Re: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970

Paul E. McKenney
2012-07-02T14:36:45Z

On Mon, Jul 02, 2012 at 06:33:34AM -0700, Paul E. McKenney wrote:
> On Mon, Jul 02, 2012 at 03:12:53PM +0200, Sasha Levin wrote:
> > On Mon, 2012-07-02 at 13:59 +0200, Peter Zijlstra wrote:

[ . . . ]

> > [ 37.048018] *** DEADLOCK ***
>
> OK, so it looks like the context-switch-time call into preemptible RCU
> needs to be outside of the runqueue-lock critical section. One way
> to do this is be reverting 616c310e (Move PREEMPT_RCU preemption to
> switch_to() invocation). Sasha, could you please try this out?

Hmmmm... Here is a patch for this reversion on top of -rcu with
conflicts resolved.

Thanx, Paul

------------------------------------------------------------------------

Revert "rcu: Move PREEMPT_RCU preemption to switch_to() invocation"

This reverts commit 616c310e83b872024271c915c1b9ab505b9efad9.
(Move PREEMPT_RCU preemption to switch_to() invocation).
Testing by Sasha Levin <levinsasha928@gmail.com> showed that this
can result in deadlock due to invoking the scheduler when one of
the runqueue locks is held. Because this commit was simply a
performance optimization, revert it.

Conflicts (resolved):

include/linux/rcutiny.h

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index 88e466b..43b39d6 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -705,7 +705,6 @@ static void stack_proc(void *arg)
struct task_struct *from = current, *to = arg;

to->thread.saved_task = from;
- rcu_switch_from(from);
switch_to(from, to, from);
}

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 2f5d89b..115ead2 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -185,7 +185,6 @@ static inline int rcu_preempt_depth(void)
/* Internal to kernel */
extern void rcu_sched_qs(int cpu);
extern void rcu_bh_qs(int cpu);
-extern void rcu_preempt_note_context_switch(void);
extern void rcu_check_callbacks(int cpu, int user);
struct notifier_block;
extern void rcu_idle_enter(void);
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 854dc4c..4e56a9c 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -87,6 +87,10 @@ static inline void kfree_call_rcu(struct rcu_head *head,

#ifdef CONFIG_TINY_RCU

+static inline void rcu_preempt_note_context_switch(void)
+{
+}
+
static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
*delta_jiffies = ULONG_MAX;
@@ -95,6 +99,7 @@ static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)

#else /* #ifdef CONFIG_TINY_RCU */

+void rcu_preempt_note_context_switch(void);
int rcu_preempt_needs_cpu(void);

static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
@@ -108,6 +113,7 @@ static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
static inline void rcu_note_context_switch(int cpu)
{
rcu_sched_qs(cpu);
+ rcu_preempt_note_context_switch();
}

/*
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4059c0f..06a4c5f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1871,22 +1871,12 @@ static inline void rcu_copy_process(struct task_struct *p)
INIT_LIST_HEAD(&p->rcu_node_entry);
}

-static inline void rcu_switch_from(struct task_struct *prev)
-{
- if (prev->rcu_read_lock_nesting != 0)
- rcu_preempt_note_context_switch();
-}
-
#else

static inline void rcu_copy_process(struct task_struct *p)
{
}

-static inline void rcu_switch_from(struct task_struct *prev)
-{
-}
-
#endif

#ifdef CONFIG_SMP
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index b9e6aaf..955f18c 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -203,6 +203,7 @@ void rcu_note_context_switch(int cpu)
{
trace_rcu_utilization("Start context switch");
rcu_sched_qs(cpu);
+ rcu_preempt_note_context_switch(cpu);
trace_rcu_utilization("End context switch");
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 86a40b5..b9065dd 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -467,6 +467,7 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);
/* Forward declarations for rcutree_plugin.h */
static void rcu_bootup_announce(void);
long rcu_batches_completed(void);
+static void rcu_preempt_note_context_switch(int cpu);
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
#ifdef CONFIG_HOTPLUG_CPU
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp,
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index dde457e..cf15a61 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -158,7 +158,7 @@ static void rcu_preempt_qs(int cpu)
*
* Caller must disable preemption.
*/
-void rcu_preempt_note_context_switch(void)
+static void rcu_preempt_note_context_switch(int cpu)
{
struct task_struct *t = current;
unsigned long flags;
@@ -169,7 +169,7 @@ void rcu_preempt_note_context_switch(void)
(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {

/* Possibly blocking in an RCU read-side critical section. */
- rdp = __this_cpu_ptr(rcu_preempt_state.rda);
+ rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
rnp = rdp->mynode;
raw_spin_lock_irqsave(&rnp->lock, flags);
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
@@ -233,7 +233,7 @@ void rcu_preempt_note_context_switch(void)
* means that we continue to block the current grace period.
*/
local_irq_save(flags);
- rcu_preempt_qs(smp_processor_id());
+ rcu_preempt_qs(cpu);
local_irq_restore(flags);
}

@@ -902,6 +902,14 @@ void rcu_force_quiescent_state(void)
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

/*
+ * Because preemptible RCU does not exist, we never have to check for
+ * CPUs being in quiescent states.
+ */
+static void rcu_preempt_note_context_switch(int cpu)
+{
+}
+
+/*
* Because preemptible RCU does not exist, there are never any preempted
* RCU readers.
*/
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d5594a4..eaead2d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2081,7 +2081,6 @@ context_switch(struct rq *rq, struct task_struct *prev,
#endif

/* Here we just switch the register state and the stack. */
- rcu_switch_from(prev);
switch_to(prev, next, prev);

barrier();