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

[Rusty Russell]

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.  The same logic
applies to any simple callback function.

Rusty.
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