On Tue, Nov 29, 2016 at 08:51:52AM -0800, Paul E. McKenney wrote:
> 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.
I think Petr's replies have addressed that now.
> > > 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. ;-)
Ha :-)
> Much nicer (but completely untested) patch below.
The kernel/rcu/tree.h changes seem intended for another patch?
Otherwise:
Reviewed-by: Josh Poimboeuf <jpoim...@redhat.com>
Also I think this will fix the KASAN warnings reported by Vince, so you
might add:
Reported-by: Vince Weaver <vincent.wea...@maine.edu>
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit d3515ee46e0cff880170e48a05e8f2791b507758
> Author: Paul E. McKenney <paul...@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 <paul...@linux.vnet.ibm.com>
> Cc: Petr Mladek <pmla...@suse.com>
> Cc: Josh Poimboeuf <jpoim...@redhat.com>
> Cc: Peter Zijlstra <pet...@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
>
