locking/lockdep: Reduce space occupied by stack traces

Although commit 669de8bd ("kernel/workqueue: Use dynamic lockdep keys
for workqueues") unregisters dynamic lockdep keys when a workqueue is
destroyed, a side effect of that commit is that all stack traces
associated with the lockdep key are leaked when a workqueue is destroyed.
Fix this by storing each unique stack trace once. Other changes in this
patch are:

- Use NULL instead of { .nr_entries = 0 } to represent 'no trace'.
- Store a pointer to a stack trace in struct lock_class and struct
  lock_list instead of storing 'nr_entries' and 'offset'.

This patch avoids that the following program triggers the "BUG:
MAX_STACK_TRACE_ENTRIES too low!" complaint:

	#include <fcntl.h>
	#include <unistd.h>

	int main()
	{
		for (;;) {
			int fd = open("/dev/infiniband/rdma_cm", O_RDWR);
			close(fd);
		}
	}
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Yuyang Du <duyuyang@gmail.com>
Link: https://lkml.kernel.org/r/20190722182443.216015-4-bvanassche@acm.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/lockdep.h

@@ -66,10 +66,7 @@ struct lock_class_key {
 
 extern struct lock_class_key __lockdep_no_validate__;
 
-struct lock_trace {
-	unsigned int		nr_entries;
-	unsigned int		offset;
-};
+struct lock_trace;
 
 #define LOCKSTAT_POINTS		4
 
@@ -105,7 +102,7 @@ struct lock_class {
 	 * IRQ/softirq usage tracking bits:
 	 */
 	unsigned long			usage_mask;
-	struct lock_trace		usage_traces[XXX_LOCK_USAGE_STATES];
+	const struct lock_trace		*usage_traces[XXX_LOCK_USAGE_STATES];
 
 	/*
 	 * Generation counter, when doing certain classes of graph walking,
@@ -193,7 +190,7 @@ struct lock_list {
 	struct list_head		entry;
 	struct lock_class		*class;
 	struct lock_class		*links_to;
-	struct lock_trace		trace;
+	const struct lock_trace		*trace;
 	int				distance;
 
 	/*

kernel/locking/lockdep_internals.h

@@ -92,6 +92,7 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =
 #define MAX_LOCKDEP_ENTRIES	16384UL
 #define MAX_LOCKDEP_CHAINS_BITS	15
 #define MAX_STACK_TRACE_ENTRIES	262144UL
+#define STACK_TRACE_HASH_SIZE	8192
 #else
 #define MAX_LOCKDEP_ENTRIES	32768UL
 
@@ -102,6 +103,7 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =
  * addresses. Protected by the hash_lock.
  */
 #define MAX_STACK_TRACE_ENTRIES	524288UL
+#define STACK_TRACE_HASH_SIZE	16384
 #endif
 
 #define MAX_LOCKDEP_CHAINS	(1UL << MAX_LOCKDEP_CHAINS_BITS)