linux-kernel - Re: [PATCH 2/2] x86/tbl/trace: Do not trace on CPU that is offline

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Message-ID: <CA+icZUUT-tmTSoY9Twj+GDp+3j0etMis=cLhX+SOnd7bgGjRbg@mail.gmail.com>
Date:	Sun, 8 Feb 2015 00:01:07 +0100
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Dave Hansen <dave@...1.net>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-next <linux-next@...r.kernel.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Kristen Carlson Accardi <kristen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] x86/tbl/trace: Do not trace on CPU that is offline

On Sat, Feb 7, 2015 at 11:14 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Sat, Feb 07, 2015 at 04:52:05PM -0500, Steven Rostedt wrote:
>> On Sat, 7 Feb 2015 12:09:48 -0800
>> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
>>
>> >    The tag sequence has the meaning of:
>> >      git cherry-pick a1f84a3
>> >      git cherry-pick 1b9508f
>> >      git cherry-pick fd21073
>> >      git cherry-pick <this commit>
>> >
>> > Does that do what you need?
>>
>> Note, for this case it really doesn't apply, because one patch does not
>> depend on the other.
>>
>> The real bug is that a tracepoint can be called when RCU is not
>> watching (cpu is offline). That bug was introduced in 3.17 and is fixed
>> by patch 2 with the conditional trace event.
>>
>> When that bug was fixed, it showed that another bug exists. That is
>> that lockdep should not complain if the conditional prevents the bad
>> RCU from happening, and this bug was introduced in 3.18. This was fixed
>> by the first patch.
>>
>> They really are two entirely separate bugs, it just happens that the
>> test case Sedat had happened to trigger both of them. This is why I
>> really don't see why the two need to reference each other.
>>
>> I'm also going to modify patch 1 to not mention porting the other
>> commit (that patch 1 fixes) to 3.17 (from 3.18), as that other commit is
>> just a debugging tool and not something that satisfies being
>> backported, and the patch that fixes it shouldn't be backported to 3.17
>> either, only to 3.18.
>
> Thank you for the explanation!  I guess I needed to have kept a scorecard
> on this one.  ;-)
>

My misunderstanding and thanks again for the clarification.
As Steve pointed out, /me needs both patches to fix my broken testcase.
My wish was not to forget affected (previous) linux-stable releases
when dealing with the issue.

- Sedat -
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