linux-kernel - Re: [RFC 2/2] workqueue: Fix work re-entrance when requeue to a different workqueue

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Date:   Sat, 9 Oct 2021 23:06:47 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [RFC 2/2] workqueue: Fix work re-entrance when requeue to a
 different workqueue

On Sat, Oct 09, 2021 at 11:21:58AM +0800, Boqun Feng wrote:
[...]
> > "Re-entrance" is defined as:
> >   work function has not been changed
> >   wq has not been changed
> >   the item has not been reinitiated.
> >   (To reduce the check complication, the workqueue subsystem often
> > considers it "Re-entrance" if the condition is changed and has changed
> > back. But the wq users should not depend on this behavior and should avoid
> > it)
> > 
> 
> Thanks for clarifiction, could you also update the documentation to
> avoid future confusion? Thanks!
> 

I come up with the following, will send a proper patch tomorrow. Looking
forwards to your and others' suggestions.

Regards,
Boqun

--------->8
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
 ======================================