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

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Message-ID: <3a89fe47-e143-885a-d116-d3805e0712a0@arm.com>
Date:   Thu, 6 Aug 2020 21:40:17 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        x86 <x86@...nel.org>
Subject: Re: [tip: perf/core] perf/core: Fix endless multiplex timer

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@...radead.org>
>>> AuthorDate:    Thu, 05 Mar 2020 13:38:51 +01:00
>>> Committer:     Peter Zijlstra <peterz@...radead.org>
>>> 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.