Commit-ID: 35732cf9dd38b1efb0f2f22c91c61b51337d1ac3 Gitweb: http://git.kernel.org/tip/35732cf9dd38b1efb0f2f22c91c61b51337d1ac3 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Wed, 5 Jul 2017 13:30:21 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Thu, 27 Jul 2017 15:53:04 -0700 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@xxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Acked-by: Lance Roy <ldr709@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> # 4.12.x --- kernel/rcu/srcutree.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 94bd6ed..c1c0ee3 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(); } /** -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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