Re: [PATCH 1/1] lockdep: fix warning: print_lock_trace defined but not used

[Paul E. McKenney]

On Tue, May 21, 2019 at 12:08:08AM -0700, john.hubbard@xxxxxxxxx wrote:
> From: John Hubbard <jhubbard@xxxxxxxxxx>
> 
> Commit 0d2cc3b34532 ("locking/lockdep: Move valid_state() inside
> CONFIG_TRACE_IRQFLAGS && CONFIG_PROVE_LOCKING") moved the only usage of
> print_lock_trace() that was originally outside of the CONFIG_PROVE_LOCKING
> case. It moved that usage into a different case: CONFIG_PROVE_LOCKING &&
> CONFIG_TRACE_IRQFLAGS. That leaves things not symmetrical, and as a result,
> the following warning fires on my build, when I have
> 
> !CONFIG_TRACE_IRQFLAGS && !CONFIG_PROVE_LOCKING
> 
> set:
> 
> kernel/locking/lockdep.c:2821:13: warning: âprint_lock_traceâ defined
>     but not used [-Wunused-function]
> 
> Fix this by only defining print_lock_trace() in cases in which is it
> called.
> 
> Fixes: 0d2cc3b34532 ("locking/lockdep: Move valid_state() inside CONFIG_TRACE_IRQFLAGS && CONFIG_PROVE_LOCKING")
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  kernel/locking/lockdep.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index d06190fa5082..3065dc36c27a 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2817,11 +2817,14 @@ static inline int validate_chain(struct task_struct *curr,
>  	return 1;
>  }
> 
> +#if defined(CONFIG_TRACE_IRQFLAGS)
>  static void print_lock_trace(struct lock_trace *trace, unsigned int spaces)

This works, but another approach is to put "__maybe_unused" in the
above declaration, which avoids the need to have "#if" in a .c file.
But this file already has quite a few #if and #ifdef commands, so maybe
it is OK here.

Also, "#ifdef CONFIG_TRACE_IRQFLAGS" is a bit more conventional than
the above, should the "__maybe_unused" be undesirable.

Yet another approach is to move this function to include/linux/lockdep.h,
where #ifdef is considered less objectionable.

But I must defer to the maintainers.

							Thanx, Paul

>  {
>  }
>  #endif
> 
> +#endif
> +
>  /*
>   * We are building curr_chain_key incrementally, so double-check
>   * it from scratch, to make sure that it's done correctly:
> -- 
> 2.21.0
>