On 08/06/2013 05:46 PM, scame...@beardog.cce.hp.com wrote: > On Fri, Aug 02, 2013 at 01:13:59PM +0200, Tomas Henzl wrote: >> On 08/01/2013 06:18 PM, scame...@beardog.cce.hp.com wrote: >>> On Thu, Aug 01, 2013 at 05:39:36PM +0200, Tomas Henzl wrote: >>>> On 08/01/2013 05:19 PM, scame...@beardog.cce.hp.com wrote: >>> [...] >>> >>>>>> Btw. on line 1284 - isn't it similar to patch 2/3 ? >>> ^^^ Oh, missed this the first time around, the sop driver uses the >>> make_request_fn() >>> interface, and it's not a stacked driver either, so there is no limit to >>> the number >>> of bios the block layer can stuff in -- make_request_fn must succeed. >>> If we get full we just chain them together using pointers already in the >>> struct >>> bio for that purpose, so storing them in the driver requires no memory >>> allocation >>> on the driver's part. So while it's somewhat similar, we already have to >>> handle >>> the case of the block layer stuffing infinite bios into the driver, so >>> getting >>> full is not terribly out of the ordinary in that driver. >> OK. >> >>> That being said, I'm poking around other bits of code lying around here >>> looking for similar problems, so thanks again for that one. >>> >>>>> find_first_zero_bit is not atomic, but the test_and_set_bit, which is what >>>>> counts, is atomic. That find_first_zero_bit is not atomic confused me >>>>> about >>>>> this code for a long time, and is why the spin lock was there in the first >>>>> place. But if there's a race on the find_first_zero_bit and it returns >>>>> the >>>>> same bit to multiple concurrent threads, only one thread will win the >>>>> test_and_set_bit, and the other threads will go back around the loop to >>>>> try >>>>> again, and get a different bit. >>>> Yes. >>>> But, let's expect just one zero bit at the end of the list. The >>>> find_first_zero_bit(ffzb) >>>> starts now, thread+1 zeroes a new bit at the beginning, ffzb continues, >>>> thread+2 takes the zero bit at the end. The result it that ffzb hasn't >>>> found a zero bit >>>> even though that at every moment that bit was there.Ffter that the >>>> function returns -EBUSY. >>>> rc = (u16) find_first_zero_bit(qinfo->request_bits, qinfo->qdepth); >>>> if (rc >= qinfo->qdepth-1) >>>> return (u16) -EBUSY; >>>> Still, I think that this is almost impossible, and if it should happen >>>> a requeue is not so bad. >>> Oh, wow. Didn't think of that. Hmm, technically no guarantee that >>> any given thread would ever get a bit, if all the other threads keep >>> snatching them away just ahead of an unlucky thread. >>> >>> Could we, instead of giving up, go back around and try again on the theory >>> that some bits should be free in there someplace and the thread shouldn't >>> be infinitely unlucky? >> In theory that gives you also no guarantee, it's likely that for a guarantee >> some >> kind of locking is needed, the spinlock, which already is there, gives you >> that. >> Otoh, a very high likelihood is probably enough and give better overall >> throughput, >> maybe some statistics/testing is needed? I don't know how much faster is it >> without the spinlock. > On thinking about this a bit more, it would be a shame if we closed the > hole allowing the "cmd_alloc returned NULL" message (the scsi_done() > cmd_free() > race) and then immediately opened up another different hole that permitted the > same problem to occur. > > So to be safe, I think we should go with your patch as is -- leave > the spin lock, but get rid of the unnecessary loop.
Thank you. I was going to write something similar - that we could use my patch as a temporary solution until a better lockless is found. tomash > > -- steve > > -- > 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 -- 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
