Re: [PATCH 4/4] tracing: Remove RCU work arounds from stack tracer

Paul E. McKenney
2017-09-23T06:07:42Z

On Fri, Sep 22, 2017 at 09:27:39PM -0400, Steven Rostedt wrote:
> On Fri, 22 Sep 2017 15:54:55 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > On Fri, Sep 22, 2017 at 06:15:47PM -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > >
> > > While debugging some RCU issues with the stack tracer, it was discovered
> > > that the problem was much more than with the stack tracer itself, but with
> > > the saving of the stack trace, which could happen from any WARN() as well.
> > > The problem was fixed within kernel_text_address().
> > >
> > > One of the bugs that was discovered was that the stack tracer called
> > > rcu_enter_irq() unconditionally. Paul McKenney said that could cause issues
> > > as well. Instead of adding logic to only call rcu_enter_irq() if RCU is not
> > > watching from within the stack tracer, since the core issue has been fixed
> > > (within save_stack_trace()), we can simply remove all the logic in the stack
> > > tracer that deals with RCU work arounds.
> >
> > I must confess that I am having some difficulty parsing this paragraph,
> > especially the last sentence...
> >
> > Does this capture it?
> >
> > One problem is that the stack tracer called rcu_irq_enter()
> > unconditionally, which is problematic if RCU's last
> > not-watching-to-watching transition was carried out by
> > rcu_nmi_enter. In that case, rcu_irq_enter() actually switches
>
> I thought the rcu_irq_enter() after rcu_nmi_enter() was a separate bug.
> Your original complaint was that I called rcu_irq_enter()
> unconditionally, and wanted me to only call it if RCU wasn't watching.
>
> But, the new code could possibly have this get called after
> rcu_nmi_enter() because we are calling it without in_nmi() being set.

You are correct.

The initial bug was rcu_irq_enter() being invoked recursively due
to tracing of some of its called functions, which caused RCU to not
have been watching when it should have. I then advised you to switch
to rcu_nmi_enter(), unaware that there would be calls to rcu_irq_enter()
between the rcu_nmi_enter() and its matching rcu_nmi_exit(). Which
led us to the second problem, described in my suggested paragraph.

> > RCU back to the not-watching state for this CPU, which results
> > in lockdep splats complaining about rcu_read_lock() being
> > used on an idle (not-watched) CPU. The first patch of this
> > series addressed this problem by having rcu_irq_enter() and
> > rcu_irq_exit() refrain from doing anything when rcu_nmi_enter()
> > caused RCU to start watching this CPU. The third patch in this
> > series caused save_stack_trace() to invoke rcu_nmi_enter() and
> > rcu_nmi_exit() as needed, so this fourth patch now removes the
> > rcu_irq_enter() and rcu_irq_exit() from within the stack tracer.
> >
> > One further question... Can I now remove the rcu_irq_enter_disabled()
> > logic?
>
> After this goes in. Yes. But that doesn't need to be a stable change.

Good, at least a little simplification out of this. ;-)

> > > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > Cc: stable@vger.kernel.org
> > > Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
> > > Suggested-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> >
> > With the hard-to-parse paragraph fixed:
> >
> > Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> Thanks, but we may need to go back and forth a bit to get the change
> log correct.
>
> Remember the first bug. The one that was fixed by changing
> rcu_irq_enter() to rcu_nmi_enter()?
>
> =============================
> WARNING: suspicious RCU usage
> 4.13.0-rc7-test+ #117 Tainted: G W
> -----------------------------
> /work/git/linux-trace.git/arch/x86/kernel/traps.c:305 entry code didn't wake RCU!
>
> other info that might help us debug this:
>
>
> RCU used illegally from idle CPU!
> rcu_scheduler_active = 2, debug_locks = 1
> RCU used illegally from extended quiescent state!
> no locks held by swapper/1/0.
>
> stack backtrace:
> CPU: 1 PID: 0 Comm: swapper/1 Tainted: G W 4.13.0-rc7-test+ #117
> Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
> Call Trace:
> dump_stack+0x86/0xcf
> lockdep_rcu_suspicious+0xc5/0x100
> do_error_trap+0x125/0x130
> ? do_error_trap+0x5/0x130
> ? trace_hardirqs_off_thunk+0x1a/0x1c
> ? do_invalid_op+0x5/0x30
> do_invalid_op+0x20/0x30
> invalid_op+0x1e/0x30
> RIP: 0010:module_assert_mutex_or_preempt+0x34/0x40
> RSP: 0018:ffffc900006abc58 EFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffffffffa000a077 RCX: 0000000000000002
> RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 0000000000000046
> RBP: ffffc900006abc58 R08: ffffc900006abf40 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: ffff8801188d8040 R15: ffffffff81ed5720
> ? 0xffffffffa000a077
> ? module_assert_mutex_or_preempt+0x30/0x40
> __module_address+0x2c/0xf0
> ? 0xffffffffa000a077
> __module_text_address+0x12/0x60
> ? 0xffffffffa000a077
> is_module_text_address+0x1f/0x50
> ? 0xffffffffa000a077
> __kernel_text_address+0x30/0x90
> unwind_get_return_address+0x1f/0x30
> __save_stack_trace+0x83/0xd0
> ? 0xffffffffa000a077
> ? rcu_dynticks_eqs_exit+0x5/0x40
> save_stack_trace+0x1b/0x20
> check_stack+0xf8/0x2f0
> ? rcu_dynticks_eqs_enter+0x30/0x30
> stack_trace_call+0x6e/0x80
> 0xffffffffa000a077
> ? ftrace_graph_caller+0x78/0xa8
> ? rcu_dynticks_eqs_exit+0x5/0x40
> rcu_dynticks_eqs_exit+0x5/0x40
> rcu_idle_exit+0xdf/0xf0
> ? rcu_dynticks_eqs_exit+0x5/0x40
> ? rcu_idle_exit+0xdf/0xf0
> do_idle+0x128/0x200
> cpu_startup_entry+0x1d/0x20
> start_secondary+0x108/0x130
> secondary_startup_64+0x9f/0x9f
>
> This was caused by just using rcu_irq_enter(). Not sure if this will
> still be an issue or not. But because we now add an rcu_nmi_enter()
> without being in_nmi(), we probably should do this. This code doesn't
> run if in_nmi() is true, but it could run from the stack trace dump
> itself, and that now calls rcu_nmi_enter().

OK, how about the following?

It turns out that functions called from rcu_irq_enter() can
be subject to various kinds of tracing, which can result in
rcu_irq_enter() being invoked recursively. This recursion
causes RCU to not have been watching when it should have,
resulting in lockdep-RCU splats. Switching from rcu_irq_enter()
to rcu_nmi_enter() still resulted in failures because of calls
to rcu_irq_enter() between the rcu_nmi_enter() and its matching
rcu_nmi_exit(). Such calls again cause RCU to not be watching
when it should have been.

In particular, the stack tracer called rcu_irq_enter()
unconditionally, which is problematic when RCU's last
not-watching-to-watching transition was carried out by
rcu_nmi_enter(), as will be the case when tracing uses
rcu_nmi_enter() to cause RCU to start watching the current CPU.
In that case, rcu_irq_enter() actually switches RCU back to
the not-watching state for this CPU, which results in lockdep
splats complaining about rcu_read_lock() being used on an idle
(not-watched) CPU. The first patch of this series addressed
this problem by having rcu_irq_enter() and rcu_irq_exit()
refrain from doing anything when rcu_nmi_enter() caused RCU to
start watching this CPU. The third patch in this series caused
save_stack_trace() to invoke rcu_nmi_enter() and rcu_nmi_exit()
as needed, so this fourth patch now removes the rcu_irq_enter()
and rcu_irq_exit() from within the stack tracer.

> Actually, thinking about this more, this doesn't need to go in stable.
> As recursive rcu_irq_enter() calls should not hurt, and you now allow
> rcu_irq_enter() to be called even after a rcu_nmi_enter() right?

Yes, it is now the case that rcu_irq_enter() can be called even after
an rcu_nmi_enter() exited idle, because rcu_irq_enter() now checks for
this and takes an early exit if so.

But what is it about older kernels prevents the tracing-induced recursive
calls to rcu_irq_enter()?

Thanx, Paul