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Subject[RT PATCH 2/3] hrtimer: Don't grab the expiry lock for non-soft hrtimer
Date
There are no guarantee the hrtimer_cancel() will be called on the same
CPU as the non-soft hrtimer is running on so the following scenario
can happen.

CPU0 | CPU1
|
| hrtimer_interrupt()
| raw_spin_lock_irqsave(&cpu_save->lock)
hrtimer_cancel() | __run_hrtimer_run_queues()
hrtimer_try_to_cancel() | __run_hrtimer()
lock_hrtimer_base() | base->running = timer;
| raw_spin_unlock_irqrestore(&cpu_save->lock)
raw_spin_lock_irqsave(cpu_base->lock) | fn(timer);
hrtimer_callback_running() |

hrtimer_callback_running() will be returning true as the callback is
running somewhere else. This means hrtimer_try_to_cancel() would return -1.
Therefore hrtimer_grab_expiry_lock() would be called.

non-soft hrtimer may be used when the timer needs to be manipulated from
a non-preemptible context. This is for instance the case of KVM Arm
timers. The following splat can be seen in the log:

[ 157.449545] 000: BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:968
[ 157.449569] 000: in_atomic(): 1, irqs_disabled(): 0, pid: 990, name: kvm-vcpu-1
[ 157.449579] 000: 2 locks held by kvm-vcpu-1/990:
[ 157.449592] 000: #0: 00000000c2fc8217 (&vcpu->mutex){+.+.}, at: kvm_vcpu_ioctl+0x70/0xae0
[ 157.449638] 000: #1: 0000000096863801 (&cpu_base->softirq_expiry_lock){+.+.}, at: hrtimer_grab_expiry_lock+0x24/0x40
[ 157.449677] 000: Preemption disabled at:
[ 157.449679] 000: [<ffff0000111a4538>] schedule+0x30/0xd8
[ 157.449702] 000: CPU: 0 PID: 990 Comm: kvm-vcpu-1 Tainted: G W 5.2.0-rt1-00001-gd368139e892f #104
[ 157.449712] 000: Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno Development Platform, BIOS EDK II Jan 23 2017
[ 157.449718] 000: Call trace:
[ 157.449722] 000: dump_backtrace+0x0/0x130
[ 157.449730] 000: show_stack+0x14/0x20
[ 157.449738] 000: dump_stack+0xbc/0x104
[ 157.449747] 000: ___might_sleep+0x198/0x238
[ 157.449756] 000: rt_spin_lock+0x5c/0x70
[ 157.449765] 000: hrtimer_grab_expiry_lock+0x24/0x40
[ 157.449773] 000: hrtimer_cancel+0x1c/0x38
[ 157.449780] 000: kvm_timer_vcpu_load+0x78/0x3e0

An hrtimer is always either running in softirq or not. This cannot be
changed after it is instantiated. So we can rely on timer->is_soft
for checking whether the lock can be grabbed.

Signed-off-by: Julien Grall <julien.grall@arm.com>
---
kernel/time/hrtimer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index b869e816e96a..119414a2f59c 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -934,7 +934,7 @@ void hrtimer_grab_expiry_lock(const struct hrtimer *timer)
{
struct hrtimer_clock_base *base = READ_ONCE(timer->base);

- if (base && base->cpu_base) {
+ if (timer->is_soft && base && base->cpu_base) {
spin_lock(&base->cpu_base->softirq_expiry_lock);
spin_unlock(&base->cpu_base->softirq_expiry_lock);
}
--
2.11.0
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