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

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Message-ID: <20150207165205.3d9d5cde@gandalf.local.home>
Date:	Sat, 7 Feb 2015 16:52:05 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Sedat Dilek <sedat.dilek@...il.com>,
	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, 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.

-- Steve
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