[PATCH 1/6] sched: Unbreak wakeups

Peter Zijlstra
2021-06-02T13:33:20Z

Remove broken task->state references and let wake_up_process() DTRT.

The anti-pattern in these patches breaks the ordering of ->state vs
COND as described in the comment near set_current_state() and can lead
to missed wakeups:

(OoO load, observes RUNNING)<-.
for (;;) { |
t->state = UNINTERRUPTIBLE; |
smp_mb(); ,-----> ,' (OoO load, observed !COND)
| |
| | COND = 1;
| `- if (t->state != RUNNING)
| wake_up_process(t); // not done
if (COND) ---------'
break;
schedule(); // forever waiting
}
t->state = TASK_RUNNING;

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
drivers/net/ethernet/qualcomm/qca_spi.c | 6 ++----
drivers/usb/gadget/udc/max3420_udc.c | 15 +++++----------
drivers/usb/host/max3421-hcd.c | 3 +--
kernel/softirq.c | 2 +-
4 files changed, 9 insertions(+), 17 deletions(-)

--- a/drivers/net/ethernet/qualcomm/qca_spi.c
+++ b/drivers/net/ethernet/qualcomm/qca_spi.c
@@ -653,8 +653,7 @@ qcaspi_intr_handler(int irq, void *data)
struct qcaspi *qca = data;

qca->intr_req++;
- if (qca->spi_thread &&
- qca->spi_thread->state != TASK_RUNNING)
+ if (qca->spi_thread)
wake_up_process(qca->spi_thread);

return IRQ_HANDLED;
@@ -777,8 +776,7 @@ qcaspi_netdev_xmit(struct sk_buff *skb,

netif_trans_update(dev);

- if (qca->spi_thread &&
- qca->spi_thread->state != TASK_RUNNING)
+ if (qca->spi_thread)
wake_up_process(qca->spi_thread);

return NETDEV_TX_OK;
--- a/drivers/usb/gadget/udc/max3420_udc.c
+++ b/drivers/usb/gadget/udc/max3420_udc.c
@@ -509,8 +509,7 @@ static irqreturn_t max3420_vbus_handler(
? USB_STATE_POWERED : USB_STATE_NOTATTACHED);
spin_unlock_irqrestore(&udc->lock, flags);

- if (udc->thread_task &&
- udc->thread_task->state != TASK_RUNNING)
+ if (udc->thread_task)
wake_up_process(udc->thread_task);

return IRQ_HANDLED;
@@ -529,8 +528,7 @@ static irqreturn_t max3420_irq_handler(i
}
spin_unlock_irqrestore(&udc->lock, flags);

- if (udc->thread_task &&
- udc->thread_task->state != TASK_RUNNING)
+ if (udc->thread_task)
wake_up_process(udc->thread_task);

return IRQ_HANDLED;
@@ -1093,8 +1091,7 @@ static int max3420_wakeup(struct usb_gad

spin_unlock_irqrestore(&udc->lock, flags);

- if (udc->thread_task &&
- udc->thread_task->state != TASK_RUNNING)
+ if (udc->thread_task)
wake_up_process(udc->thread_task);
return ret;
}
@@ -1117,8 +1114,7 @@ static int max3420_udc_start(struct usb_
udc->todo |= UDC_START;
spin_unlock_irqrestore(&udc->lock, flags);

- if (udc->thread_task &&
- udc->thread_task->state != TASK_RUNNING)
+ if (udc->thread_task)
wake_up_process(udc->thread_task);

return 0;
@@ -1137,8 +1133,7 @@ static int max3420_udc_stop(struct usb_g
udc->todo |= UDC_START;
spin_unlock_irqrestore(&udc->lock, flags);

- if (udc->thread_task &&
- udc->thread_task->state != TASK_RUNNING)
+ if (udc->thread_task)
wake_up_process(udc->thread_task);

return 0;
--- a/drivers/usb/host/max3421-hcd.c
+++ b/drivers/usb/host/max3421-hcd.c
@@ -1169,8 +1169,7 @@ max3421_irq_handler(int irq, void *dev_i
struct spi_device *spi = to_spi_device(hcd->self.controller);
struct max3421_hcd *max3421_hcd = hcd_to_max3421(hcd);

- if (max3421_hcd->spi_thread &&
- max3421_hcd->spi_thread->state != TASK_RUNNING)
+ if (max3421_hcd->spi_thread)
wake_up_process(max3421_hcd->spi_thread);
if (!test_and_set_bit(ENABLE_IRQ, &max3421_hcd->todo))
disable_irq_nosync(spi->irq);
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -76,7 +76,7 @@ static void wakeup_softirqd(void)
/* Interrupts are disabled: no need to stop preemption */
struct task_struct *tsk = __this_cpu_read(ksoftirqd);

- if (tsk && tsk->state != TASK_RUNNING)
+ if (tsk)
wake_up_process(tsk);
}