linux-kernel - [PATCH 1/2] Fix pdflush thread creation upper bound.

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Date:	Tue, 30 Dec 2008 16:12:31 -0700
From:	Peter W Morreale <pmorreale@...ell.com>
To:	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] Fix pdflush thread creation upper bound.

From: \"Peter W. Morreale\" <pmorreale@...ell.com>

This patch fixes a race on creating pdflush threads.  Without the patch,
it is possible to create more than MAX_PDFLUSH_THREADS threads, and this
has been observed in practice on IO loaded SMP machines.

The fix involves moving the lock around to protect the check against the
thread count and correctly dealing with thread creation failure.
---

Signed-off-by: Peter W Morreale <pmorreale@...ell.com>

 mm/pdflush.c |   27 +++++++++++++++++++++------
 1 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/mm/pdflush.c b/mm/pdflush.c
index 0cbe0c6..481680f 100644
--- a/mm/pdflush.c
+++ b/mm/pdflush.c
@@ -98,7 +98,6 @@ static int __pdflush(struct pdflush_work *my_work)
 	INIT_LIST_HEAD(&my_work->list);
 
 	spin_lock_irq(&pdflush_lock);
-	nr_pdflush_threads++;
 	for ( ; ; ) {
 		struct pdflush_work *pdf;
 
@@ -126,20 +125,23 @@ static int __pdflush(struct pdflush_work *my_work)
 
 		(*my_work->fn)(my_work->arg0);
 
+		spin_lock_irq(&pdflush_lock);
+
 		/*
 		 * Thread creation: For how long have there been zero
 		 * available threads?
 		 */
 		if (time_after(jiffies, last_empty_jifs + 1 * HZ)) {
-			/* unlocked list_empty() test is OK here */
 			if (list_empty(&pdflush_list)) {
-				/* unlocked test is OK here */
-				if (nr_pdflush_threads < MAX_PDFLUSH_THREADS)
+				if (nr_pdflush_threads < MAX_PDFLUSH_THREADS) {
+					nr_pdflush_threads++;
+					spin_unlock_irq(&pdflush_lock);
 					start_one_pdflush_thread();
+					spin_lock_irq(&pdflush_lock);
+				}
 			}
 		}
 
-		spin_lock_irq(&pdflush_lock);
 		my_work->fn = NULL;
 
 		/*
@@ -226,13 +228,26 @@ int pdflush_operation(void (*fn)(unsigned long), unsigned long arg0)
 
 static void start_one_pdflush_thread(void)
 {
-	kthread_run(pdflush, NULL, "pdflush");
+	struct task_struct *k;
+
+	k = kthread_run(pdflush, NULL, "pdflush");
+	if (unlikely(IS_ERR(k))) {
+		spin_lock_irq(&pdflush_lock);
+		nr_pdflush_threads--;
+		spin_unlock_irq(&pdflush_lock);
+	}
 }
 
 static int __init pdflush_init(void)
 {
 	int i;
 
+	/*
+	 * Pre-set nr_pdflush_threads...  If we fail to create, 
+	 * the count will be decremented.
+	 */
+	nr_pdflush_threads = MIN_PDFLUSH_THREADS;
+
 	for (i = 0; i < MIN_PDFLUSH_THREADS; i++)
 		start_one_pdflush_thread();
 	return 0;

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