workqueue: use irqsafe timer for delayed_work

Up to now, for delayed_works, try_to_grab_pending() couldn't be used
from IRQ handlers because IRQs may happen while
delayed_work_timer_fn() is in progress leading to indefinite -EAGAIN.

This patch makes delayed_work use the new TIMER_IRQSAFE flag for
delayed_work->timer.  This makes try_to_grab_pending() and thus
mod_delayed_work_on() safe to call from IRQ handlers.

Signed-off-by: Tejun Heo <tj@kernel.org>

2 files changed, 16 insertions, 12 deletions

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index e84ebb69607..d86b320319e 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -136,7 +136,8 @@ struct execute_work {
 #define __DELAYED_WORK_INITIALIZER(n, f, tflags) {                      \
         .work = __WORK_INITIALIZER((n).work, (f)),                      \
         .timer = __TIMER_INITIALIZER(delayed_work_timer_fn,             \
-                        0, (unsigned long)&(n), (tflags)),              \
+                                     0, (unsigned long)&(n),            \
+                                     (tflags) | TIMER_IRQSAFE),         \
         }
 
 #define DECLARE_WORK(n, f)                                              \
@@ -214,7 +215,8 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
         do {                                                            \
                 INIT_WORK(&(_work)->work, (_func));                     \
                 __setup_timer(&(_work)->timer, delayed_work_timer_fn,   \
-                              (unsigned long)(_work), (_tflags));       \
+                              (unsigned long)(_work),                   \
+                              (_tflags) | TIMER_IRQSAFE);               \
         } while (0)
 
 #define __INIT_DELAYED_WORK_ONSTACK(_work, _func, _tflags)              \
@@ -223,7 +225,7 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
                 __setup_timer_on_stack(&(_work)->timer,                 \
                                        delayed_work_timer_fn,           \
                                        (unsigned long)(_work),          \
-                                       (_tflags));                      \
+                                       (_tflags) | TIMER_IRQSAFE);      \
         } while (0)
 
 #define INIT_DELAYED_WORK(_work, _func)                                 \
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 85bd3409b9f..b394df8beae 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1048,16 +1048,14 @@ static void cwq_dec_nr_in_flight(struct cpu_workqueue_struct *cwq, int color,
  *              for arbitrarily long
  *
  * On >= 0 return, the caller owns @work's PENDING bit.  To avoid getting
- * preempted while holding PENDING and @work off queue, preemption must be
- * disabled on entry.  This ensures that we don't return -EAGAIN while
- * another task is preempted in this function.
+ * interrupted while holding PENDING and @work off queue, irq must be
+ * disabled on entry.  This, combined with delayed_work->timer being
+ * irqsafe, ensures that we return -EAGAIN for finite short period of time.
  *
  * On successful return, >= 0, irq is disabled and the caller is
  * responsible for releasing it using local_irq_restore(*@flags).
  *
- * This function is safe to call from any context other than IRQ handler.
- * An IRQ handler may run on top of delayed_work_timer_fn() which can make
- * this function return -EAGAIN perpetually.
+ * This function is safe to call from any context including IRQ handler.
  */
 static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
                                unsigned long *flags)
@@ -1072,6 +1070,11 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
         if (is_dwork) {
                 struct delayed_work *dwork = to_delayed_work(work);
 
+                /*
+                 * dwork->timer is irqsafe.  If del_timer() fails, it's
+                 * guaranteed that the timer is not queued anywhere and not
+                 * running on the local CPU.
+                 */
                 if (likely(del_timer(&dwork->timer)))
                         return 1;
         }
@@ -1327,9 +1330,8 @@ void delayed_work_timer_fn(unsigned long __data)
         struct delayed_work *dwork = (struct delayed_work *)__data;
         struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);
 
-        local_irq_disable();
+        /* should have been called from irqsafe timer with irq already off */
         __queue_work(dwork->cpu, cwq->wq, &dwork->work);
-        local_irq_enable();
 }
 EXPORT_SYMBOL_GPL(delayed_work_timer_fn);
 
@@ -1444,7 +1446,7 @@ EXPORT_SYMBOL_GPL(queue_delayed_work);
  * Returns %false if @dwork was idle and queued, %true if @dwork was
  * pending and its timer was modified.
  *
- * This function is safe to call from any context other than IRQ handler.
+ * This function is safe to call from any context including IRQ handler.
  * See try_to_grab_pending() for details.
  */
 bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,