On Feb 28 James Bottomley wrote: > On Tue, 2012-02-28 at 15:32 +0100, Stefan Richter wrote: > > Commit 2a48fc0ab242 "block: autoconvert trivial BKL users to private > > mutex" and other commits at the time mechanically swapped BKL for > > per-driver global mutexes. If the sr driver is any indication, these > > replacements have still not been checked by anybody for their > > necessessity, removed where possible, or the sections they serialize > > reduced to a necessary minimum. > > > > The sr_mutex in particular very noticably degraded performance of > > CD-DA ripping with multiple drives in parallel. When several > > instances of "grip" are used with two or more drives, their GUIs > > became laggier, as did the KDE file manager GUI, and drive utilization > > was reduced. (During ripping, drive lights flicker instead of staying > > on most of the time.) IOW time to rip a stack of CDs was increased. > > I didn't measure this but it is highly noticeable. > > > > On the other hand, I don't see what state sr_mutex would protect. > > So I removed it entirely and that works fine for me. > > > I'm afraid you can't do that: The problem is that we have an entangled > set of reference counts that need to be taken and released atomically. > If we don't surround them with a mutex you get undefined results from > racing last release with new acquire. You can see this usage in sd.c. While I do remove sr_mutex aroud scsi_cd_get/put() calls, these ones internally use another lock: sr_ref_mutex. Always did, still do, since neither Arnd's mechanical BKL pushdown and BKL-to-mutex conversions patches nor my patch changed that. This sr_ref_mutex also protects sr's reference counting outside of the three block_device_operations methods which I changed. I suppose I could have mentioned right away in the changelog that the sr driver's own reference counting serialization remains in place, via that other mutex. > The sr.c use case looks like bd_mutex would mediate ... but that's > because it doesn't use driver shutdown and has no power management > functions ... I think I have vague memories that someone is working on > pm for cdroms? > > I don't think the mutex needs to be on the ioctls, though, which is > what's causing your performance problems, right? I guess sr_block_open/release are less of an issue; after all they are still partly serialized across all sr devices (the sections which are under the mentioned sr_ref_mutex protection). -- Stefan Richter -=====-===-- --=- ===-- http://arcgraph.de/sr/ -- 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
