[PATCH AUTOSEL 3.18 55/98] rcu: Clear need_qs flag to prevent splat

Sasha Levin
2018-10-25T14:18:10Z

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

[ Upstream commit c0135d07b013fa8f7ba9ec91b4369c372e6a28cb ]

If the scheduling-clock interrupt sets the current tasks need_qs flag,
but if the current CPU passes through a quiescent state in the meantime,
then rcu_preempt_qs() will fail to clear the need_qs flag, which can fool
RCU into thinking that additional rcu_read_unlock_special() processing
is needed. This commit therefore clears the need_qs flag before checking
for additional processing.

For this problem to occur, we need rcu_preempt_data.passed_quiesce equal
to true and current->rcu_read_unlock_special.b.need_qs also equal to true.
This condition can occur as follows:

1. CPU 0 is aware of the current preemptible RCU grace period,
but has not yet passed through a quiescent state. Among other
things, this means that rcu_preempt_data.passed_quiesce is false.

2. Task A running on CPU 0 enters a preemptible RCU read-side
critical section.

3. CPU 0 takes a scheduling-clock interrupt, which notices the
RCU read-side critical section and the need for a quiescent state,
and thus sets current->rcu_read_unlock_special.b.need_qs to true.

4. Task A is preempted, enters the scheduler, eventually invoking
rcu_preempt_note_context_switch() which in turn invokes
rcu_preempt_qs().

Because rcu_preempt_data.passed_quiesce is false,
control enters the body of the "if" statement, which sets
rcu_preempt_data.passed_quiesce to true.

5. At this point, CPU 0 takes an interrupt. The interrupt
handler contains an RCU read-side critical section, and
the rcu_read_unlock() notes that current->rcu_read_unlock_special
is nonzero, and thus invokes rcu_read_unlock_special().

6. Once in rcu_read_unlock_special(), the fact that
current->rcu_read_unlock_special.b.need_qs is true becomes
apparent, so rcu_read_unlock_special() invokes rcu_preempt_qs().
Recursively, given that we interrupted out of that same
function in the preceding step.

7. Because rcu_preempt_data.passed_quiesce is now true,
rcu_preempt_qs() does nothing, and simply returns.

8. Upon return to rcu_read_unlock_special(), it is noted that
current->rcu_read_unlock_special is still nonzero (because
the interrupted rcu_preempt_qs() had not yet gotten around
to clearing current->rcu_read_unlock_special.b.need_qs).

9. Execution proceeds to the WARN_ON_ONCE(), which notes that
we are in an interrupt handler and thus duly splats.

The solution, as noted above, is to make rcu_read_unlock_special()
clear out current->rcu_read_unlock_special.b.need_qs after calling
rcu_preempt_qs(). The interrupted rcu_preempt_qs() will clear it again,
but this is harmless. The worst that happens is that we clobber another
attempt to set this field, but this is not a problem because we just
got done reporting a quiescent state.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix embarrassing build bug noted by Sasha Levin. ]
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/tree_plugin.h | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c1d7f27bd38f..c038831bfa57 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -328,6 +328,7 @@ void rcu_read_unlock_special(struct task_struct *t)
special = t->rcu_read_unlock_special;
if (special.b.need_qs) {
rcu_preempt_qs();
+ t->rcu_read_unlock_special.b.need_qs = false;
if (!t->rcu_read_unlock_special.s) {
local_irq_restore(flags);
return;
--
2.17.1