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Date:   Mon, 18 Oct 2021 09:31:17 +0800
From:   Boqun Feng <>
Cc:     Tejun Heo <>, Lai Jiangshan <>,
        "Paul E . McKenney" <>,
        Thomas Gleixner <>,
        Peter Zijlstra <>,
        Frederic Weisbecker <>,
        Jonathan Corbet <>,
        Boqun Feng <>
Subject: [PATCH] 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 improve the doc.


Signed-off-by: Boqun Feng <>
 Documentation/core-api/workqueue.rst | 21 +++++++++++++++++----
 1 file 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.
@@ -391,6 +387,23 @@ the stack trace of the offending worker thread. ::
 The work item's function should be trivially visible in the stack
+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

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