Re: [PATCH v2] cxl/hdm: fix a warning in devm_remove_action()

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

Sungwoo Kim wrote:
> In the following race scenario, devm_remove_action() can be called
> before devm_add_action(), triggering a warning because there is no
> action to remove.
> 
> To fix this, __cxl_dpa_reserve() performs devm_add_action(). This
> extends the critical section that embraces cxled->dpa_res = res and
> devm_add_action(), so cxl_dpa_free() cannot observe dpa_res without
> the devres action.
> 
> task 1:
> cxl_dpa_alloc()
>   __cxl_dpa_alloc()
>     guard(&cxl_rwsem.dpa)
>     cxled->dpa_res = res;  ...(1)
>   devm_add_action()        ...(4)
> 
> task 2:
> cxl_dpa_free()
>   guard(&cxl_rwsem.dpa)
>   if (!cxled->dpa_res)     ...(2) pass, due to (1)
>     return 0;
>   devm_cxl_dpa_release()
>     devm_remove_action()   ...(3) warning, no action is added yet
> 
> Splat:
> 
> WARNING: ./include/linux/device/devres.h:160 at devm_remove_action include/linux/device/devres.h:160 [inline], CPU#0: syz.1.6464/25993
> WARNING: ./include/linux/device/devres.h:160 at devm_cxl_dpa_release drivers/cxl/core/hdm.c:290 [inline], CPU#0: syz.1.6464/25993
> WARNING: ./include/linux/device/devres.h:160 at cxl_dpa_free+0x2a4/0x320 drivers/cxl/core/hdm.c:572, CPU#0: syz.1.6464/25993
> 
> Fixes: cf880423b6a0 ("cxl/hdm: Add support for allocating DPA to an endpoint decoder")
> Fixes: 9c57cde0dcbd ("cxl/hdm: Enumerate allocated DPA")
> Signed-off-by: Sungwoo Kim <iam@sung-woo.kim>
> ---
> V1: https://lore.kernel.org/linux-cxl/20260309000810.2632065-2-iam@sung-woo.kim/
> V1->V2:
> - Let __cxl_dpa_reserve() handle devm_remove_action() to reduce
>   duplicated patches. (Thanks to Dan for the suggestion)
> - Make the comment concise (Reflect Dan and Alison's comments)
> - Make a warning log in a commit message concise
> 
>  drivers/cxl/core/hdm.c | 33 +++++++++++++--------------------
>  1 file changed, 13 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index cb5d5a047a9d..63bd0d5f31df 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -408,7 +408,13 @@ static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
>  
>  	port->hdm_end++;
>  	get_device(&cxled->cxld.dev);
> -	return 0;
> +
> +	/*
> +	 * Perform devres registration while holding cxl_rwsem.dpa so
> +	 * cxl_dpa_free() cannot observe dpa_res without a matching devres
> +	 * action.
> +	 */
> +	return devm_add_action_or_reset(&port->dev, cxl_dpa_release, cxled);

This needs the pattern you identified in v1 and comments should be
reserved for cases where the code needs more explanation. Something
like:

/* 
 * Invoke the release action with the unlocked __cxl_dpa_release() since
 * cxl_dpa_release() acquires the the dpa rwsem
 */
rc = devm_add_action(..., cxl_dpa_release())
if (rc)
    __cxl_dpa_release()
return rc;