[RFC PATCH 01/11] workqueue: Add a decrement-after-return and wake if 0 facility

From: David Howells <dhowells@redhat.com>
To: linux-afs@lists.infradead.org
Cc: Tejun Heo <tj@kernel.org>,
	linux-fsdevel@vger.kernel.org, dhowells@redhat.com,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org
Subject: [RFC PATCH 01/11] workqueue: Add a decrement-after-return and wake if 0 facility
Date: Fri, 01 Sep 2017 16:40:53 +0100	[thread overview]
Message-ID: <150428045304.25051.1778333106306853298.stgit@warthog.procyon.org.uk> (raw)

Add a facility to the workqueue subsystem whereby an atomic_t can be
registered by a work function such that the work function dispatcher will
decrement the atomic after the work function has returned and then call
wake_up_atomic() on it if it reached 0.

This is analogous to complete_and_exit() for kernel threads and is used to
avoid a race between notifying that a work item is about to finish and the
.text segment from a module being discarded.

The way this is used is that the work function calls:

	dec_after_work(atomic_t *counter);

to register the counter and then process_one_work() calls it, potentially
wakes it and clears the registration.

The reason I've used an atomic_t rather than a completion is that (1) it
takes up less space and (2) it can monitor multiple objects.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Tejun Heo <tj@kernel.org>
cc: Lai Jiangshan <jiangshanlai@gmail.com>
---

 include/linux/workqueue.h   |    1 +
 kernel/workqueue.c          |   25 +++++++++++++++++++++++++
 kernel/workqueue_internal.h |    1 +
 3 files changed, 27 insertions(+)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index db6dc9dc0482..ceaed1387e9b 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -451,6 +451,7 @@ extern bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
 
 extern void flush_workqueue(struct workqueue_struct *wq);
 extern void drain_workqueue(struct workqueue_struct *wq);
+extern void dec_after_work(atomic_t *counter);
 
 extern int schedule_on_each_cpu(work_func_t func);
 
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ca937b0c3a96..2936ad0ab293 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2112,6 +2112,12 @@ __acquires(&pool->lock)
 		dump_stack();
 	}
 
+	if (worker->dec_after) {
+		if (atomic_dec_and_test(worker->dec_after))
+			wake_up_atomic_t(worker->dec_after);
+		worker->dec_after = NULL;
+	}
+
 	/*
 	 * The following prevents a kworker from hogging CPU on !PREEMPT
 	 * kernels, where a requeueing work item waiting for something to
@@ -3087,6 +3093,25 @@ int schedule_on_each_cpu(work_func_t func)
 }
 
 /**
+ * dec_after_work - Register counter to dec and wake after work func returns
+ * @counter: The counter to decrement and wake
+ *
+ * Register an atomic counter to be decremented after a work function returns
+ * to the core.  The counter is 'woken' if it is decremented to 0.  This allows
+ * synchronisation to be effected by one or more work functions in a module
+ * without leaving a window in which the work function code can be unloaded.
+ */
+void dec_after_work(atomic_t *counter)
+{
+	struct worker *worker = current_wq_worker();
+
+	BUG_ON(!worker);
+	BUG_ON(worker->dec_after);
+	worker->dec_after = counter;
+}
+EXPORT_SYMBOL(dec_after_work);
+
+/**
  * execute_in_process_context - reliably execute the routine with user context
  * @fn:		the function to execute
  * @ew:		guaranteed storage for the execute work structure (must
diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h
index 8635417c587b..94ea1ca9b01f 100644
--- a/kernel/workqueue_internal.h
+++ b/kernel/workqueue_internal.h
@@ -28,6 +28,7 @@ struct worker {
 
 	struct work_struct	*current_work;	/* L: work being processed */
 	work_func_t		current_func;	/* L: current_work's fn */
+	atomic_t		*dec_after;	/* Decrement after func returns */
 	struct pool_workqueue	*current_pwq; /* L: current_work's pwq */
 	bool			desc_valid;	/* ->desc is valid */
 	struct list_head	scheduled;	/* L: scheduled works */
