linux-kernel - Re: [PATCH 01/45] Create a dynamically sized pool of threads for doing very slow work items [ver #41]

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Date:	Fri, 19 Dec 2008 12:12:34 +0000
From:	David Howells <dhowells@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
Cc:	dhowells@...hat.com, "Serge E. Hallyn" <serue@...ibm.com>,
	trond.myklebust@....uio.no, viro@...IV.linux.org.uk,
	nfsv4@...ux-nfs.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 01/45] Create a dynamically sized pool of threads for doing very slow work items [ver #41]

David Howells <dhowells@...hat.com> wrote:

> I guess I need to stick a comment in slow_work_enqueue() to detail this,
> though the comments in slow_work_execute() do talk about it.

How about this:

diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index f638f36..adf9b78 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -195,12 +195,32 @@ int slow_work_enqueue(struct slow_work *work)
 	BUG_ON(!work->ops);
 	BUG_ON(!work->ops->get_ref);
 
+	/* when honouring an enqueue request, we only promise that we will run
+	 * the work function in the future; we do not promise to run it once
+	 * per enqueue request
+	 *
+	 * we use the PENDING bit to merge together repeat requests without
+	 * having to disable IRQs and take the spinlock, whilst still
+	 * maintaining our promise
+	 */
 	if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
 		spin_lock_irqsave(&slow_work_queue_lock, flags);
 
+		/* we promise that we will not attempt to execute the work
+		 * function in more than one thread simultaneously
+		 *
+		 * this, however, leaves us with a problem if we're asked to
+		 * enqueue the work whilst someone is executing the work
+		 * function as simply queueing the work immediately means that
+		 * another thread may try executing it whilst it is already
+		 * under execution
+		 *
+		 * to deal with this, we set the ENQ_DEFERRED bit instead of
+		 * enqueueing, and the thread currently executing the work
+		 * function will enqueue the work item when the work function
+		 * returns and it has cleared the EXECUTING bit
+		 */
 		if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
-			/* can't queue lest we cause multiple threads to try
-			 * executing this item, so defer for later */
 			set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
 		} else {
 			if (work->ops->get_ref(work) < 0)

David
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