linux-kernel - RE: [PATCH 4/6] ftrace syscalls: Allow arch specific syscallsymbol matching

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Date:	Wed, 02 Feb 2011 09:28:13 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Ian Munsie <imunsie@....ibm.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Andreas Schwab <schwab@...ux-m68k.org>,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	Namhyung Kim <namhyung@...il.com>,
	Jiri Kosina <jkosina@...e.cz>, Jason Baron <jbaron@...hat.com>,
	linux-kernel@...r.kernel.org, Alexander Graf <agraf@...e.de>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Scott Wood <scottwood@...escale.com>,
	Nathan Lynch <ntl@...ox.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Gibson <dwg@....ibm.com>,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Avantika Mathur <mathur@...ux.vnet.ibm.com>
Subject: RE: [PATCH 4/6] ftrace syscalls: Allow arch specific syscallsymbol
 matching

On Wed, 2011-02-02 at 14:15 +0000, David Laight wrote:
> > +#define arch_syscall_match_sym_name(sym, name) !strcmp(sym + 3, name
> + 3)
> 
> Whenever you use a #define macro arg, you should enclose it in ().
> About the only time you don't need to is when it is being
> passed as an argument to another function
> (ie when it's use is also ',' separated).
> 
> So the above ought to be:
> #define arch_syscall_match_sym_name(sym, name) (!strcmp((sym) + 3,
> (name) + 3))

I would have mentioned this if I wanted it to stay a macro ;)

> 
> Whether an inline function is better or worse is much more subtle!
> For instance I've used:
>    asm volatile ( "# line " STR(__LINE__) :: )
> to stop gcc merging the tails of conditionals.
> Useful when the conditional is at the end of a loop (etc),
> it might increase code size slightly, but removes a branch.
> 
> If I put one of those in an 'inline' function separate copies
> of the function end up sharing code.
> With a #define __LINE__ differs so they don't.
> 
> (I had some code to get below 190 clocks, these changes
> were significant!)

For what you were doing, this may have helped. But the code in question
is the "default" version of the function. I much more prefer it to be a
static inline. The issues you experience could change from gcc to gcc.
But static inlined functions are much cleaner and easier to read than
macros.

Using a macro for this purpose is just too messy.

Again, look at include/trace/ftrace.h. If I'm saying using a macro is
ugly, then don't use it! Listen to me, because I'm Mr. Ugly Macro Man.

-- Steve


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