workqueue: Fixes for v6.13-rc5

- Suppress a corner case spurious flush dependency warning.
 
 - Two trivial changes.
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Merge tag 'wq-for-6.13-rc5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq

Pull workqueue fixes from Tejun Heo:

 - Suppress a corner case spurious flush dependency warning

 - Two trivial changes

* tag 'wq-for-6.13-rc5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
  workqueue: add printf attribute to __alloc_workqueue()
  workqueue: Do not warn when cancelling WQ_MEM_RECLAIM work from !WQ_MEM_RECLAIM worker
  rust: add safety comment in workqueue traits

kernel/workqueue.c

@@ -3680,23 +3680,27 @@ void workqueue_softirq_dead(unsigned int cpu)
  * check_flush_dependency - check for flush dependency sanity
  * @target_wq: workqueue being flushed
  * @target_work: work item being flushed (NULL for workqueue flushes)
+ * @from_cancel: are we called from the work cancel path
  *
  * %current is trying to flush the whole @target_wq or @target_work on it.
- * If @target_wq doesn't have %WQ_MEM_RECLAIM, verify that %current is not
- * reclaiming memory or running on a workqueue which doesn't have
- * %WQ_MEM_RECLAIM as that can break forward-progress guarantee leading to
- * a deadlock.
+ * If this is not the cancel path (which implies work being flushed is either
+ * already running, or will not be at all), check if @target_wq doesn't have
+ * %WQ_MEM_RECLAIM and verify that %current is not reclaiming memory or running
+ * on a workqueue which doesn't have %WQ_MEM_RECLAIM as that can break forward-
+ * progress guarantee leading to a deadlock.
  */
 static void check_flush_dependency(struct workqueue_struct *target_wq,
-				   struct work_struct *target_work)
+				   struct work_struct *target_work,
+				   bool from_cancel)
 {
-	work_func_t target_func = target_work ? target_work->func : NULL;
+	work_func_t target_func;
 	struct worker *worker;
 
-	if (target_wq->flags & WQ_MEM_RECLAIM)
+	if (from_cancel || target_wq->flags & WQ_MEM_RECLAIM)
 		return;
 
 	worker = current_wq_worker();
+	target_func = target_work ? target_work->func : NULL;
 
 	WARN_ONCE(current->flags & PF_MEMALLOC,
 		  "workqueue: PF_MEMALLOC task %d(%s) is flushing !WQ_MEM_RECLAIM %s:%ps",
@@ -3980,7 +3984,7 @@ void __flush_workqueue(struct workqueue_struct *wq)
 		list_add_tail(&this_flusher.list, &wq->flusher_overflow);
 	}
 
-	check_flush_dependency(wq, NULL);
+	check_flush_dependency(wq, NULL, false);
 
 	mutex_unlock(&wq->mutex);
 
@@ -4155,7 +4159,7 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
 	}
 
 	wq = pwq->wq;
-	check_flush_dependency(wq, work);
+	check_flush_dependency(wq, work, from_cancel);
 
 	insert_wq_barrier(pwq, barr, work, worker);
 	raw_spin_unlock_irq(&pool->lock);
@@ -5641,6 +5645,7 @@ static void wq_adjust_max_active(struct workqueue_struct *wq)
 	} while (activated);
 }
 
+__printf(1, 0)
 static struct workqueue_struct *__alloc_workqueue(const char *fmt,
 						  unsigned int flags,
 						  int max_active, va_list args)

rust/kernel/workqueue.rs

@@ -519,7 +519,15 @@ impl_has_work! {
     impl{T} HasWork<Self> for ClosureWork<T> { self.work }
 }
 
-// SAFETY: TODO.
+// SAFETY: The `__enqueue` implementation in RawWorkItem uses a `work_struct` initialized with the
+// `run` method of this trait as the function pointer because:
+//   - `__enqueue` gets the `work_struct` from the `Work` field, using `T::raw_get_work`.
+//   - The only safe way to create a `Work` object is through `Work::new`.
+//   - `Work::new` makes sure that `T::Pointer::run` is passed to `init_work_with_key`.
+//   - Finally `Work` and `RawWorkItem` guarantee that the correct `Work` field
+//     will be used because of the ID const generic bound. This makes sure that `T::raw_get_work`
+//     uses the correct offset for the `Work` field, and `Work::new` picks the correct
+//     implementation of `WorkItemPointer` for `Arc<T>`.
 unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Arc<T>
 where
     T: WorkItem<ID, Pointer = Self>,
@@ -537,7 +545,13 @@ where
     }
 }
 
-// SAFETY: TODO.
+// SAFETY: The `work_struct` raw pointer is guaranteed to be valid for the duration of the call to
+// the closure because we get it from an `Arc`, which means that the ref count will be at least 1,
+// and we don't drop the `Arc` ourselves. If `queue_work_on` returns true, it is further guaranteed
+// to be valid until a call to the function pointer in `work_struct` because we leak the memory it
+// points to, and only reclaim it if the closure returns false, or in `WorkItemPointer::run`, which
+// is what the function pointer in the `work_struct` must be pointing to, according to the safety
+// requirements of `WorkItemPointer`.
 unsafe impl<T, const ID: u64> RawWorkItem<ID> for Arc<T>
 where
     T: WorkItem<ID, Pointer = Self>,