Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends

Le 04/03/2021 à 20:24, Segher Boessenkool a écrit :

On Thu, Mar 04, 2021 at 09:54:44AM -0800, Nick Desaulniers wrote:

quoted

On Thu, Mar 4, 2021 at 9:42 AM Marco Elver [off-list ref] wrote:
include/linux/compiler.h:246:
prevent_tail_call_optimization

commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")

https://github.com/linuxppc/linux/commit/a9a3ed1eff36

That is much heavier than needed (an mb()).  You can just put an empty
inline asm after a call before a return, and that call cannot be
optimised to a sibling call: (the end of a function is an implicit
return:)

Instead of:

void g(void);
void f(int x)
	if (x)
		g();
}

Do:

void g(void);
void f(int x)
	if (x)
		g();
	asm("");
}

This costs no extra instructions, and certainly not something as heavy
as an mb()!  It works without the "if" as well, of course, but with it
it is a more interesting example of a tail call.

In the commit mentionned at the top, it is said:

The next attempt to prevent compilers from tail-call optimizing
the last function call cpu_startup_entry(), ... , was to add an empty asm("").

This current solution was short and sweet, and reportedly, is supported
by both compilers but we didn't get very far this time: future (LTO?)
optimization passes could potentially eliminate this, which leads us
to the third attempt: having an actual memory barrier there which the
compiler cannot ignore or move around etc.

Christophe

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