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    bsg: add support for tail queuing · 05378940
    Boaz Harrosh 提交于
    Currently inherited from sg.c bsg will submit asynchronous request
     at the head-of-the-queue, (using "at_head" set in the call to
     blk_execute_rq_nowait()). This is bad in situation where the queues
     are full, requests will execute out of order, and can cause
     starvation of the first submitted requests.
    
    The sg_io_v4->flags member is used and a bit is allocated to denote the
    Q_AT_TAIL. Zero is to queue at_head as before, to be compatible with old
    code at the write/read path. SG_IO code path behavior was changed so to
    be the same as write/read behavior. SG_IO was very rarely used and breaking
    compatibility with it is OK at this stage.
    
    sg_io_hdr at sg.h also has a flags member and uses 3 bits from the first
    nibble and one bit from the last nibble. Even though none of these bits
    are supported by bsg, The second nibble is allocated for use by bsg. Just
    in case.
    Signed-off-by: NBoaz Harrosh <bharrosh@panasas.com>
    CC: Douglas Gilbert <dgilbert@interlog.com>
    Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
    05378940
bsg.c 23.4 KB