[tip:locking/core] lockdep: Make the lock debug output more useful

[tip-bot for Tetsuo Handa]

Commit-ID:  b3c39758c8a6972f02b43f83dba7fe7a352371b9
Gitweb:     https://git.kernel.org/tip/b3c39758c8a6972f02b43f83dba7fe7a352371b9
Author:     Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 27 Mar 2018 19:41:41 +0900
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 29 Mar 2018 14:41:40 +0200

lockdep: Make the lock debug output more useful

The lock debug output in print_lock() has a few shortcomings:

 - It prints the hlock->acquire_ip field in %px and %pS format. That's
   redundant information.

 - It lacks information about the lock object itself. The lock class is
   not helpful to identify a particular instance of a lock.

Change the output so it prints:

 - hlock->instance to allow identification of a particular lock instance.

 - only the %pS format of hlock->ip_acquire which is sufficient to decode
   the actual code line with faddr2line.

The resulting output is:

3 locks held by a.out/31106:
#0: 00000000b0f753ba (&mm->mmap_sem){++++}, at: copy_process.part.41+0x10d5/0x1fe0
#1: 00000000ef64d539 (&mm->mmap_sem/1){+.+.}, at: copy_process.part.41+0x10fe/0x1fe0
#2: 00000000b41a282e (&mapping->i_mmap_rwsem){++++}, at: copy_process.part.41+0x12f2/0x1fe0

[ tglx: Massaged changelog ]

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/201803271941.GBE57310.tVSOJLQOFFOHFM@xxxxxxxxxxxxxxxxxxx

---
 kernel/locking/lockdep.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 12a2805dd64a..023386338269 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -556,9 +556,9 @@ static void print_lock(struct held_lock *hlock)
 		return;
 	}
 
+	printk(KERN_CONT "%p", hlock->instance);
 	print_lock_name(lock_classes + class_idx - 1);
-	printk(KERN_CONT ", at: [<%px>] %pS\n",
-		(void *)hlock->acquire_ip, (void *)hlock->acquire_ip);
+	printk(KERN_CONT ", at: %pS\n", (void *)hlock->acquire_ip);
 }
 
 static void lockdep_print_held_locks(struct task_struct *curr)