Re: [PATCH v2] cxl/memdev: fix deadlock in cxl_memdev_autoremove() on attach failure

[<dan.j.williams@intel.com>]

Gregory Price wrote:
> cxl_memdev_autoremove() takes device_lock(&cxlmd->dev) via guard(device)
> and then calls cxl_memdev_unregister() when the attach callback was
> provided but cxl_mem_probe() failed to bind.
> 
> cxl_memdev_unregister() calls
>   cdev_device_del()
>     device_del()
>       bus_remove_device()
>         device_release_driver()
> 
> This path is reached when a driver uses the @attach parameter to
> devm_cxl_add_memdev() and the CXL topology fails to enumerate (e.g.
> DVSEC range registers decode outside platform-defined CXL ranges,
> causing the endpoint port probe to fail).
> 
> Fixes: 29317f8dc6ed ("cxl/mem: Introduce cxl_memdev_attach for CXL-dependent operation")
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>  drivers/cxl/core/memdev.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
> index af3d0cc65138..281acb3b9abe 100644
> --- a/drivers/cxl/core/memdev.c
> +++ b/drivers/cxl/core/memdev.c
> @@ -1089,10 +1089,8 @@ static int cxlmd_add(struct cxl_memdev *cxlmd, struct cxl_dev_state *cxlds)
>  DEFINE_FREE(put_cxlmd, struct cxl_memdev *,
>  	    if (!IS_ERR_OR_NULL(_T)) put_device(&_T->dev))
>  
> -static struct cxl_memdev *cxl_memdev_autoremove(struct cxl_memdev *cxlmd)
> +static bool cxl_memdev_did_attach(struct cxl_memdev *cxlmd)
>  {
> -	int rc;
> -
>  	/*
>  	 * If @attach is provided fail if the driver is not attached upon
>  	 * return. Note that failure here could be the result of a race to
> @@ -1100,7 +1098,14 @@ static struct cxl_memdev *cxl_memdev_autoremove(struct cxl_memdev *cxlmd)
>  	 * succeeded and then cxl_mem unbound before the lock is acquired.
>  	 */
>  	guard(device)(&cxlmd->dev);
> -	if (cxlmd->attach && !cxlmd->dev.driver) {
> +	return (cxlmd->attach && !cxlmd->dev.driver);
> +}
> +
> +static struct cxl_memdev *cxl_memdev_autoremove(struct cxl_memdev *cxlmd)
> +{
> +	int rc;
> +
> +	if (cxl_memdev_did_attach(cxlmd)) {

Oh, looks like the polarity of the return is flipped from the name. So
either:

    s/cxl_memdev_did_attach/cxl_memdev_attach_failed/

...or make the return:

    if (!cxlmd->attach)
    	return true;
    if (cxlmd->dev.driver)
    	return true;
    return false;

...and the make the check:

    if (!cxl_memdev_did_attach(cxlmd))

I am partial to cxl_memdev_attach_failed() because that avoids the
ambiguity of the !cxlmd->attach case which is ok with "cxlmd->dev.driver
== NULL".