linux-kernel - Re: [PATCH 1/8] ftrace: add ftrace_init

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Date:   Mon, 21 Oct 2019 14:07:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        amit.kachhap@....com, ard.biesheuvel@...aro.org,
        catalin.marinas@....com, deller@....de, duwe@...e.de,
        james.morse@....com, jeyu@...nel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, mingo@...hat.com, peterz@...radead.org,
        svens@...ckframe.org, takahiro.akashi@...aro.org, will@...nel.org
Subject: Re: [PATCH 1/8] ftrace: add ftrace_init_nop()

On Mon, 21 Oct 2019 17:34:19 +0100
Mark Rutland <mark.rutland@....com> wrote:

> Architectures may need to perform special initialization of ftrace
> callsites, and today they do so by special-casing ftrace_make_nop() when
> the expected branch address is MCOUNT_ADDR. In some cases (e.g. for
> patchable-function-entry), we don't have an mcount-like symbol and don't
> want a synthetic MCOUNT_ADDR, but we may need to perform some
> initialization of callsites.
> 
> To make it possible to separate initialization from runtime
> modification, and to handle cases without an mcount-like symbol, this
> patch adds an optional ftrace_init_nop() function that architectures can
> implement, which does not pass a branch address.
> 
> Where an architecture does not provide ftrace_init_nop(), we will fall
> back to the existing behaviour of calling ftrace_make_nop() with
> MCOUNT_ADDR.
> 
> At the same time, ftrace_code_disable() is renamed to
> ftrace_code_init_disabled() to make it clearer that it is intended to
> intialize a callsite into a disabled state, and is not for disabling a
> callsite that has been runtime enabled.

To make the name even better, let's just rename it to:

 ftrace_nop_initialization()

I think that may be the best description for it.

> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Torsten Duwe <duwe@...e.de>
> ---
>  kernel/trace/ftrace.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index f296d89be757..afd7e210e595 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2493,15 +2493,22 @@ struct dyn_ftrace *ftrace_rec_iter_record(struct ftrace_rec_iter *iter)
>  	return &iter->pg->records[iter->index];
>  }
>  
> +#ifndef ftrace_init_nop
> +static int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> +{
> +	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> +}
> +#endif

Can you place the above in the ftrace.h header. That's where that would
belong.

#ifndef ftrace_init_nop
struct module;
static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
{
	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
}
#endif

-- Steve

> +
>  static int
> -ftrace_code_disable(struct module *mod, struct dyn_ftrace *rec)
> +ftrace_code_init_disabled(struct module *mod, struct dyn_ftrace *rec)
>  {
>  	int ret;
>  
>  	if (unlikely(ftrace_disabled))
>  		return 0;
>  
> -	ret = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> +	ret = ftrace_init_nop(mod, rec);
>  	if (ret) {
>  		ftrace_bug_type = FTRACE_BUG_INIT;
>  		ftrace_bug(ret, rec);
> @@ -2943,7 +2950,7 @@ static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
>  			 * to the NOP instructions.
>  			 */
>  			if (!__is_defined(CC_USING_NOP_MCOUNT) &&
> -			    !ftrace_code_disable(mod, p))
> +			    !ftrace_code_init_disabled(mod, p))
>  				break;
>  
>  			update_cnt++;