linux-kernel - Re: bug in tty ldisc and friends

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Message-ID: <20090616103606.GA3497@elte.hu>
Date:	Tue, 16 Jun 2009 12:36:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Joerg Roedel <joerg.roedel@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: bug in tty ldisc and friends


* Ingo Molnar <mingo@...e.hu> wrote:

> This is something we noticed recently: dma-debug uses a lot of 
> lock classes and thus creates a really large lock-graph, depleting 
> the reserves quickly.

yep, i think dma-debug is the culprit. It has:

 #define HASH_SIZE       1024ULL

each hash entry has a separate lock.

But this should be easily to solve: a special lock-class for these 
locks. Patch below (warning: utterly untested!).

(Btw., that SPIN_LOCK_UNLOCKED init method should really be 
deprecated for real.)

	Ingo

diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index 3b93129..012c240 100644
--- a/lib/dma-debug.c
+++ b/lib/dma-debug.c
@@ -62,6 +62,8 @@ struct dma_debug_entry {
 #endif
 };
 
+static struct lock_class_key hash_bucket_class;
+
 struct hash_bucket {
 	struct list_head list;
 	spinlock_t lock;
@@ -716,7 +718,8 @@ void dma_debug_init(u32 num_entries)
 
 	for (i = 0; i < HASH_SIZE; ++i) {
 		INIT_LIST_HEAD(&dma_entry_hash[i].list);
-		dma_entry_hash[i].lock = SPIN_LOCK_UNLOCKED;
+		spin_lock_init(&dma_entry_hash[i].lock);
+		lockdep_set_lock_class(&dma_entry_hash[i].lock, &hash_bucket_class);
 	}
 
 	if (dma_debug_fs_init() != 0) {
--
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