[PATCH 1/2] irq_work: Introduce irq_work_raw_sync()

[Marco Elver]

Introduce a non-sleeping spinning variant of irq_work_sync(), called
irq_work_raw_sync(). Its usage is limited to contexts where interrupts
are disabled, and unlike irq_work_sync(), may fail if the work is
pending in the current CPU.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
v2:
* New patch.
---
 include/linux/irq_work.h |  1 +
 kernel/irq_work.c        | 41 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
index 8cd11a223260..490adecbb4be 100644
--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -59,6 +59,7 @@ bool irq_work_queue_on(struct irq_work *work, int cpu);
 
 void irq_work_tick(void);
 void irq_work_sync(struct irq_work *work);
+bool irq_work_raw_sync(struct irq_work *work);
 
 #ifdef CONFIG_IRQ_WORK
 #include <asm/irq_work.h>
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 7afa40fe5cc4..b251b3437db1 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -290,6 +290,47 @@ void irq_work_sync(struct irq_work *work)
 }
 EXPORT_SYMBOL_GPL(irq_work_sync);
 
+/*
+ * Synchronize against the irq_work @work, ensuring the entry is not currently
+ * in use after returning true; returns false if it's impossible to synchronize
+ * due to being queued on the current CPU. Requires that interrupts are already
+ * disabled (prefer irq_work_sync() in all other cases).
+ */
+bool irq_work_raw_sync(struct irq_work *work)
+{
+	struct llist_node *head;
+	struct irq_work *entry;
+
+	/*
+	 * Interrupts should be disabled, so that we can be sure the current
+	 * CPU's work queues aren't concurrently run, cleared, and potentially
+	 * some of its entries becoming invalid in the below iterations.
+	 */
+	lockdep_assert_irqs_disabled();
+
+	while (irq_work_is_busy(work)) {
+		/*
+		 * It is only safe to wait if the work is not on this CPU's work
+		 * queues. Also beware of concurrent irq_work_queue_on(), so we
+		 * need to keep re-checking this CPU's queues in this busy loop.
+		 */
+		head = READ_ONCE(this_cpu_ptr(&raised_list)->first);
+		llist_for_each_entry(entry, head, node.llist) {
+			if (entry == work)
+				return false;
+		}
+		head = READ_ONCE(this_cpu_ptr(&lazy_list)->first);
+		llist_for_each_entry(entry, head, node.llist) {
+			if (entry == work)
+				return false;
+		}
+		cpu_relax();
+	}
+
+	return true;
+}
+EXPORT_SYMBOL_GPL(irq_work_raw_sync);
+
 static void run_irq_workd(unsigned int cpu)
 {
 	irq_work_run_list(this_cpu_ptr(&lazy_list));
-- 
2.37.3.998.g577e59143f-goog


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