Re: [PATCH v5 2/5] misc: fastrpc: Remove buffer from list prior to unmap operation

[Jianping Li <jianping.li@oss.qualcomm.com>]

On 5/25/2026 4:30 PM, Dmitry Baryshkov wrote:
> On Fri, May 22, 2026 at 02:55:29PM +0800, Jianping Li wrote:
>> On 5/15/2026 9:36 PM, Dmitry Baryshkov wrote:
>>> On Fri, May 15, 2026 at 08:42:14PM +0800, Jianping Li wrote:
>>>> From: Ekansh Gupta<ekansh.gupta@oss.qualcomm.com>
>>>>
>>>> fastrpc_req_munmap_impl() is called to unmap any buffer. The buffer is
>>>> getting removed from the list after it is unmapped from DSP. This can
>>>> create potential race conditions if any other thread removes the entry
>>>> from list while unmap operation is ongoing. Remove the entry before
>>> How can it remove the entry from the list?
>> Multiple threads sharing the same file descriptor may invoke unmap concurrently.
> => commit message
>
>>>> @@ -1898,7 +1897,14 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>>>>    		return -EINVAL;
>>>>    	}
>>>> -	return fastrpc_req_munmap_impl(fl, buf);
>>>> +	err = fastrpc_req_munmap_impl(fl, buf);
>>>> +	if (err) {
>>>> +		spin_lock(&fl->lock);
>>>> +		list_add_tail(&buf->node, &fl->mmaps);
>>>> +		spin_unlock(&fl->lock);
>>>> +	}
>>> Is it expected that userspace tries to unmap it again? Or why is it
>>> being added to the list?
>> User process can call unmap and fastrpc library won't call the unmap again.
> In the other email you wrote that the driver can be used by random apps.
> So... what happens if userspace unmaps it again? What if the userspace
> _doesn't_ unmap it (although you've just readded it back)?

If the same buf is unmapped again, because it has already been added back to the list, the unmap logic will be executed again.
If userspace no longer performs unmap, the driver will not unmap it proactively.
The Fastrpc driver will free up this list during fastrpc user-free.

>
>> Fastrpc driver will free up this list during fastrpc user-free.