On Thu, 2010-11-11 at 13:37 -0800, Nicholas A. Bellinger wrote: > 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..? It's less expensive than the explicit lock around just the serial number, yes ... but if all use cases use an explicit lock, we can just use a simple inc in there without bothering with atomics. > > 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: Yes, that's what I meant by "eliminate the overloading of the serial number zero value" above. This needs fixing before the serial number can be dumped for fast hba drivers. James > 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
