linux-kernel - Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain

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Date:   Tue, 4 Feb 2020 13:36:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain_hlocks entries

On Mon, Feb 03, 2020 at 11:41:46AM -0500, Waiman Long wrote:
> +	if (unlikely(size < 2))
> +		return; // XXX leak!

Stuck this on top...

--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2631,7 +2631,8 @@ struct lock_chain lock_chains[MAX_LOCKDE
 static DECLARE_BITMAP(lock_chains_in_use, MAX_LOCKDEP_CHAINS);
 static u16 chain_hlocks[MAX_LOCKDEP_CHAIN_HLOCKS];
 unsigned long nr_zapped_lock_chains;
-unsigned int nr_free_chain_hlocks;	/* Free cfhain_hlocks in buckets */
+unsigned int nr_free_chain_hlocks;	/* Free chain_hlocks in buckets */
+unsigned int nr_lost_chain_hlocks;	/* Lost chain_hlocks */
 unsigned int nr_large_chain_blocks;	/* size > MAX_CHAIN_BUCKETS */
 
 /*
@@ -2718,8 +2719,17 @@ static inline void add_chain_block(int o
 	int bucket = size_to_bucket(size);
 	int next = chain_block_buckets[bucket];
 
-	if (unlikely(size < 2))
-		return; // XXX leak!
+	if (unlikely(size < 2)) {
+		/*
+		 * We can't store single entries on the freelist. Leak them.
+		 *
+		 * One possible way out would be to uniquely mark them, other
+		 * than with CHAIN_BLK_FLAG, such that we can recover them when
+		 * the block before it is re-added.
+		 */
+		nr_lost_chain_hlocks++;
+		return;
+	}
 
 	init_chain_block(offset, next, bucket, size);
 	chain_block_buckets[bucket] = offset;
@@ -2798,8 +2808,8 @@ static int alloc_chain_hlocks(int req)
 
 search:
 	/*
-	 * linear search in the 'dump' bucket; look for an exact match or the
-	 * largest block.
+	 * linear search of the variable sized freelist; look for an exact
+	 * match or the largest block.
 	 */
 	for_each_chain_block(0, prev, curr, next) {
 		size = chain_block_size(curr);
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -141,6 +141,7 @@ extern unsigned int nr_hardirq_chains;
 extern unsigned int nr_softirq_chains;
 extern unsigned int nr_process_chains;
 extern unsigned int nr_free_chain_hlocks;
+extern unsigned int nr_lost_chain_hlocks;
 extern unsigned int nr_large_chain_blocks;
 
 extern unsigned int max_lockdep_depth;
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -278,9 +278,11 @@ static int lockdep_stats_show(struct seq
 #ifdef CONFIG_PROVE_LOCKING
 	seq_printf(m, " dependency chains:             %11lu [max: %lu]\n",
 			lock_chain_count(), MAX_LOCKDEP_CHAINS);
-	seq_printf(m, " dependency chain hlocks:       %11lu [max: %lu]\n",
-			MAX_LOCKDEP_CHAIN_HLOCKS - nr_free_chain_hlocks,
+	seq_printf(m, " dependency chain hlocks used:  %11lu [max: %lu]\n",
+			MAX_LOCKDEP_CHAIN_HLOCKS - (nr_free_chain_hlocks - nr_lost_chain_hlocks),
 			MAX_LOCKDEP_CHAIN_HLOCKS);
+	seq_printf(m, " dependency chain hlocks free:  %11lu\n", nr_free_chain_hlocks);
+	seq_printf(m, " dependency chain hlocks lost:  %11lu\n", nr_lost_chain_hlocks);
 #endif
 
 #ifdef CONFIG_TRACE_IRQFLAGS