on 2010-3-10 3:42, Paul Menage wrote: > On Sat, Mar 6, 2010 at 6:33 PM, Miao Xie <miaox@xxxxxxxxxxxxxx> wrote: >> >> Before applying this patch, cpuset updates task->mems_allowed just like >> what you said. But the allocator is still likely to see an empty nodemask. >> This problem have been pointed out by Nick Piggin. >> >> The problem is following: >> The size of nodemask_t is greater than the size of long integer, so loading >> and storing of nodemask_t are not atomic operations. If task->mems_allowed >> don't intersect with new_mask, such as the first word of the mask is empty >> and only the first word of new_mask is not empty. When the allocator >> loads a word of the mask before >> >> current->mems_allowed |= new_mask; >> >> and then loads another word of the mask after >> >> current->mems_allowed = new_mask; >> >> the allocator gets an empty nodemask. > > Couldn't that be solved by having the reader read the nodemask twice > and compare them? In the normal case there's no race, so the second > read is straight from L1 cache and is very cheap. In the unlikely case > of a race, the reader would keep trying until it got two consistent > values in a row. I think this method can't fix the problem because we can guarantee the second read is after the update of mask completes. Thanks! Miao > > Paul > > > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>