Merge branches 'expcb.2026.07.24a', 'misc.2026.07.30a', 'rcu-tasks.2026.07.30a', 'srcu.2026.08.11a' and 'torture.2026.08.14a' into HEADgrafted

Changes:

Make expedited grace periods expedite normal RCU callbacks

Miscellaneous fixes:
 * Improve diagnostic output with character task states.
 * Mark accesses to inform KCSAN of concurrency design.
 * Move from kmalloc() to kmalloc_obj().
 * Documentation updates.
 * Improve handling of RCU deferred quiescent states.
 * Clean up unused function arguments and structure fields.
 * Reduce show_rcu_gp_kthreads() stack space.

Tasks RCU updates:
 * Clean up after SRCU re-implementation of Tasks Trace RCU.
 * Mark accesses to inform KCSAN of concurrency design.
 * Add ->lazy_timer status to diagnostic output.
 * Remove an unnecessary memory barrier.
 * Fix a data race, courtesy of KCSAN.
 * Documentation updates.
 * Convert cond_resched_tasks_rcu_qs() from macro to static inline
   function.

SRCU updates:
 * Add Rust helpers for SRCU.
 * Avoid losing queued work at cleanup_srcu_struct() time.

Torture-test updates:
 * Preparation work for immediate RCU priority deboosting.
 * Test RCU readers from real interrupt handlers (as opposed to softirq).
 * Simplify code through use of cpumask_next_wrap().
 * Improve diagnostic output with character task states.
 * Add rcutorture.nwriters parameter to allow lightweight stall testing,
   and rcutorture.stall_only to make doing so easier.
 * Test an RCU Tasks Trace grace period implying an RCU grace period.
 * Make RCU Tasks Trace torturing track reader batches.
 * Fix a data race, courtesy of KCSAN.
 * Plug a shuffle_tmp_mask memory leak on kthread spawn failure.

1 files changed, 186 insertions, 0 deletions

diff --git a/lib/refcount.c b/lib/refcount.c
new file mode 100644
index 000000000..a207a8f22
--- /dev/null
+++ b/lib/refcount.c
@@ -0,0 +1,186 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Out-of-line refcount functions.
+ */
+
+#include <linux/mutex.h>
+#include <linux/refcount.h>
+#include <linux/spinlock.h>
+#include <linux/bug.h>
+
+#define REFCOUNT_WARN(str)	WARN_ONCE(1, "refcount_t: " str ".\n")
+
+void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
+{
+	refcount_set(r, REFCOUNT_SATURATED);
+
+	switch (t) {
+	case REFCOUNT_ADD_NOT_ZERO_OVF:
+		REFCOUNT_WARN("saturated; leaking memory");
+		break;
+	case REFCOUNT_ADD_OVF:
+		REFCOUNT_WARN("saturated; leaking memory");
+		break;
+	case REFCOUNT_ADD_UAF:
+		REFCOUNT_WARN("addition on 0; use-after-free");
+		break;
+	case REFCOUNT_SUB_UAF:
+		REFCOUNT_WARN("underflow; use-after-free");
+		break;
+	case REFCOUNT_DEC_LEAK:
+		REFCOUNT_WARN("decrement hit 0; leaking memory");
+		break;
+	default:
+		REFCOUNT_WARN("unknown saturation event!?");
+	}
+}
+EXPORT_SYMBOL(refcount_warn_saturate);
+
+/**
+ * refcount_dec_if_one - decrement a refcount if it is 1
+ * @r: the refcount
+ *
+ * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
+ * success thereof.
+ *
+ * Like all decrement operations, it provides release memory order and provides
+ * a control dependency.
+ *
+ * It can be used like a try-delete operator; this explicit case is provided
+ * and not cmpxchg in generic, because that would allow implementing unsafe
+ * operations.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
+ */
+bool refcount_dec_if_one(refcount_t *r)
+{
+	int val = 1;
+
+	return atomic_try_cmpxchg_release(&r->refs, &val, 0);
+}
+EXPORT_SYMBOL(refcount_dec_if_one);
+
+/**
+ * refcount_dec_not_one - decrement a refcount if it is not 1
+ * @r: the refcount
+ *
+ * No atomic_t counterpart, it decrements unless the value is 1, in which case
+ * it will return false.
+ *
+ * Was often done like: atomic_add_unless(&var, -1, 1)
+ *
+ * Return: true if the decrement operation was successful, false otherwise
+ */
+bool refcount_dec_not_one(refcount_t *r)
+{
+	unsigned int new, val = atomic_read(&r->refs);
+
+	do {
+		if (unlikely(val == REFCOUNT_SATURATED))
+			return true;
+
+		if (val == 1)
+			return false;
+
+		new = val - 1;
+		if (new > val) {
+			WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
+			return true;
+		}
+
+	} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
+
+	return true;
+}
+EXPORT_SYMBOL(refcount_dec_not_one);
+
+/**
+ * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
+ *                               refcount to 0
+ * @r: the refcount
+ * @lock: the mutex to be locked
+ *
+ * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
+ * to decrement when saturated at REFCOUNT_SATURATED.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before, and provides a control dependency such that free() must come after.
+ * See the comment on top.
+ *
+ * Return: true and hold mutex if able to decrement refcount to 0, false
+ *         otherwise
+ */
+bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
+{
+	if (refcount_dec_not_one(r))
+		return false;
+
+	mutex_lock(lock);
+	if (!refcount_dec_and_test(r)) {
+		mutex_unlock(lock);
+		return false;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
+
+/**
+ * refcount_dec_and_lock - return holding spinlock if able to decrement
+ *                         refcount to 0
+ * @r: the refcount
+ * @lock: the spinlock to be locked
+ *
+ * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
+ * decrement when saturated at REFCOUNT_SATURATED.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before, and provides a control dependency such that free() must come after.
+ * See the comment on top.
+ *
+ * Return: true and hold spinlock if able to decrement refcount to 0, false
+ *         otherwise
+ */
+bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
+{
+	if (refcount_dec_not_one(r))
+		return false;
+
+	spin_lock(lock);
+	if (!refcount_dec_and_test(r)) {
+		spin_unlock(lock);
+		return false;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(refcount_dec_and_lock);
+
+/**
+ * refcount_dec_and_lock_irqsave - return holding spinlock with disabled
+ *                                 interrupts if able to decrement refcount to 0
+ * @r: the refcount
+ * @lock: the spinlock to be locked
+ * @flags: saved IRQ-flags if the is acquired
+ *
+ * Same as refcount_dec_and_lock() above except that the spinlock is acquired
+ * with disabled interrupts.
+ *
+ * Return: true and hold spinlock if able to decrement refcount to 0, false
+ *         otherwise
+ */
+bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock,
+				   unsigned long *flags)
+{
+	if (refcount_dec_not_one(r))
+		return false;
+
+	spin_lock_irqsave(lock, *flags);
+	if (!refcount_dec_and_test(r)) {
+		spin_unlock_irqrestore(lock, *flags);
+		return false;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(refcount_dec_and_lock_irqsave);