linux-kernel - Re: [PATCH] async: make sure independent async domains can't accidentally entangle.

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Date:	Sun, 24 May 2009 13:50:43 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] async: make sure independent async domains can't
	accidentally entangle.

On Sun, 2009-05-24 at 09:29 -0500, James Bottomley wrote:
> The problem occurs when async_synchronize_full_domain() is called when
> the async_pending list is not empty.  This will cause lowest_running()
> to return the cookie of the first entry on the async_pending list, which
> might be nothing at all to do with the domain being asked for and thus
> cause the domain synchronization to wait for an unrelated domain.   This
> can cause a deadlock if domain synchronization is used from one domain
> to wait for another.
> 
> Fix by running over the async_pending list to see if any pending items
> actually belong to our domain (and return their cookies if they do).
> 
> Signed-off-by: James Bottomley <James.Bottomley@...senPartnership.com>

OK, so that version locked up under testing ... this version doesn't ---
I think the phrase "MUST be called with the lock held!" was supposed to
be some sort of clue ...

James

---

diff --git a/kernel/async.c b/kernel/async.c
index 968ef94..13ed571 100644
--- a/kernel/async.c
+++ b/kernel/async.c
@@ -92,19 +92,21 @@ extern int initcall_debug;
 static async_cookie_t  __lowest_in_progress(struct list_head *running)
 {
 	struct async_entry *entry;
+	async_cookie_t ret = next_cookie; /* begin with "infinity" value */
+
 	if (!list_empty(running)) {
 		entry = list_first_entry(running,
 			struct async_entry, list);
-		return entry->cookie;
+		ret = entry->cookie;
 	} else if (!list_empty(&async_pending)) {
-		entry = list_first_entry(&async_pending,
-			struct async_entry, list);
-		return entry->cookie;
-	} else {
-		/* nothing in progress... next_cookie is "infinity" */
-		return next_cookie;
+		list_for_each_entry(entry, &async_pending, list)
+			if (entry->running == running) {
+				ret = entry->cookie;
+				break;
+			}
 	}
 
+	return ret;
 }
 
 static async_cookie_t  lowest_in_progress(struct list_head *running)


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