[RFC] drivercore: deferral race condition fix

[Peter Ujfalusi]

When the kernel is built with CONFIG_PREEMPT it is possible to reach a state
when all modules are loaded but some driver still stuck in the deferred list
and there is a need for external event needed to kick the deferred queue
to probe these drivers.

The issue has been observed on embedded systems with CONFIG_PREEMPT enabled
audio support built as modules and using nfsroot for root filesystem.

The following fragment of a log shows such sequence when all audio modules
were loaded but the sound card is not present since the machine driver has
failed to probe due to missing dependency during it's probe.
The board is am335x-evmsk (McASP<->tlv320aic3106 codec) with davinci-evm
machine driver:

...
[   12.615118] davinci-mcasp 4803c000.mcasp: davinci_mcasp_probe: ENTER
[   12.719969] davinci_evm sound.3: davinci_evm_probe: ENTER
[   12.725753] davinci_evm sound.3: davinci_evm_probe: snd_soc_register_card
[   12.753846] davinci-mcasp 4803c000.mcasp: davinci_mcasp_probe: snd_soc_register_component
[   12.922051] davinci-mcasp 4803c000.mcasp: davinci_mcasp_probe: snd_soc_register_component DONE
[   12.950839] davinci_evm sound.3: ASoC: platform (null) not registered
[   12.957898] davinci_evm sound.3: davinci_evm_probe: snd_soc_register_card DONE (-517)
[   13.099026] davinci-mcasp 4803c000.mcasp: Kicking the deferred list
[   13.177838] davinci-mcasp 4803c000.mcasp: really_probe: probe_count = 2
[   13.194130] davinci_evm sound.3: snd_soc_register_card failed (-517)
[   13.346755] davinci_mcasp_driver_init: LEAVE
[   13.377446] platform sound.3: Driver davinci_evm requests probe deferral
[   13.592527] platform sound.3: really_probe: probe_count = 0

In the log the machine driver enters it's probe at 12.719969 (this point it
has been removed from the deferred lists). McASP driver already executing
it's probing (12.615118) and finishes first as well.
The machine driver tries to construct the sound card (12.950839) but did
not found one of the components so it fails. After this McASP driver
registers all the ASoC components and the deferred work is prepared at
13.099026 (not that this time the machine driver is not in the lists so it
is not going to be handled when the work is executing).
Lastly the machine driver exit from it's probe and the core places it to the
deferred list but there will be no other driver going to load and the
deferred queue is not going to be kicked again - till we have external event
like connecting USB stick, etc.

The proposed solution is to try the deferred queue once more when the last
driver is asking for deferring and we had drivers probed while this last
driver was probing.

This way we can avoid drivers stuck in the deferred queue.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>
---
 drivers/base/dd.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 06051767393f..80703de6e6ad 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -53,6 +53,10 @@ static LIST_HEAD(deferred_probe_pending_list);
 static LIST_HEAD(deferred_probe_active_list);
 static struct workqueue_struct *deferred_wq;
 
+static atomic_t probe_count = ATOMIC_INIT(0);
+static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
+static bool deferral_retry;
+
 /**
  * deferred_probe_work_func() - Retry probing devices in the active list.
  */
@@ -141,6 +145,11 @@ static void driver_deferred_probe_trigger(void)
 	if (!driver_deferred_probe_enable)
 		return;
 
+	if (atomic_read(&probe_count) > 1)
+		deferral_retry = true;
+	else
+		deferral_retry = false;
+
 	/*
 	 * A successful probe means that all the devices in the pending list
 	 * should be triggered to be reprobed.  Move all the deferred devices
@@ -259,9 +268,6 @@ int device_bind_driver(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(device_bind_driver);
 
-static atomic_t probe_count = ATOMIC_INIT(0);
-static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
-
 static int really_probe(struct device *dev, struct device_driver *drv)
 {
 	int ret = 0;
@@ -310,6 +316,16 @@ probe_failed:
 		/* Driver requested deferred probing */
 		dev_info(dev, "Driver %s requests probe deferral\n", drv->name);
 		driver_deferred_probe_add(dev);
+		/*
+		 * This is the last driver to load and asking to be deferred.
+		 * If other driver(s) loaded while this driver was loading, we
+		 * should try the deferred modules again to avoid missing
+		 * dependency for this driver.
+		 */
+		if (atomic_read(&probe_count) == 1 && deferral_retry) {
+			deferral_retry = false;
+			driver_deferred_probe_trigger();
+		}
 	} else if (ret != -ENODEV && ret != -ENXIO) {
 		/* driver matched but the probe failed */
 		printk(KERN_WARNING
-- 
1.8.5.2

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