[v3.6 3/3] iommu/tegra: smmu: Fix unsleepable memory allocation at alloc_pdir()

[joerg.roedel at amd.com]

On Tue, Jul 17, 2012 at 02:25:24PM +0200, Hiroshi Doyu wrote:
> The above spin_lock is always necessary. "as->lock" should be held to
> protect "as->pdir_page". Only when "as->pdir_page" is NULL,
> "as->pdir_page" would be allocated in "alloc_pdir()". Without this
> lock, the following race could happen:
> 
> 
> Without as->lock:
> A:			B:
> i == 3
> pdir_page == NULL
> 			i == 3
> 	     		pdir_page == NULL
> pdir_page = a;
> 			pdir_page = b;	!!!!!! OVERWRITTEN !!!!!!
>

Unless I am missing something, this is not the correct situation with my
patch. It would look more like this:


 A:					B:
 i == 3
 pdir_page == NULL
 					i == 3
 	     				pdir_page == NULL

 take as->lock

 /* race check */
 pdir_page == NULL -> proceed		/* spinning on as->lock */

 pdir_page = a;

 release as->lock

					 take as->lock

					 /* race check */
					 pdir_page != NULL -> return

This should be fine, no? Do I miss something?


	Joerg