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Date: Tue, 16 Jun 2009 12:36:06 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>, Joerg Roedel <joerg.roedel@amd.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Steven Rostedt <rostedt@goodmis.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: bug in tty ldisc and friends
Message-ID: <20090616103606.GA3497@elte.hu>
References: <alpine.DEB.2.00.0906160144340.26673@gandalf.stny.rr.com> <20090616093741.0d5b36bc@lxorguk.ukuu.org.uk> <20090616085028.GA13771@elte.hu> <20090616095706.36f01282@lxorguk.ukuu.org.uk> <20090616100015.GD6432@elte.hu> <20090616110419.14bc9485@lxorguk.ukuu.org.uk> <20090616101240.GB28204@elte.hu> <20090616102509.GA32717@elte.hu> <20090616102910.GA11011@elte.hu>
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* Ingo Molnar <mingo@elte.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) {