Commit f1a2a9b6 authored by Committed by Dave AirlieBrowse files
drm: Preserve SHMLBA bits in hash key for _DRM_SHM mappings.
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. Signed-off-by: David S. Miller <firstname.lastname@example.org> Signed-off-by: Dave Airlie <email@example.com>