linux-kernel - RE: [PATCH 03/16] IA64: use ACCESS

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Message-ID: <57C9024A16AD2D4C97DC78E552063EA3E38E71DD@orsmsx505.amr.corp.intel.com>
Date:	Wed, 18 Nov 2009 10:56:19 -0800
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Jiri Slaby <jslaby@...ell.com>,
	"jirislaby@...il.com" <jirislaby@...il.com>
CC:	"mingo@...e.hu" <mingo@...e.hu>,
	"nhorman@...driver.com" <nhorman@...driver.com>,
	"sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"marcin.slusarz@...il.com" <marcin.slusarz@...il.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	James Morris <jmorris@...ei.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>
Subject: RE: [PATCH 03/16] IA64: use ACCESS_ONCE for rlimits

> Make sure compiler won't do weird things with limits. E.g. fetching
> them twice may return 2 different values after writable limits are
> implemented.

-	if (size > task->signal->rlim[RLIMIT_MEMLOCK].rlim_cur)
+	if (size > ACCESS_ONCE(task->signal->rlim[RLIMIT_MEMLOCK].rlim_cur))

I don't see how this helps.  If someone else is changing limits while
we are looking at them, then there is a race.  We either get the old
or the new value.  Using ACCESS_ONCE (which on ia64 forces a "volatile"
access, which will make the compiler generate "ld.acq" rather than a
plain "ld") won't make any difference to this race.

Please explain what issue you see with the current code.

-Tony

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