Date: Tue, 29 Nov 2016 08:51:52 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Vince Weaver <vincent.weaver@maine.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        "dvyukov@google.com" <dvyukov@google.com>, pmladek@suse.com
Subject: Re: perf: fuzzer BUG: KASAN: stack-out-of-bounds in __unwind_start
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On Tue, Nov 29, 2016 at 09:09:17AM -0600, Josh Poimboeuf wrote:
> On Tue, Nov 29, 2016 at 06:07:34AM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 29, 2016 at 10:16:50AM +0100, Peter Zijlstra wrote:
> > > On Mon, Nov 28, 2016 at 11:52:41PM -0600, Josh Poimboeuf wrote:
> > > > > We used to do that, but the resulting NMIs were problematic on some
> > > > > platforms.  Perhaps things have gotten better?
> > > > 
> > > > Did a little digging on git blame and found the following commit (which
> > > > seems to be the cause of the KASAN warning and missing stack dump):
> > > > 
> > > >   bc1dce514e9b ("rcu: Don't use NMIs to dump other CPUs' stacks")
> > > > 
> > > > I presume this commit is still needed because of the NMI printk deadlock
> > > > issues which were discussed at Kernel Summit.  I guess those issues need
> > > > to be sorted out before the above commit can be reverted.
> > > 
> > > so printk should more or less work from NMI, esp. after:
> > > 
> > >   42a0bb3f7138 ("printk/nmi: generic solution for safe printk in NMI")
> > 
> > And of course bc1dce514e9b doesn't revert cleanly, but see hand reversion
> > below.  Also, 42a0bb3f7138's commit log calls out MN10300 and Xtensa as
> > needing more work.  Has that happened?
> 
> Petr M, any idea?

My Not-yet-signed-off-by is due to this concern, FWIW.

> > But I really like the fact that RCU CPU stall warnings dump only those
> > stacks that are likely to be involved, and the patch below goes back
> > to dumping everyone.  Shouldn't be that hard to fix, though...
> 
> There's a new trigger_single_cpu_backtrace() function which can be used
> for that.

Even better, thank you!  Killed an hour or so of coding, but I must
confess that it was a mercy killing.  ;-)

Much nicer (but completely untested) patch below.

							Thanx, Paul

------------------------------------------------------------------------

commit d3515ee46e0cff880170e48a05e8f2791b507758
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Tue Nov 29 05:49:06 2016 -0800

    rcu: Once again use NMI-based stack traces in stall warnings
    
    This commit is for all intents and purposes a revert of bc1dce514e9b
    ("rcu: Don't use NMIs to dump other CPUs' stacks").  The reason to suppose
    that this can now safely be reverted is the presence of 42a0bb3f7138
    ("printk/nmi: generic solution for safe printk in NMI"), which is said
    to have made NMI-based stack dumps safe.
    
    However, this reversion keeps one nice property of bc1dce514e9b
    ("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
    only those CPUs blocking the grace period are dumped.  The new
    trigger_single_cpu_backtrace() is used to make this happen, as
    suggested by Josh Poimboeuf.
    
    Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Petr Mladek <pmladek@suse.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 91a68e4e6671..ba0e4825be9d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1396,7 +1396,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
 }
 
 /*
- * Dump stacks of all tasks running on stalled CPUs.
+ * Dump stacks of all tasks running on stalled CPUs.  First try using
+ * NMIs, but fall back to manual remote stack tracing on architectures
+ * that don't support NMI-based stack dumps.  The NMI-triggered stack
+ * traces are more accurate because they are printed by the target CPU.
  */
 static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 {
@@ -1406,11 +1409,10 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 
 	rcu_for_each_leaf_node(rsp, rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-		if (rnp->qsmask != 0) {
-			for_each_leaf_node_possible_cpu(rnp, cpu)
-				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+		for_each_leaf_node_possible_cpu(rnp, cpu)
+			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+				if (!trigger_single_cpu_backtrace(cpu))
 					dump_cpu_task(cpu);
-		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 7dcdd59d894c..c0a4bf8f1ed0 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -691,18 +691,6 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
 #endif /* #ifdef CONFIG_RCU_TRACE */
 
 /*
- * Place this after a lock-acquisition primitive to guarantee that
- * an UNLOCK+LOCK pair act as a full barrier.  This guarantee applies
- * if the UNLOCK and LOCK are executed by the same CPU or if the
- * UNLOCK and LOCK operate on the same lock variable.
- */
-#ifdef CONFIG_PPC
-#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
-#else /* #ifdef CONFIG_PPC */
-#define smp_mb__after_unlock_lock()	do { } while (0)
-#endif /* #else #ifdef CONFIG_PPC */
-
-/*
  * Wrappers for the rcu_node::lock acquire and release.
  *
  * Because the rcu_nodes form a tree, the tree traversal locking will observe