Re: [PATCH 4.4 40/57] tpm: Provide strong locking for device removal

[Ben Hutchings]

On Wed, 2017-07-19 at 13:12 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx>
> 
> commit 4e26195f240d73150e8308ae42874702e3df8d2c upstream.
> 
> Add a read/write semaphore around the ops function pointers so
> ops can be set to null when the driver un-registers.
[...]
> @@ -49,10 +99,10 @@ struct tpm_chip *tpm_chip_find_get(int c
>  		if (chip_num != TPM_ANY_NUM && chip_num != pos->dev_num)
>  			continue;
>  
> -		if (try_module_get(pos->dev.parent->driver->owner)) {
> +		/* rcu prevents chip from being free'd */
> +		if (!tpm_try_get_ops(pos))
[...]

But an RCU read-side critical section is an atomic context, and
semaphore operations can block!  Fixed upstream by:

commit 15516788e581eb32ec1c50e5f00aba3faf95d817
Author: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
Date:   Mon Feb 29 08:53:02 2016 -0500

    tpm: Replace device number bitmap with IDR

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.