Subject: Re: possible circular locking dependency detected [was: linux-next: Tree for Aug 22]

From: Peter Zijlstra <[email-masked]>
Date: Wed, 30 Aug 2017 10:47:46 +0200

On Wed, Aug 30, 2017 at 10:42:07AM +0200, Peter Zijlstra wrote:
> 
> So the overhead looks to be spread out over all sorts, which makes it
> harder to find and fix.
> 
> stack unwinding is done lots and is fairly expensive, I've not yet
> checked if crossrelease does too much of that.

Aah, we do an unconditional stack unwind for every __lock_acquire() now.
It keeps a trace in the xhlocks[].

Does the below cure most of that overhead?

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 44c8d0d17170..7b872036b72e 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4872,7 +4872,7 @@ static void add_xhlock(struct held_lock *hlock)
 	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
 	xhlock->trace.entries = xhlock->trace_entries;
 	xhlock->trace.skip = 3;
-	save_stack_trace(&xhlock->trace);
+	/* save_stack_trace(&xhlock->trace); */
 }
 
 static inline int same_context_xhlock(struct hist_lock *xhlock)