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SubjectRe: [PATCH 4.18 207/235] rcu: Fix grace-period hangs due to race with CPU offline
On Mon, Sep 24, 2018 at 01:53:13PM +0200, Greg Kroah-Hartman wrote:
> 4.18-stable review patch. If anyone has any objections, please let me know.

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);
> @@ -1925,11 +1926,13 @@ 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;
> }
>
> @@ -1964,6 +1967,7 @@ static bool rcu_gp_init(struct rcu_state
> }
>
> raw_spin_unlock_irq_rcu_node(rnp);
> + spin_unlock(&rsp->ofl_lock);
> }
>
> /*
> @@ -3725,9 +3729,11 @@ 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);
> }
>
> /*
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -384,6 +384,10 @@ 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. */
>
>

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