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SubjectRe: [RFC PATCH] x86, mce: change the mce notifier to 'blocking' from 'atomic'
On Wed, Apr 12, 2017 at 11:50:45PM +0200, Thomas Gleixner wrote:
> On Wed, 12 Apr 2017, Borislav Petkov wrote:
> > On Wed, Apr 12, 2017 at 08:27:05PM +0000, Verma, Vishal L wrote:
> > > But isn't the atomic notifier call chain always called in atomic
> > > context?
> >
> > No, it isn't. We're calling it in normal process context in
> > mce_gen_pool_process() too.
> >
> > So this early exit will avoid any sleeping in atomic context. And since
> > there's nothing you can do about the errors reported in atomic context,
> > we can actually use that fact.
> No, you can't.
> CONFIG_RCU_PREEMPT=n + CONFIG_PREEMPT_COUNT will disable preemption from
> within __atomic_notifier_call_chain() via rcu_read_lock(). Ergo you wont
> ever enter the handler.
> The behaviour in the RCU code is inconsistent. CONFIG_RCU_PREEMPT=y does
> obviouly not disable preemption, but it should still trigger the
> might_sleep() check when a blocking function is called from within a rcu
> read side critical section.

Maybe something like the (untested) patch below. Please note that this
would need some help to work correctly in -rt. This applies only against
-rcu tip, but in that case you can just get it directly from -rcu.

Thanx, Paul


commit 122acec803471468d8a453d08219ca2fc94f5556
Author: Paul E. McKenney <>
Date: Wed Apr 12 15:29:14 2017 -0700

rcu: Complain if blocking in preemptible RCU read-side critical section

Although preemptible RCU allows its read-side critical sections to be
preempted, general blocking is forbidden. The reason for this is that
excessive preemption times can be handled by CONFIG_RCU_BOOST=y, but a
voluntarily blocked task doesn't care how high you boost its priority.
Because preemptible RCU is a global mechanism, one ill-behaved reader
hurts everyone. Hence the prohibition against general blocking in
RCU-preempt read-side critical sections. Preemption yes, blocking no.

This commit enforces this prohibition.

There is a special exception for the -rt patchset (which they kindly
volunteered to implement): It is OK to block (as opposed to merely being
preempted) within an RCU-preempt read-side critical section, but only if
the blocking is subject to priority inheritance. This exception permits
CONFIG_RCU_BOOST=y to get -rt RCU readers out of trouble.

Why doesn't this exception also apply to mainline's rt_mutex? Because
of the possibility that someone does general blocking while holding
an rt_mutex. Yes, the priority boosting will affect the rt_mutex,
but it won't help with the task doing general blocking while holding
that rt_mutex.

Reported-by: Thomas Gleixner <>
Signed-off-by: Paul E. McKenney <>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d013bd4767a7..abc09d368b3a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -465,7 +465,7 @@ void rcu_note_context_switch(bool preempt)
barrier(); /* Avoid RCU read-side critical sections leaking down. */
trace_rcu_utilization(TPS("Start context switch"));
- rcu_preempt_note_context_switch();
+ rcu_preempt_note_context_switch(preempt);
/* Load rcu_urgent_qs before other flags. */
if (!smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs)))
goto out;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 0e598ab08fea..781fe684f230 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -476,7 +476,7 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);

/* Forward declarations for rcutree_plugin.h */
static void rcu_bootup_announce(void);
-static void rcu_preempt_note_context_switch(void);
+static void rcu_preempt_note_context_switch(bool preempt);
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6d8f7f82259c..67a90158f32e 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -286,12 +286,13 @@ static void rcu_preempt_qs(void)
* Caller must disable interrupts.
-static void rcu_preempt_note_context_switch(void)
+static void rcu_preempt_note_context_switch(bool preempt)
struct task_struct *t = current;
struct rcu_data *rdp;
struct rcu_node *rnp;

+ WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
if (t->rcu_read_lock_nesting > 0 &&
!t->rcu_read_unlock_special.b.blocked) {

@@ -738,7 +739,7 @@ static void __init rcu_bootup_announce(void)
* Because preemptible RCU does not exist, we never have to check for
* CPUs being in quiescent states.
-static void rcu_preempt_note_context_switch(void)
+static void rcu_preempt_note_context_switch(bool preempt)

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