Re: [PATCH v2] nvmem: fix registration vs use race

[Hector Martin <marcan@marcan.st>]

On 03/01/2023 23.42, Russell King (Oracle) wrote:
> The i.MX6 CPU frequency driver sometimes fails to register at boot time
> due to nvmem_cell_read_u32() sporadically returning -ENOENT.
> 
> This happens because there is a window where __nvmem_device_get() in
> of_nvmem_cell_get() is able to return the nvmem device, but as cells
> have been setup, nvmem_find_cell_entry_by_node() returns NULL.
> 
> The occurs because the nvmem core registration code violates one of the
> fundamental principles of kernel programming: do not publish data
> structures before their setup is complete.
> 
> Fix this by making nvmem core code conform with this principle.
> 
> Fixes: eace75cfdcf7 ("nvmem: Add a simple NVMEM framework for nvmem providers")
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> ---
> v2: add fixes tag
> 
>  drivers/nvmem/core.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 321d7d63e068..6b89fb6fa582 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -835,22 +835,16 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>  	nvmem->dev.groups = nvmem_dev_groups;
>  #endif
>  
> -	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> -
> -	rval = device_register(&nvmem->dev);
> -	if (rval)
> -		goto err_put_device;
> -
>  	if (nvmem->nkeepout) {
>  		rval = nvmem_validate_keepouts(nvmem);
>  		if (rval)
> -			goto err_device_del;
> +			goto err_put_device;

You can't call put_device() on a device that hasn't gone through
device_initialize() yet.

>  	}
>  
>  	if (config->compat) {
>  		rval = nvmem_sysfs_setup_compat(nvmem, config);
>  		if (rval)
> -			goto err_device_del;
> +			goto err_put_device;

Ditto.

>  	}
>  
>  	if (config->cells) {
> @@ -867,6 +861,12 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>  	if (rval)
>  		goto err_remove_cells;
>  
> +	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> +
> +	rval = device_register(&nvmem->dev);
> +	if (rval)
> +		goto err_remove_cells;
> +
>  	blocking_notifier_call_chain(&nvmem_notifier, NVMEM_ADD, nvmem);
>  
>  	return nvmem;
> @@ -876,8 +876,6 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>  err_teardown_compat:
>  	if (config->compat)
>  		nvmem_sysfs_remove_compat(nvmem, config);
> -err_device_del:
> -	device_del(&nvmem->dev);
>  err_put_device:
>  	put_device(&nvmem->dev);
>  


- Hector