Since commit a2c1c57be8d9fd5b716113c8991d3d702eeacf77,
workqueue: consider work function when searching for busy work items
work items whose work functions are re-assigned are no longer guaranteed
non-reentrancy with the previously assigned work function. For example,
PREPARE_WORK(&work, funcA)
schedule_work(&work)
.
< funcA starts >
.
PREPARE_WORK(&work, funcB)
schedule_work(&work)
funcA() may run concurrently with funcB().
The work item non-reentrancy guarantee is a crucial design guarantee
which, if violated, may not have obvious consequences. For example,
the entire firewire subsystem expects the as-documented per-work item
non-reentrancy guarantee, which was the behavior prior to the above
commit, not the per-work function + per-work item behavior since.
Document the known exceptions to this guarantee.
Cc: Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
Documentation/workqueue.txt | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/Documentation/workqueue.txt b/Documentation/workqueue.txt
index f81a65b..5b49491 100644
--- a/Documentation/workqueue.txt
+++ b/Documentation/workqueue.txt
@@ -223,9 +223,17 @@ 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.
+Note that the flag WQ_NON_REENTRANT no longer exists.
+Generally, workqueues are non-reentrant -- a work item will be executed
+by at most one worker system-wide at any given time. However, this
+non-reentrancy guarantee does not extend to work items whose function
+is re-assigned (for example, via PREPARE_WORK/PREPARE_DELAYED_WORK)
+and re-queued for execution; the newly scheduled work may run concurrently
+with the previous work item function.
+
+Note that WQ_UNBOUND workqueues, such as system_unbound_wq, have no
+non-reentrancy guarantees as these workqueues do not provide concurrency
+management.
@max_active:
--
1.8.1.2
--
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