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:
CPU 1 goes offline.
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.
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.
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.
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.
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);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. */