On Thu, Oct 25, 2018 at 10:18:10AM -0400, Sasha Levin wrote:
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.
Given that this produced a splat that someone (you, in fact) actually encountered, no objection to it going to -stable.
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:
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.
Task A running on CPU 0 enters a preemptible RCU read-side critical section.
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.
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.
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().
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.
Because rcu_preempt_data.passed_quiesce is now true, rcu_preempt_qs() does nothing, and simply returns.
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).
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;-- 2.17.1