• David Rientjes's avatar
    cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask · b246272e
    David Rientjes authored
    Kernels where MAX_NUMNODES > BITS_PER_LONG may temporarily see an empty
    nodemask in a tsk's mempolicy if its previous nodemask is remapped onto a
    new set of allowed cpuset nodes where the two nodemasks, as a result of
    the remap, are now disjoint.
    
    c0ff7453 ("cpuset,mm: fix no node to alloc memory when changing
    cpuset's mems") adds get_mems_allowed() to prevent the set of allowed
    nodes from changing for a thread.  This causes any update to a set of
    allowed nodes to stall until put_mems_allowed() is called.
    
    This stall is unncessary, however, if at least one node remains unchanged
    in the update to the set of allowed nodes.  This was addressed by
    89e8a244 ("cpusets: avoid looping when storing to mems_allowed if one
    node remains set"), but it's still possible that an empty nodemask may be
    read from a mempolicy because the old nodemask may be remapped to the new
    nodemask during rebind.  To prevent this, only avoid the stall if there is
    no mempolicy for the thread being changed.
    
    This is a temporary solution until all reads from mempolicy nodemasks can
    be guaranteed to not be empty without the get_mems_allowed()
    synchronization.
    
    Also moves the check for nodemask intersection inside task_lock() so that
    tsk->mems_allowed cannot change.  This ensures that nothing can set this
    tsk's mems_allowed out from under us and also protects tsk->mempolicy.
    Reported-by: default avatarMiao Xie <miaox@cn.fujitsu.com>
    Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Paul Menage <paul@paulmenage.org>
    Cc: Stephen Rothwell <sfr@canb.auug.org.au>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    b246272e
cpuset.c 74.7 KB