Re: suspicious RCU usage (perf)

[Dave Jones <davej@redhat.com>]

On Mon, Aug 26, 2013 at 01:30:41PM -0400, Steven Rostedt wrote:

 > > On Mon, Aug 26, 2013 at 10:58:38AM -0400, Dave Jones wrote:
 > > > Another day, another rcu backtrace..
 > > > This says rc6, but it's pretty darn close to rc7, I think it was running
 > > > a build from Friday.
 > > 
 > > Could you please send your .config?  Also, were you running ftrace,
 > > perf, RCU event tracing, or what?
 > > 
 > > Looks like you are running ftrace, but I though Steven had set that
 > > up so that it could be called from an extended quiescent state.
 > > 
 > 
 > I know exactly what the issue is. Yes ftrace is safe to call even from
 > these extended quiescent states, the problem is that ftrace is also the
 > infrastructure of other users, where some of those users are not safe.
 > Namely, perf.
 > 
 > Right now perf is not safe to trace all functions, as some of those
 > functions have this issue. I was going to add something like:
 > 
 > FTRACE_NON_SAFE(rcu_eqs_enter);
 > 
 > where it will record locations that are not safe for all users, such
 > that unless a function registers to ftrace with a flag of
 > "FTRACE_FL_NON_SAFE_OK", anything that is on the non safe list (from
 > the macro) will not be traced.
 > 
 > Now, how urgent is this fix? perf can only trace functions as root, and
 > there's no reason for perf to be tracing all functions at the moment.
 > But yes, a root user could run that and get this warning. Because I was
 > going to implement this for 3.12 and not for this release.

This was triggered as a regular user fwiw.
I had not been running perf, or any other tracing. It was just left
fuzzing over the weekend with no interaction at all.

	Dave