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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-07-05 13:30:21 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-07-27 15:53:04 -0700
commit35732cf9dd38b1efb0f2f22c91c61b51337d1ac3 (patch)
tree891e471ae7b9834a969dcac165722757dd9a2ca8 /kernel/rcu/srcutree.c
parent0d8a1e831e21d955af68f4ae5658b58b7ec25557 (diff)
srcu: Provide ordering for CPU not involved in grace period
Tree RCU guarantees that every online CPU has a memory barrier between any given grace period and any of that CPU's RCU read-side sections that must be ordered against that grace period. Since RCU doesn't always know where read-side critical sections are, the actual implementation guarantees order against prior and subsequent non-idle non-offline code, whether in an RCU read-side critical section or not. As a result, there does not need to be a memory barrier at the end of synchronize_rcu() and friends because the ordering internal to the grace period has ordered every CPU's post-grace-period execution against each CPU's pre-grace-period execution, again for all non-idle online CPUs. In contrast, SRCU can have non-idle online CPUs that are completely uninvolved in a given SRCU grace period, for example, a CPU that never runs any SRCU read-side critical sections and took no part in the grace-period processing. It is in theory possible for a given synchronize_srcu()'s wakeup to be delivered to a CPU that was completely uninvolved in the prior SRCU grace period, which could mean that the code following that synchronize_srcu() would end up being unordered with respect to both the grace period and any pre-existing SRCU read-side critical sections. This commit therefore adds an smp_mb() to the end of __synchronize_srcu(), which prevents this scenario from occurring. Reported-by: Lance Roy <ldr709@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Lance Roy <ldr709@gmail.com> Cc: <stable@vger.kernel.org> # 4.12.x
Diffstat (limited to 'kernel/rcu/srcutree.c')
-rw-r--r--kernel/rcu/srcutree.c9
1 files changed, 9 insertions, 0 deletions
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 94bd6ed43ea3..c1c0ee3cce3b 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -897,6 +897,15 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool do_norm)
__call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
wait_for_completion(&rcu.completion);
destroy_rcu_head_on_stack(&rcu.head);
+
+ /*
+ * Make sure that later code is ordered after the SRCU grace
+ * period. This pairs with the raw_spin_lock_irq_rcu_node()
+ * in srcu_invoke_callbacks(). Unlike Tree RCU, this is needed
+ * because the current CPU might have been totally uninvolved with
+ * (and thus unordered against) that grace period.
+ */
+ smp_mb();
}
/**