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

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Date:	Fri, 5 Dec 2014 14:30:12 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	"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
> 
> 
> 

Thanks for the hint David!

I'm going to drop that unlikely and simply replace in_atomic() by
pagefault_disabled() - will also keep the change minimal!

David

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