[patch V2 18/29] lockdep: Move stack trace logic into check_prev_add()

[Peter Zijlstra]

On Thu, Apr 18, 2019 at 10:41:37AM +0200, Thomas Gleixner wrote:
> There is only one caller of check_prev_add() which hands in a zeroed struct
> stack trace and a function pointer to save_stack(). Inside check_prev_add()
> the stack_trace struct is checked for being empty, which is always
> true. Based on that one code path stores a stack trace which is unused. The
> comment there does not make sense either. It's all leftovers from
> historical lockdep code (cross release).

I was more or less expecting a revert of:

ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace")

And then I read the comment that went with the "static struct
stack_trace trace" that got removed (in the above commit) and realized
that your patch will consume more stack entries.

The problem is when the held lock stack in check_prevs_add() has multple
trylock entries on top, in that case we call check_prev_add() multiple
times, and this patch will then save the exact same stack-trace multiple
times, consuming static resources.

Possibly we should copy what stackdepot does (but we cannot use it
directly because stackdepot uses locks; but possible we can share bits),
but that is a patch for another day I think.

So while convoluted, perhaps we should retain this code for now.