On Thu, Nov 15, 2012 at 02:36:30PM -0800, Nicholas A. Bellinger wrote: > However, setting the default ibr->pending=2 value before dispatch, and > making a extra iblock_complete_cmd() call from iblock_execute_rw(), > while doing the normal iblock_complete_cmd() calls from > iblock_bio_done() is AFAICT a pointless extra atomic_dec_and_test() call > per I/O. > > Was there a reason why you changed ->pending from 1 -> 2 during the > se_task removal in commit 5787cacd0bd5ee01..? It is to avoid having the request completed before even submitting all bios. As soon as one is submitted it could on a very fast device complete before we've incremented the count for the next bio. It's a very common scheme used all over the kernel when submitting I/O in smaller subdivisions. > So the case this patch tries to avoid is where iblock_submit_bio() is > called multiple times before the final ibr->pending value is set, this > could potentially cause bios completion calls to decrement the value + > complete to core before iblock_execute_rw() is done incrementing > ibr->pending. That's exacltly what we try to avoid here. > How about returning an exception here instead when IBLOCK_MAX_BIOS in > reached..? Why? As soon as we kicked the first batch off we're guaranteed to make progress allocating more as sson as the first of the submitted ones completes. > Btw, where did the default of 32 for this come from..? It's a random number, with the important property that it's considerably smaller than IBLOCK_BIO_POOL_SIZE. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
