

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.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 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