1. 19 Mar, 2008 2 commits
    • Randy Dunlap's avatar
      mm: fix various kernel-doc comments · 7682486b
      Randy Dunlap authored
      
      
      Fix various kernel-doc notation in mm/:
      
      filemap.c: add function short description; convert 2 to kernel-doc
      fremap.c: change parameter 'prot' to @prot
      pagewalk.c: change "-" in function parameters to ":"
      slab.c: fix short description of kmem_ptr_validate()
      swap.c: fix description & parameters of put_pages_list()
      swap_state.c: fix function parameters
      vmalloc.c: change "@returns" to "Returns:" since that is not a parameter
      Signed-off-by: default avatarRandy Dunlap <randy.dunlap@oracle.com>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      7682486b
    • Quentin Barnes's avatar
      aio: bad AIO race in aio_complete() leads to process hang · 6cb2a210
      Quentin Barnes authored
      
      
      My group ran into a AIO process hang on a 2.6.24 kernel with the process
      sleeping indefinitely in io_getevents(2) waiting for the last wakeup to come
      and it never would.
      
      We ran the tests on x86_64 SMP.  The hang only occurred on a Xeon box
      ("Clovertown") but not a Core2Duo ("Conroe").  On the Xeon, the L2 cache isn't
      shared between all eight processors, but is L2 is shared between between all
      two processors on the Core2Duo we use.
      
      My analysis of the hang is if you go down to the second while-loop
      in read_events(), what happens on processor #1:
      	1) add_wait_queue_exclusive() adds thread to ctx->wait
      	2) aio_read_evt() to check tail
      	3) if aio_read_evt() returned 0, call [io_]schedule() and sleep
      
      In aio_complete() with processor #2:
      	A) info->tail = tail;
      	B) waitqueue_active(&ctx->wait)
      	C) if waitqueue_active() returned non-0, call wake_up()
      
      The way the code is written, step 1 must be seen by all other processors
      before processor 1 checks for pending events in step 2 (that were recorded by
      step A) and step A by processor 2 must be seen by all other processors
      (checked in step 2) before step B is done.
      
      The race I believed I was seeing is that steps 1 and 2 were
      effectively swapped due to the __list_add() being delayed by the L2
      cache not shared by some of the other processors.  Imagine:
      proc 2: just before step A
      proc 1, step 1: adds to ctx->wait, but is not visible by other processors yet
      proc 1, step 2: checks tail and sees no pending events
      proc 2, step A: updates tail
      proc 1, step 3: calls [io_]schedule() and sleeps
      proc 2, step B: checks ctx->wait, but sees no one waiting, skips wakeup
                      so proc 1 sleeps indefinitely
      
      My patch adds a memory barrier between steps A and B.  It ensures that the
      update in step 1 gets seen on processor 2 before continuing.  If processor 1
      was just before step 1, the memory barrier makes sure that step A (update
      tail) gets seen by the time processor 1 makes it to step 2 (check tail).
      
      Before the patch our AIO process would hang virtually 100% of the time.  After
      the patch, we have yet to see the process ever hang.
      Signed-off-by: default avatarQuentin Barnes <qbarnes+linux@yahoo-inc.com>
      Reviewed-by: default avatarZach Brown <zach.brown@oracle.com>
      Cc: Benjamin LaHaise <bcrl@kvack.org>
      Cc: <stable@kernel.org>
      Cc: Nick Piggin <nickpiggin@yahoo.com.au>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      [ We should probably disallow that "if (waitqueue_active()) wake_up()"
        coding pattern, because it's so often buggy wrt memory ordering ]
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      6cb2a210
  2. 18 Mar, 2008 33 commits
  3. 17 Mar, 2008 5 commits