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Date:   Fri, 24 May 2019 13:11:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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        Joel Fernandes <joel@joelfernandes.org>,
        Andy Lutomirski <luto@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Namhyung Kim <namhyung@kernel.org>,
        "Frank Ch. Eigler" <fche@redhat.com>
Subject: Re: [RFC][PATCH 01/14 v2] function_graph: Convert ret_stack to a
 series of longs
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References: <20190520142001.270067280@goodmis.org>
 <20190520142156.704372433@goodmis.org>
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On Mon, May 20, 2019 at 10:20:02AM -0400, Steven Rostedt wrote:

> +#define FGRAPH_RET_SIZE (sizeof(struct ftrace_ret_stack))
> +#define FGRAPH_RET_INDEX (ALIGN(FGRAPH_RET_SIZE, sizeof(long)) / sizeof(long))

I think you want to write that like:

	BUILD_BUG_ON(sizeof(ftrace_ret_stack) % sizeof(long));

It'd be very weird for that sizeof not to be right.

> +#define SHADOW_STACK_SIZE (PAGE_SIZE)

Do we really need that big a shadow stack?

> +#define SHADOW_STACK_INDEX			\
> +	(ALIGN(SHADOW_STACK_SIZE, sizeof(long)) / sizeof(long))
> +/* Leave on a buffer at the end */
> +#define SHADOW_STACK_MAX_INDEX (SHADOW_STACK_INDEX - FGRAPH_RET_INDEX)
> +
> +#define RET_STACK(t, index) ((struct ftrace_ret_stack *)(&(t)->ret_stack[index]))
> +#define RET_STACK_INC(c) ({ c += FGRAPH_RET_INDEX; })
> +#define RET_STACK_DEC(c) ({ c -= FGRAPH_RET_INDEX; })

I'm thinking something like:

#define RET_PUSH(s, val)				\
do {							\
	(s) -= sizeof(val);				\
	(typeof(val) *)(s) = val;			\
} while (0)

#define RET_POP(s, type)				\
({							\
	type *__ptr = (void *)(s);			\
	(s) += sizeof(type);				\
	*__ptr;						\
})

Would me clearer?