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

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Message-ID: <20150207221407.GF5418@linux.vnet.ibm.com>
Date:	Sat, 7 Feb 2015 14:14:07 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, 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.  ;-)

							Thanx, Paul

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