linux-kernel - Re: [PATCH 1/5] Split wait_noreap

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Date:	Mon, 11 May 2009 14:17:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Hellwig <hch@...radead.org>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc:	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/5] Split wait_noreap_copyout()


* Christoph Hellwig <hch@...radead.org> wrote:

> On Mon, May 11, 2009 at 12:12:40PM +0200, Vitaly Mayatskikh wrote:
> > +static int wait_copyout(struct wait_opts *wo, struct task_struct *p,
> > +			pid_t pid, uid_t uid, int why, int status, int signal)
> >  {
> > -	struct siginfo __user *infop;
> > +	struct siginfo __user *infop = wo->wo_info;
> >  	int retval = wo->wo_rusage
> >  		? getrusage(p, RUSAGE_BOTH, wo->wo_rusage) : 0;
> >  
> > +	if (!retval && infop) {
> > +		retval = put_user(signal, &infop->si_signo);
> > +		if (!retval)
> > +			retval = put_user(0, &infop->si_errno);
> > +		if (!retval)
> > +			retval = put_user((short)why, &infop->si_code);
> > +		if (!retval)
> > +			retval = put_user(pid, &infop->si_pid);
> > +		if (!retval)
> > +			retval = put_user(uid, &infop->si_uid);
> > +		if (!retval)
> > +			retval = put_user(status, &infop->si_status);
> > +	}
> > +	return retval;
> 
> wouldn't this better be written as:
> 
> static int wait_copyout(struct wait_opts *wo, struct task_struct *p,
> 			pid_t pid, uid_t uid, int why, int status, int signal)
> {
> 	struct siginfo __user *infop = wo->wo_info;
> 
> 	if (wo->wo_rusage) {
> 		int retval = getrusage(p, RUSAGE_BOTH, wo->wo_rusage);
> 		if (retval)
> 			return retval;
> 	}
> 
> 	if (!infop)
> 		return 0;
> 
> 	if (put_user(signal, &infop->si_signo) ||
> 	    put_user(0, &infop->si_errno) ||
> 	    put_user((short)why, &infop->si_code) ||
> 	    put_user(pid, &infop->si_pid) ||
> 	    put_user(uid, &infop->si_uid) ||
> 	    put_user(status, &infop->si_status))
> 		return -EFAULT;

For best assembly code this should generally be written as a series 
of:

   __uaccess_err |= __put_user(x, ptr);
   __uaccess_err |= __put_user(y, ptr);
   __uaccess_err |= __put_user(z, ptr);

As this generates non-dependent, compressed, branch-less code.

See the (new) put_user_try / put_user_ex() / put_user_catch() 
abstraction in arch/x86/include/asm/uaccess.h, and how all the x86 
signal code makes use of that to optimize such patterns of per field 
user copies.

	Ingo
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