• David L Stevens's avatar
    sunvnet - add missing rmb() for sunvnet driver · 78dcff7b
    David L Stevens authored
    
    
    The sunvnet driver does not have an rmb() in the ring consumer corresponding
    to the wmb() in the producer. According to Documentation/memory-barriers.txt:
    
    "When dealing with CPU-CPU interactions, certain types of memory barrier should
    always be paired.  A lack of appropriate pairing is almost certainly an error."
    
    In cases where an rmb() is not a no-op and a consumer is removing data from
    the ring while a producer is adding new entries, a load reorder would allow
    
    CPU1						CPU2
    ----						----
    						LOAD desc.size [e.g]
    STORE desc.size
    <wmb>
    set desc.hdr.state = VIO_DESC_READY
    						LOAD desc.hdr.state
    						[because VIO_DESC_READY, use
    						 old desc.size, already loaded
    						 out of order]
    
    [CPU2 has reordered apparently unrelated LOADs]
    
    To ensure other desc fields are not loaded before checking VIO_DESC_READY, we
    need an rmb() between the check and desc data accesses.
    
    I've also moved the viodbg() call to after the rmb() so that it, too, has
    current descriptor data even with reordering, which has the side effect that
    it won't print anything for descriptors that are not VIO_DESC_READY as before.
    That's a) probably a good thing, since the fields are not necessarily set and,
    b) better than adding another rmb() just for viodbg().
    
    This would not be possible if strict-ordering is enforced, but then the
    memory barriers should be no-ops in that case.
    Signed-off-by: default avatarDavid L Stevens <david.stevens@oracle.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    78dcff7b
sunvnet.c 29.6 KB