On Sat, 31 Jan 2009 08:29:15 +1030 Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > On Friday 30 January 2009 17:00:42 Andrew Morton wrote: > > On Fri, 30 Jan 2009 16:33:53 +1030 Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > > > > > On Thursday 29 January 2009 12:42:05 Andrew Morton wrote: > > > > On Thu, 29 Jan 2009 12:13:32 +1030 Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > > > > > > > > > On Thursday 29 January 2009 06:14:40 Andrew Morton wrote: > > > > > > It's vulnerable to the same deadlock, I think? Suppose we have: > > > > > ... > > > > > > - A calls work_on_cpu() and takes woc_mutex. > > > > > > > > > > > > - Before function_which_takes_L() has started to execute, task B takes L > > > > > > then calls work_on_cpu() and task B blocks on woc_mutex. > > > > > > > > > > > > - Now function_which_takes_L() runs, and blocks on L > > > > > > > > > > Agreed, but now it's a fairly simple case. Both sides have to take lock L, and both have to call work_on_cpu. > > > > > > > > > > Workqueues are more generic and widespread, and an amazing amount of stuff gets called from them. That's why I felt uncomfortable with removing the one known problematic caller. > > > > > > > > > > > > > hm. it's a bit of a timebomb. > > > > > > > > y'know, the original way in which acpi-cpufreq did this is starting to > > > > look attractive. Migrate self to that CPU then just call the dang > > > > function. Slow, but no deadlocks (I think)? > > > > > > Just buggy. What random thread was it mugging? If there's any path where > > > it's not a kthread, what if userspace does the same thing at the same time? > > > We risk running on the wrong cpu, *then* overriding userspace when we restore > > > it. > > > > hm, Ok, not unficable but not pleasant. > > > > > In general these cpumask games are a bad idea. > > > > So we still don't have any non-buggy proposal. > > I disagree about the avoiding-workqueue one being buggy. > I assume you're talking about the patch I looked at a couple of days ago. It's vulnerable to the same deadlock as work_on_cpu() has always been. Just as an example, take a look at allocate_threshold_blocks(). That function way down in the innards of x86 has blotted out large amounts of kernel code, so that code can now not use work_on_cpu(). Anything which happens inside ext3 commit (the entire block layer and all drivers underneath it). Large lumps of networking code. Parts of the page allocator and the VFS which I haven't started to think about yet. > The same logic > applies to any simple callback function. Not! The difference here is the queueing and serialisation which introduces dependencies between unrelated subsystems which happen to use this piece of core infrastructure. -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
