1. 14 May, 2008 2 commits
    • Nick Piggin's avatar
      fix SMP data race in pagetable setup vs walking · 362a61ad
      Nick Piggin authored
      
      
      There is a possible data race in the page table walking code. After the split
      ptlock patches, it actually seems to have been introduced to the core code, but
      even before that I think it would have impacted some architectures (powerpc
      and sparc64, at least, walk the page tables without taking locks eg. see
      find_linux_pte()).
      
      The race is as follows:
      The pte page is allocated, zeroed, and its struct page gets its spinlock
      initialized. The mm-wide ptl is then taken, and then the pte page is inserted
      into the pagetables.
      
      At this point, the spinlock is not guaranteed to have ordered the previous
      stores to initialize the pte page with the subsequent store to put it in the
      page tables. So another Linux page table walker might be walking down (without
      any locks, because we have split-leaf-ptls), and find that new pte we've
      inserted. It might try to take the spinlock before the store from the other
      CPU initializes it. And subsequently it might read a pte_t out before stores
      from the other CPU have cleared the memory.
      
      There are also similar races in higher levels of the page tables. They
      obviously don't involve the spinlock, but could see uninitialized memory.
      
      Arch code and hardware pagetable walkers that walk the pagetables without
      locks could see similar uninitialized memory problems, regardless of whether
      split ptes are enabled or not.
      
      I prefer to put the barriers in core code, because that's where the higher
      level logic happens, but the page table accessors are per-arch, and open-coding
      them everywhere I don't think is an option. I'll put the read-side barriers
      in alpha arch code for now (other architectures perform data-dependent loads
      in order).
      Signed-off-by: default avatarNick Piggin <npiggin@suse.de>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      362a61ad
    • Nick Piggin's avatar
      read_barrier_depends arch fixlets · 73f10281
      Nick Piggin authored
      
      
      read_barrie_depends has always been a noop (not a compiler barrier) on all
      architectures except SMP alpha. This brings UP alpha and frv into line with all
      other architectures, and fixes incorrect documentation.
      Signed-off-by: default avatarNick Piggin <npiggin@suse.de>
      Acked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      73f10281
  2. 13 May, 2008 38 commits