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. 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? 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
