Re: [PATCH 4.9 094/111] rcu: Fix grace-period hangs due to race with CPU offline

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, Sep 24, 2018 at 01:53:01PM +0200, Greg Kroah-Hartman wrote:
> 4.9-stable review patch.  If anyone has any objections, please let me know.

And here as well...

This should not be needed in 4.18 because of a number of crude but
effective grace-period forward-progress failsafes.  I have not tested
it in isolation.  It looks harmless enough, but all testing has been in
conjunction with a large number of preceding patches.

I therefore strongly recommend against backporting this one.

							Thanx, Paul

> ------------------
> 
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> [ Upstream commit 1e64b15a4b102e1cd059d4d798b7a78f93341333 ]
> 
> Without special fail-safe quiescent-state-propagation checks, grace-period
> hangs can result from the following scenario:
> 
> 1.	CPU 1 goes offline.
> 
> 2.	Because CPU 1 is the only CPU in the system blocking the current
> 	grace period, the grace period ends as soon as
> 	rcu_cleanup_dying_idle_cpu()'s call to rcu_report_qs_rnp()
> 	returns.
> 
> 3.	At this point, the leaf rcu_node structure's ->lock is no longer
> 	held: rcu_report_qs_rnp() has released it, as it must in order
> 	to awaken the RCU grace-period kthread.
> 
> 4.	At this point, that same leaf rcu_node structure's ->qsmaskinitnext
> 	field still records CPU 1 as being online.  This is absolutely
> 	necessary because the scheduler uses RCU (in this case on the
> 	wake-up path while awakening RCU's grace-period kthread), and
> 	->qsmaskinitnext contains RCU's idea as to which CPUs are online.
> 	Therefore, invoking rcu_report_qs_rnp() after clearing CPU 1's
> 	bit from ->qsmaskinitnext would result in a lockdep-RCU splat
> 	due to RCU being used from an offline CPU.
> 
> 5.	RCU's grace-period kthread awakens, sees that the old grace period
> 	has completed and that a new one is needed.  It therefore starts
> 	a new grace period, but because CPU 1's leaf rcu_node structure's
> 	->qsmaskinitnext field still shows CPU 1 as being online, this new
> 	grace period is initialized to wait for a quiescent state from the
> 	now-offline CPU 1.
> 
> 6.	Without the fail-safe force-quiescent-state checks, there would
> 	be no quiescent state from the now-offline CPU 1, which would
> 	eventually result in RCU CPU stall warnings and memory exhaustion.
> 
> It would be good to get rid of the special fail-safe quiescent-state
> propagation checks, and thus it would be good to fix things so that
> the above scenario cannot happen.  This commit therefore adds a new
> ->ofl_lock to the rcu_state structure.  This lock is held by rcu_gp_init()
> across the applying of buffered online and offline operations to the
> rcu_node tree, and it is also held by rcu_cleanup_dying_idle_cpu()
> when buffering a new offline operation.  This prevents rcu_gp_init()
> from acquiring the leaf rcu_node structure's lock during the interval
> between when rcu_cleanup_dying_idle_cpu() invokes rcu_report_qs_rnp(),
> which releases ->lock and the re-acquisition of that same lock.
> This in turn prevents the failure scenario outlined above, and will
> hopefully eventually allow removal of the offline-CPU checks from the
> force-quiescent-state code path.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  kernel/rcu/tree.c |    6 ++++++
>  kernel/rcu/tree.h |    4 ++++
>  2 files changed, 10 insertions(+)
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -102,6 +102,7 @@ struct rcu_state sname##_state = { \
>  	.abbr = sabbr, \
>  	.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
>  	.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
> +	.ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \
>  }
> 
>  RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
> @@ -1966,11 +1967,13 @@ static bool rcu_gp_init(struct rcu_state
>  	 */
>  	rcu_for_each_leaf_node(rsp, rnp) {
>  		rcu_gp_slow(rsp, gp_preinit_delay);
> +		spin_lock(&rsp->ofl_lock);
>  		raw_spin_lock_irq_rcu_node(rnp);
>  		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
>  		    !rnp->wait_blkd_tasks) {
>  			/* Nothing to do on this leaf rcu_node structure. */
>  			raw_spin_unlock_irq_rcu_node(rnp);
> +			spin_unlock(&rsp->ofl_lock);
>  			continue;
>  		}
> 
> @@ -2005,6 +2008,7 @@ static bool rcu_gp_init(struct rcu_state
>  		}
> 
>  		raw_spin_unlock_irq_rcu_node(rnp);
> +		spin_unlock(&rsp->ofl_lock);
>  	}
> 
>  	/*
> @@ -3915,9 +3919,11 @@ static void rcu_cleanup_dying_idle_cpu(i
> 
>  	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
>  	mask = rdp->grpmask;
> +	spin_lock(&rsp->ofl_lock);
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
>  	rnp->qsmaskinitnext &= ~mask;
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> +	spin_unlock(&rsp->ofl_lock);
>  }
> 
>  void rcu_report_dead(unsigned int cpu)
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -550,6 +550,10 @@ struct rcu_state {
>  	const char *name;			/* Name of structure. */
>  	char abbr;				/* Abbreviated name. */
>  	struct list_head flavors;		/* List of RCU flavors. */
> +
> +	spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
> +						/* Synchronize offline with */
> +						/*  GP pre-initialization. */
>  };
> 
>  /* Values for rcu_state structure's gp_flags field. */
> 
>