Re: [tip: perf/core] perf/core: Fix endless multiplex timer

[Robin Murphy]

On 2020-08-06 19:53, Greg KH wrote:
> On Thu, Aug 06, 2020 at 07:11:24PM +0100, Robin Murphy wrote:
> > On 2020-03-20 12:58, tip-bot2 for Peter Zijlstra wrote:
> > > The following commit has been merged into the perf/core branch of tip:
> > >
> > > Commit-ID:     90c91dfb86d0ff545bd329d3ddd72c147e2ae198
> > > Gitweb:        https://git.kernel.org/tip/90c91dfb86d0ff545bd329d3ddd72c147e2ae198
> > > Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > AuthorDate:    Thu, 05 Mar 2020 13:38:51 +01:00
> > > Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > CommitterDate: Fri, 20 Mar 2020 13:06:22 +01:00
> > >
> > > perf/core: Fix endless multiplex timer
> > >
> > > Kan and Andi reported that we fail to kill rotation when the flexible
> > > events go empty, but the context does not. XXX moar
> > >
> > > Fixes: fd7d55172d1e ("perf/cgroups: Don't rotate events for cgroups unnecessarily")
> >
> > Can this patch (commit 90c91dfb86d0 ("perf/core: Fix endless multiplex
> > timer") upstream) be applied to stable please? For PMU drivers built as
> > modules, the bug can actually kill the system, since the runaway hrtimer
> > loop keeps calling pmu->{enable,disable} after all the events have been
> > closed and dropped their references to pmu->module. Thus legitimately
> > unloading the module once things have got into this state quickly results in
> > a crash when those callbacks disappear.
> >
> > (FWIW I spent about two days fighting with this while testing a new driver
> > as a module against the 5.3 kernel installed on someone else's machine,
> > assuming it was a bug in my code...)
>
> What exactly kernel(s) do you wish for it to be applied to?  It's
> already in the latest stable releases of 5.7.y.

Sorry, I implicitly meant 5.4.y there - the buggy commit was merged in 
5.3, the fix in 5.7, so I think that's the only "stable" branch in 
between that warrants explicit action. Apologies if I'm getting the 
terminology wrong.

Cheers,
Robin.