linux-kernel - RE: [PATCH v1 3/5] mm, uaccess: trigger might_sleep() in might

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Date:	Fri, 5 Dec 2014 12:08:19 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'David Hildenbrand' <dahi@...ux.vnet.ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"borntraeger@...ibm.com" <borntraeger@...ibm.com>,
	"mst@...hat.com" <mst@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"hughd@...gle.com" <hughd@...gle.com>,
	"hocko@...e.cz" <hocko@...e.cz>
Subject: RE: [PATCH v1 3/5] mm, uaccess: trigger might_sleep() in
 might_fault() when pagefaults are disabled

From: David Hildenbrand [...
> > This should be likely() instead of unlikely(), no?
> > I'd rather write this
> >
> > 	if (pagefault_disabled())
> >  		return;
> > 	__might_sleep(file, line, 0);
> >
> > and leave the likely stuff completely away.
> 
> Makes perfect sense!

>From my experience of getting (an older version of) gcc to emit
'correctly' statically predicted branches I found that code that
looks like (I don't think return/goto make any difference):

	If (unlikely(condition)) {
		code;
	}
	more_code;

is compile with a forwards conditional branch (ie ignoring the unlikely()).
Similarly 'if () continue' is likely to generate a 'predicted taken'
backwards conditional branch.

To get the desired effect you need a non-empty 'else' part, an assembler
comment will suffice, eg: asm volatile("# comment").

	David



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