On Wed, 18 Feb 2009 15:41:02 -0800 (PST) David Miller <da...@davemloft.net> wrote:
> > Platforms such as sparc64 have D-cache aliasing issues. We > cannot allow virtual mappings in different contexts to be such > that two cache lines can be loaded for the same backing data. > Updates to one cache line won't be seen by accesses to the other > cache line. > > Code in sparc64 and other architectures solve this problem by > making sure that all userland mappings of MAP_SHARED objects have > the same virtual address base. They implement this by keying > off of the page offset, and using that to choose a suitably > consistent virtual address for mmap() requests. > > Making things even worse, getting this wrong on sparc64 can result > in hangs during DRM lock acquisition. This is because, at least on > UltraSPARC-III, normal loads consult the D-cache but atomics such > as 'cas' (which is what cmpxchg() is implement using) only consult > the L2 cache. So if a D-cache alias is inserted, the load can > see different data than the atomic, and we'll loop forever because > the atomic compare-and-exchange will never complete successfully. > > So to make this all work properly, we need to make sure that the > hash address computed by drm_map_handle() preserves the SHMLBA > relevant bits, and that's what this patch does for _DRM_SHM mappings. > > As a historical note, many years ago this bug didn't exist because we > used to just use the low 32-bits of the address as the hash and just > hope for the best. This preserved the SHMLBA bits properly. But when > the hashtab code was added to DRM, this was no longer the case. > > ... > > #include <linux/vmalloc.h> > +#include <linux/log2.h> > +#include <asm/shmparam.h> > #include "drmP.h" > The inclusion of asm/shmparam.h direct from a driver is a bit risky. It assumes that asm/shmparam.h is compileable in isolation from the additional things which include/linux/shm.h includes. In particular, asm/page.h. eg: arch/xtensa/include/asm/shmparam.h #define SHMLBA ((PAGE_SIZE > DCACHE_WAY_SIZE)? PAGE_SIZE : DCACHE_WAY_SIZE) But including linux/shm.h here seems a bit silly. We'll see.. ------------------------------------------------------------------------------ Open Source Business Conference (OSBC), March 24-25, 2009, San Francisco, CA -OSBC tackles the biggest issue in open source: Open Sourcing the Enterprise -Strategies to boost innovation and cut costs with open source participation -Receive a $600 discount off the registration fee with the source code: SFAD http://p.sf.net/sfu/XcvMzF8H -- _______________________________________________ Dri-devel mailing list Dri-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/dri-devel
