On Thu, Sep 18, 2014 at 12:48 AM, Christoph Hellwig <h...@lst.de> wrote: > On Wed, Sep 17, 2014 at 05:47:58PM +0800, Ming Lei wrote: >> From: Ming Lei <ming....@canoical.com> >> >> This patch removes two unnecessary blk_clear_rq_complete(), >> the REQ_ATOM_COMPLETE flag is cleared inside blk_mq_start_request(), >> so: >> >> - The blk_clear_rq_complete() in blk_flush_restore_request() >> needn't because the request will be freed later, and clearing >> it here may open a small race window with timeout. > > This one is defintively correct, blk_mq_end_io should take care of this. > >> - The blk_clear_rq_complete() in blk_mq_requeue_request() isn't >> necessary too, even though REQ_ATOM_STARTED is cleared in >> __blk_mq_requeue_request(), in theory it still may cause a small >> race window with timeout since the two clear_bit() may be >> reordered. > > Why yo you think it's not nessecary? The request is not in the drivers
The COMPLETED flag will be cleared in blk_mq_start_request(), so it needn't to be cleared here, and it is a bit early and might cause a tiny race window. > hand at this point, so it should not be marked started. Maybe I'm missing > something, but this sounds like it could very likely cause regressions. The STARTED flag has been cleared in __blk_mq_requeue_request(), I mean the COMPLETED flag, consider the following situation: - one req is queued to driver, and the req is completed from the device just before its timeout expired - the driver checks the result and finds it need to requeue for some reason - blk_mq_requeue_request() is called to requeue the req - COMPLETED is cleared before clearing STARTED because of writes reorder - timeout just comes between the two writes - then the request may be completed another time because of timeout Thanks, -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/