From 1e64b15a4b102e1cd059d4d798b7a78f93341333 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 25 May 2018 19:23:09 -0700 Subject: [PATCH] rcu: Fix grace-period hangs due to race with CPU offline 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 --- kernel/rcu/tree.c | 6 ++++++ kernel/rcu/tree.h | 4 ++++ 2 files changed, 10 insertions(+) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 7f872721c54e..50e4f7ebf8cf 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -101,6 +101,7 @@ 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); @@ -1900,11 +1901,13 @@ static bool rcu_gp_init(struct rcu_state *rsp) */ 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; } @@ -1940,6 +1943,7 @@ static bool rcu_gp_init(struct rcu_state *rsp) } raw_spin_unlock_irq_rcu_node(rnp); + spin_unlock(&rsp->ofl_lock); } /* @@ -3749,6 +3753,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp) /* 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. */ if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */ /* Report quiescent state -before- changing ->qsmaskinitnext! */ @@ -3757,6 +3762,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp) } rnp->qsmaskinitnext &= ~mask; raw_spin_unlock_irqrestore_rcu_node(rnp, flags); + spin_unlock(&rsp->ofl_lock); } /* diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 3def94fc9c74..6683da6e4ecc 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -363,6 +363,10 @@ 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. */