On Thu, 2010-11-11 at 09:53 -0600, James Bottomley wrote: > On Thu, 2010-11-11 at 02:46 -0800, Nicholas A. Bellinger wrote: > > From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx> > > > > This patch converts struct scsi_host->cmd_serial_number to a 'odd incremented by 2' > > atomic_t counter in scsi_cmd_get_serial(), and moves the host_lock held call in > > jgarzik's DEF_SCSI_QCMD() wrapper back into it's original location in scsi_dispatch_cmd(). > > Actually, no ... this isn't really a good idea; you're lengthening our > critical path here (an atomic costs a lot more than a simple add under > spinlock). > Fair enough, but this is only less expensive with a spinlock w/p interrupts disabled, yes..? > There are only a few drivers left that actually make use of a serial > number. Of those, the only modern ones are qla4, lpfc, mpt2sas and > megaraid. > I am not exactly sure that qla4 or lpfc is on the list of LLDs that use and still need a cmd->serial_number for anything beyond simple printk() purposes.. > So the next logical step seems to be eliminate the overloading of the > serial number zero value, which removes the last mid-layer use (dpt_i2o > seems to abuse this unnecessarily as well), then the serial number code > can be pushed down into the queuecommand routines of only those drivers > that actually use it. Correct, this is what we where originally doing in the host_lock-less-for-37-v6 series here: http://git.kernel.org/?p=linux/kernel/git/nab/lio-4.0.git;a=commitdiff;h=72a72033661506ead54e0f366218fd0aef2e5339 The LLDs that ended up requring the explict calls for: mpt2sas: Add scsi_cmd_get_serial() call and set SHT->queuecommand_unlocked() mpt/fusion: Add scsi_cmd_get_serial() call and set SHT->queuecommand_unlocked() dpt_i2o: Add scsi_cmd_get_serial() call eata: Add scsi_cmd_get_serial() call u14-34f: Add scsi_cmd_get_serial() call > None of the modern ones seems to have a > legitimate use, so I think their uses can mostly be eliminated. >From the above 5 LLDs, they all appear to be releated to using cmd->serial_number in their internal error recovery handling. > Thus, > we might be able to get away with a simple queuecommand push down and > never bother with atomics for this (since it's unlikely the legacy users > would convert away from a lock wrapping their queuecommand routines). > Sounds good to me, but you will recall the last attempt to make scsi_cmd_get_serial() optional for the special case LLDs, that we started running quickly in the legacy usage of cmd->serial_number in scsi_softirq_done() and the side effects in scsi_try_to_abort_cmd(), who use is complex enough that we have not found a proper resolution sufficent to andmike discussed here: http://marc.info/?l=linux-scsi&m=128535319915212&w=2 http://marc.info/?l=linux-scsi&m=128820726325009&w=2 Any takers..? ;) --nab > James > > > -- > 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 -- 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
