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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>
This commit is contained in:
Boqun Feng 2021-10-22 08:42:08 +08:00 committed by Tejun Heo
parent 55df0933be
commit f9eaaa82b4
1 changed files with 17 additions and 4 deletions
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21
Documentation/core-api/workqueue.rst
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@ -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
======================================