linux-kernel - RE: [PATCH] cxl/region: Fix cxl_region_rwsem lock held when returning to user space

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Date:   Wed, 25 Oct 2023 15:04:59 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Li Zhijian <lizhijian@...itsu.com>, <alison.schofield@...el.com>,
        <vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
        <bwidawsk@...nel.org>, <dan.j.williams@...el.com>,
        <linux-cxl@...r.kernel.org>
CC:     <dave.jiang@...el.com>, <Jonathan.Cameron@...wei.com>,
        <fan.ni@...sung.com>, <linux-kernel@...r.kernel.org>,
        Li Zhijian <lizhijian@...itsu.com>
Subject: RE: [PATCH] cxl/region: Fix cxl_region_rwsem lock held when
 returning to user space

Li Zhijian wrote:
> goto out to release cxl_region_rwsem. Fix below warning:

Hi, this change looks good, the changelog would benefit from more
detail so that backporters can determine how important this fix is. I
will add the following:

---

cxl_region_invalidate_memregion() on x86 invokes wbinvd which is a
problematic instruction for virtualized environments. So, on virtualized
x86, cxl_region_invalidate_memregion() returns an error. This failure
case got missed because CXL memory-expander device passthrough is not a
production use case, and emulation of CXL devices is typically limited
to kernel development builds with CONFIG_CXL_REGION_INVALIDATION_TEST=y,
that makes cxl_region_invalidate_memregion() succeed.

In other words, the expected exposure of this bug is limited to CXL
subsystem development environments using QEMU that neglected
CONFIG_CXL_REGION_INVALIDATION_TEST=y.

> ================================================
> WARNING: lock held when returning to user space!
> 6.6.0-rc3-lizhijian+ #213 Not tainted
> ------------------------------------------------
> cxl/673 is leaving the kernel with locks still held!
> 1 lock held by cxl/673:
>  #0: ffffffffa013b9d0 (cxl_region_rwsem){++++}-{3:3}, at: commit_store+0x7d/0x3e0 [cxl_core]
> 
> Fixes: d1257d098a5a ("cxl/region: Move cache invalidation before region teardown, and before setup")
> Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
> ---
>  drivers/cxl/core/region.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 6d63b8798c29..18da77f8f22b 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -294,7 +294,7 @@ static ssize_t commit_store(struct device *dev, struct device_attribute *attr,
>  	 */
>  	rc = cxl_region_invalidate_memregion(cxlr);
>  	if (rc)
> -		return rc;
> +		goto out;
>  
>  	if (commit) {
>  		rc = cxl_region_decode_commit(cxlr);
> -- 
> 2.29.2
>