Il 07/05/2014 16:34, Ming Lei ha scritto: > * > - * An interesting effect of this policy is that only writes to req_vq need to > - * take the tgt_lock. Read can be done outside the lock because: > - * > - * - writes of req_vq only occur when atomic_inc_return(&tgt->reqs) returns > 1. > - * In that case, no other CPU is reading req_vq: even if they were in > - * virtscsi_queuecommand_multi, they would be spinning on tgt_lock. > - * > - * - reads of req_vq only occur when the target is not idle (reqs != 0). > - * A CPU that enters virtscsi_queuecommand_multi will not modify req_vq. > + * tgt_lock is held to serialize reading and writing req_vq. Reading req_vq > + * could be done locklessly, but we do not do it yet. > * > * Similarly, decrements of reqs are never concurrent with writes of req_vq. > * Thus they can happen outside the tgt_lock, provided of course we make reqs
The part you deleted explains _why_ decrements of reqs are never concurrent with writes of req_vq. Perhaps: * An interesting effect of this policy is that only increments to reqs and writes * to req_vq need to take the tgt_lock. Reads of req_vq and decrements or req_vq * can be done outside the lock because: * * - writes of req_vq only occur when atomic_inc_return(&tgt->reqs) returns 1. * In that case, no other CPU is reading req_vq: even if they were in * virtscsi_queuecommand_multi, they would be spinning on tgt_lock. * * - reads of req_vq only occur when the target is not idle (reqs != 0). * A CPU that enters virtscsi_queuecommand_multi will not modify req_vq. * * - likewise, decrements of reqs only occur when reqs != 0. If the decremented * value is zero, the first CPU that enters virtscsi_queuecommand_multi will * modify req_vq and the others will spin on tgt_lock. * * We do not try to read req_vq locklessly for simplicity, so tgt_lock is used * to serialize reads of req_vq too. Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
