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SubjectRe: [PATCH 2/3] work_on_cpu: Use our own workqueue.

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 30 Jan 2009 16:33:53 +1030 Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> > On Thursday 29 January 2009 12:42:05 Andrew Morton wrote:
> > > On Thu, 29 Jan 2009 12:13:32 +1030 Rusty Russell <rusty@rustcorp.com.au> 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.

Current upstream code is not pretty (due to the extra workqueue) but not
buggy either. You'd be right to point out that it is easy to insert a bug
into it and thus it's not pleasant (more of a workaround than a real fix)
but if it's outright buggy then please talk up.

Ingo


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