linux-kernel - RE: [PATCH] Staging: memrar: Moved memrar_allocator struct into memrar

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Date:	Thu, 24 Jun 2010 10:46:41 -0700
From:	"Othman, Ossama" <ossama.othman@...el.com>
To:	Henri Häkkinen <henrih81@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	"gregkh@...e.de" <gregkh@...e.de>,
	Matti Lammi <mattij.lammi@...il.com>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] Staging: memrar: Moved memrar_allocator struct into
 memrar_allocator.c

Hi,

> >> size_t memrar_allocator_largest_free_area(struct memrar_allocator
> *allocator)
> >> {
> >> -	if (allocator == NULL)
> >> -		return 0;
> >> -	return allocator->largest_free_area;
> >> +	size_t tmp = 0;
> >> +
> >> +	if (allocator != NULL) {
> >> +		mutex_lock(&allocator->lock);
> >> +		tmp = allocator->largest_free_area;
> >> +		mutex_unlock(&allocator->lock);
> >
> > This doesn't seem to make any sense (in either version). The moment
> you
> > drop the lock the value in "tmp" becomes stale as the allocator could
> > change it. ?
> >
> 
> The idea was proposed by Ossama Othman in his earlier reply.

:-) 

[OO] > > Certainly the allocator->largest_free_area value could be updated
> after the lock is released and by the time it is returned to the user
> (for statistical purposes), but at least the internal allocator state
> would remain consistent in the presences of multiple threads.

My suggestion focused solely on hiding the allocator lock from the caller.  The TOCTOU race I alluded to above exists in the current version of the code, and was not introduced with the change I proposed to your patch.

HTH,
-Ossama
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