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authorBoqun Feng <boqun.feng@gmail.com>2021-10-22 08:42:08 +0800
committerTejun Heo <tj@kernel.org>2021-10-25 07:18:40 -1000
commitf9eaaa82b474350aa8436d15a7ae150a3c8b9d5c (patch)
tree41c1f24760c9aeb8a155dae35ed0b6917158de8c
parent55df0933be74bd2e52aba0b67eb743ae0feabe7e (diff)
workqueue: doc: Call out the non-reentrance conditions
The current doc of workqueue API suggests that work items are non-reentrant: any work item is guaranteed to be executed by at most one worker system-wide at any given time. However this is not true, the following case can cause a work item W executed by two workers at the same time: queue_work_on(0, WQ1, W); // after a worker picks up W and clear the pending bit queue_work_on(1, WQ2, W); // workers on CPU0 and CPU1 will execute W in the same time. , which means the non-reentrance of a work item is conditional, and Lai Jiangshan provided a nice summary[1] of the conditions, therefore use it to describe a work item instance and improve the doc. [1]: https://lore.kernel.org/lkml/CAJhGHyDudet_xyNk=8xnuO2==o-u06s0E0GZVP4Q67nmQ84Ceg@mail.gmail.com/ Suggested-by: Matthew Wilcox <willy@infradead.org> Suggested-by: Tejun Heo <tj@kernel.org> Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Tejun Heo <tj@kernel.org>
-rw-r--r--Documentation/core-api/workqueue.rst21
1 files changed, 17 insertions, 4 deletions
diff --git a/Documentation/core-api/workqueue.rst b/Documentation/core-api/workqueue.rst
index 541d31de8926..3b22ed137662 100644
--- a/Documentation/core-api/workqueue.rst
+++ b/Documentation/core-api/workqueue.rst
@@ -216,10 +216,6 @@ resources, scheduled and executed.
This flag is meaningless for unbound wq.
-Note that the flag ``WQ_NON_REENTRANT`` no longer exists as all
-workqueues are now non-reentrant - any work item is guaranteed to be
-executed by at most one worker system-wide at any given time.
-
``max_active``
--------------
@@ -391,6 +387,23 @@ the stack trace of the offending worker thread. ::
The work item's function should be trivially visible in the stack
trace.
+Non-reentrance Conditions
+=========================
+
+Workqueue guarantees that a work item cannot be re-entrant if the following
+conditions hold after a work item gets queued:
+
+ 1. The work function hasn't been changed.
+ 2. No one queues the work item to another workqueue.
+ 3. The work item hasn't been reinitiated.
+
+In other words, if the above conditions hold, the work item is guaranteed to be
+executed by at most one worker system-wide at any given time.
+
+Note that requeuing the work item (to the same queue) in the self function
+doesn't break these conditions, so it's safe to do. Otherwise, caution is
+required when breaking the conditions inside a work function.
+
Kernel Inline Documentations Reference
======================================