On Mon, 29 Oct 2012, Ming Lei wrote:
> The patch introduces the flag of memalloc_noio_resume in
> 'struct dev_pm_info' to help PM core to teach mm not allocating
> memory with GFP_KERNEL flag for avoiding probable deadlock
> problem.
>
> As explained in the comment, any GFP_KERNEL allocation inside
> runtime_resume on any one of device in the path from one block
> or network device to the root device in the device tree may cause
> deadlock, the introduced pm_runtime_set_memalloc_noio() sets or
> clears the flag on device of the path recursively.
>
> This patch also introduces pm_runtime_get_memalloc_noio() because
> the flag may be accessed in block device's error handling path
> (for example, usb device reset)
> +/*
> + * pm_runtime_get_memalloc_noio - Get a device's memalloc_noio flag.
> + * @dev: Device to handle.
> + *
> + * Return the device's memalloc_noio flag.
> + *
> + * The device power lock is held because bitfield is not SMP-safe.
> + */
> +bool pm_runtime_get_memalloc_noio(struct device *dev)
> +{
> + bool ret;
> + spin_lock_irq(&dev->power.lock);
> + ret = dev->power.memalloc_noio_resume;
> + spin_unlock_irq(&dev->power.lock);
> + return ret;
> +}
You don't need to acquire and release a spinlock just to read the
value. Reading bitfields _is_ SMP-safe; writing them is not.
> +/*
> + * pm_runtime_set_memalloc_noio - Set a device's memalloc_noio flag.
> + * @dev: Device to handle.
> + * @enable: True for setting the flag and False for clearing the flag.
> + *
> + * Set the flag for all devices in the path from the device to the
> + * root device in the device tree if @enable is true, otherwise clear
> + * the flag for devices in the path which sibliings don't set the flag.
s/which/whose/
s/ii/i
> + *
> + * The function should only be called by block device, or network
> + * device driver for solving the deadlock problem during runtime
> + * resume:
> + * if memory allocation with GFP_KERNEL is called inside runtime
> + * resume callback of any one of its ancestors(or the block device
> + * itself), the deadlock may be triggered inside the memory
> + * allocation since it might not complete until the block device
> + * becomes active and the involed page I/O finishes. The situation
> + * is pointed out first by Alan Stern. Network device are involved
> + * in iSCSI kind of situation.
> + *
> + * The lock of dev_hotplug_mutex is held in the function for handling
> + * hotplug race because pm_runtime_set_memalloc_noio() may be called
> + * in async probe().
> + */
> +void pm_runtime_set_memalloc_noio(struct device *dev, bool enable)
> +{
> + static DEFINE_MUTEX(dev_hotplug_mutex);
> +
> + mutex_lock(&dev_hotplug_mutex);
> + while (dev) {
Unless you think somebody is likely to call this function with dev
equal to NULL, this can simply be
for (;;) {
> + /* hold power lock since bitfield is not SMP-safe. */
> + spin_lock_irq(&dev->power.lock);
> + dev->power.memalloc_noio_resume = enable;
> + spin_unlock_irq(&dev->power.lock);
> +
> + dev = dev->parent;
> +
> + /* only clear the flag for one device if all
> + * children of the device don't set the flag.
> + */
> + if (!dev || (!enable &&
... thanks to this test.
> + device_for_each_child(dev, NULL,
> + dev_memalloc_noio)))
> + break;
> + }
> + mutex_unlock(&dev_hotplug_mutex);
> +}
This might not work if somebody calls pm_runtime_set_memalloc_noio(dev,
true) and then afterwards registers dev at the same time as someone
else calls pm_runtime_set_memalloc_noio(dev2, false), if dev and dev2
have the same parent.
Perhaps the kerneldoc should mention that this function must not be
called until after dev is registered.
Alan Stern
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