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Subject[PATCH] notifier: Make atomic_notifiers use raw_spinlock
Date
Booting a recent PREEMPT_RT kernel (v5.10-rc3-rt7-rebase) on my arm64 Juno
leads to the idle task blocking on an RT sleeping spinlock down some
notifier path:

[ 1.809101] BUG: scheduling while atomic: swapper/5/0/0x00000002
[ 1.809116] Modules linked in:
[ 1.809123] Preemption disabled at:
[ 1.809125] secondary_start_kernel (arch/arm64/kernel/smp.c:227)
[ 1.809146] CPU: 5 PID: 0 Comm: swapper/5 Tainted: G W 5.10.0-rc3-rt7 #168
[ 1.809153] Hardware name: ARM Juno development board (r0) (DT)
[ 1.809158] Call trace:
[ 1.809160] dump_backtrace (arch/arm64/kernel/stacktrace.c:100 (discriminator 1))
[ 1.809170] show_stack (arch/arm64/kernel/stacktrace.c:198)
[ 1.809178] dump_stack (lib/dump_stack.c:122)
[ 1.809188] __schedule_bug (kernel/sched/core.c:4886)
[ 1.809197] __schedule (./arch/arm64/include/asm/preempt.h:18 kernel/sched/core.c:4913 kernel/sched/core.c:5040)
[ 1.809204] preempt_schedule_lock (kernel/sched/core.c:5365 (discriminator 1))
[ 1.809210] rt_spin_lock_slowlock_locked (kernel/locking/rtmutex.c:1072)
[ 1.809217] rt_spin_lock_slowlock (kernel/locking/rtmutex.c:1110)
[ 1.809224] rt_spin_lock (./include/linux/rcupdate.h:647 kernel/locking/rtmutex.c:1139)
[ 1.809231] atomic_notifier_call_chain_robust (kernel/notifier.c:71 kernel/notifier.c:118 kernel/notifier.c:186)
[ 1.809240] cpu_pm_enter (kernel/cpu_pm.c:39 kernel/cpu_pm.c:93)
[ 1.809249] psci_enter_idle_state (drivers/cpuidle/cpuidle-psci.c:52 drivers/cpuidle/cpuidle-psci.c:129)
[ 1.809258] cpuidle_enter_state (drivers/cpuidle/cpuidle.c:238)
[ 1.809267] cpuidle_enter (drivers/cpuidle/cpuidle.c:353)
[ 1.809275] do_idle (kernel/sched/idle.c:132 kernel/sched/idle.c:213 kernel/sched/idle.c:273)
[ 1.809282] cpu_startup_entry (kernel/sched/idle.c:368 (discriminator 1))
[ 1.809288] secondary_start_kernel (arch/arm64/kernel/smp.c:273)

Two points worth noting:

1) That this is conceptually the same issue as pointed out in:
313c8c16ee62 ("PM / CPU: replace raw_notifier with atomic_notifier")
2) Only the _robust() variant of atomic_notifier callchains suffer from
this

AFAICT only the cpu_pm_notifier_chain really needs to be changed, but
singling it out would mean introducing a new (truly) non-blocking API. At
the same time, callers that are fine with any blocking within the call
chain should use blocking notifiers, so patching up all atomic_notifier's
doesn't seem *too* crazy to me.

Fixes: 70d932985757 ("notifier: Fix broken error handling pattern")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
include/linux/notifier.h | 6 +++---
kernel/notifier.c | 12 ++++++------
2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 2fb373a5c1ed..723bc2df6388 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -58,7 +58,7 @@ struct notifier_block {
};

struct atomic_notifier_head {
- spinlock_t lock;
+ raw_spinlock_t lock;
struct notifier_block __rcu *head;
};

@@ -78,7 +78,7 @@ struct srcu_notifier_head {
};

#define ATOMIC_INIT_NOTIFIER_HEAD(name) do { \
- spin_lock_init(&(name)->lock); \
+ raw_spin_lock_init(&(name)->lock); \
(name)->head = NULL; \
} while (0)
#define BLOCKING_INIT_NOTIFIER_HEAD(name) do { \
@@ -95,7 +95,7 @@ extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
cleanup_srcu_struct(&(name)->srcu);

#define ATOMIC_NOTIFIER_INIT(name) { \
- .lock = __SPIN_LOCK_UNLOCKED(name.lock), \
+ .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \
.head = NULL }
#define BLOCKING_NOTIFIER_INIT(name) { \
.rwsem = __RWSEM_INITIALIZER((name).rwsem), \
diff --git a/kernel/notifier.c b/kernel/notifier.c
index 1b019cbca594..c20782f07643 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -142,9 +142,9 @@ int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
unsigned long flags;
int ret;

- spin_lock_irqsave(&nh->lock, flags);
+ raw_spin_lock_irqsave(&nh->lock, flags);
ret = notifier_chain_register(&nh->head, n);
- spin_unlock_irqrestore(&nh->lock, flags);
+ raw_spin_unlock_irqrestore(&nh->lock, flags);
return ret;
}
EXPORT_SYMBOL_GPL(atomic_notifier_chain_register);
@@ -164,9 +164,9 @@ int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
unsigned long flags;
int ret;

- spin_lock_irqsave(&nh->lock, flags);
+ raw_spin_lock_irqsave(&nh->lock, flags);
ret = notifier_chain_unregister(&nh->head, n);
- spin_unlock_irqrestore(&nh->lock, flags);
+ raw_spin_unlock_irqrestore(&nh->lock, flags);
synchronize_rcu();
return ret;
}
@@ -182,9 +182,9 @@ int atomic_notifier_call_chain_robust(struct atomic_notifier_head *nh,
* Musn't use RCU; because then the notifier list can
* change between the up and down traversal.
*/
- spin_lock_irqsave(&nh->lock, flags);
+ raw_spin_lock_irqsave(&nh->lock, flags);
ret = notifier_call_chain_robust(&nh->head, val_up, val_down, v);
- spin_unlock_irqrestore(&nh->lock, flags);
+ raw_spin_unlock_irqrestore(&nh->lock, flags);

return ret;
}
--
2.27.0
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