Re: [PATCH v2 3/4] misc: fastrpc: Remove buffer from list prior to unmap operation

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

On 2/4/2026 5:15 AM, Dmitry Baryshkov wrote:
> On Mon, Feb 02, 2026 at 02:51:33PM +0800, Jianping wrote:
>>
>>
>> On 1/16/2026 4:47 AM, Dmitry Baryshkov wrote:
>>> On Thu, Jan 15, 2026 at 04:28:50PM +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
>>>> calling unmap operation.
>>>>
>>>> Fixes: 2419e55e532de ("misc: fastrpc: add mmap/unmap support")
>>>> Cc: stable@kernel.org
>>>> Co-developed-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>>>> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>>>> Signed-off-by: Jianping Li <jianping.li@oss.qualcomm.com>
>>>> ---
>>>>    drivers/misc/fastrpc.c | 28 ++++++++++++++++++++--------
>>>>    1 file changed, 20 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
>>>> index 4f12fa5a05aa..833c265add5e 100644
>>>> --- a/drivers/misc/fastrpc.c
>>>> +++ b/drivers/misc/fastrpc.c
>>>> @@ -202,6 +202,8 @@ struct fastrpc_buf {
>>>>    	/* mmap support */
>>>>    	struct list_head node; /* list of user requested mmaps */
>>>>    	uintptr_t raddr;
>>>> +	/* Lock for buf->node */
>>>> +	spinlock_t *list_lock;
>>>
>>> Why do you need to lock this? Isn't fl->lock enough?
>>
>> According to the discussion in v1 patch:
>> https://lore.kernel.org/all/p6cc5lxufmefeulx5bhlh6q6ivwluqf2muj3hu5e5526fsppuu@brcy6arm7epg/
>>
>> The lock is stored in fastrpc_buf here.
> 
> That was a separate topic. So, why fl->lock isn't enough? What is the
> race that isn't prevented by it?
"Is fl->lock not enough?" — At the granularity of the lock, fl->lock is 
sufficient. And you can see buf->list_lock reuses fl->lock.

The purpose of doing this is to pass along lock together with the buf 
via the buf->list_lock pointer, so that all operations on buf->node no 
longer need to look for fl, reducing coupling and lowering the chance of 
errors.

If you think buf->list_lock is unnecessary, I can remove it and use 
fl->lock that makes more sense.

> 
>>>
>>>>    };
>>>>    struct fastrpc_dma_buf_attachment {
>>>> @@ -441,6 +443,7 @@ static int __fastrpc_buf_alloc(struct fastrpc_user *fl, struct device *dev,
>>>>    	buf->size = size;
>>>>    	buf->dev = dev;
>>>>    	buf->raddr = 0;
>>>> +	buf->list_lock = &fl->lock;
>>>>    	buf->virt = dma_alloc_coherent(dev, buf->size, &buf->dma_addr,
>>>>    				       GFP_KERNEL);
>>>> @@ -1865,9 +1868,6 @@ static int fastrpc_req_munmap_impl(struct fastrpc_user *fl, struct fastrpc_buf *
>>>>    				      &args[0]);
>>>>    	if (!err) {
>>>>    		dev_dbg(dev, "unmmap\tpt 0x%09lx OK\n", buf->raddr);
>>>> -		spin_lock(&fl->lock);
>>>> -		list_del(&buf->node);
>>>> -		spin_unlock(&fl->lock);
>>>>    		fastrpc_buf_free(buf);
>>>>    	} else {
>>>>    		dev_err(dev, "unmmap\tpt 0x%09lx ERROR\n", buf->raddr);
>>>> @@ -1881,6 +1881,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>>>>    	struct fastrpc_buf *buf = NULL, *iter, *b;
>>>>    	struct fastrpc_req_munmap req;
>>>>    	struct device *dev = fl->sctx->dev;
>>>> +	int err;
>>>>    	if (copy_from_user(&req, argp, sizeof(req)))
>>>>    		return -EFAULT;
>>>> @@ -1888,6 +1889,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>>>>    	spin_lock(&fl->lock);
>>>>    	list_for_each_entry_safe(iter, b, &fl->mmaps, node) {
>>>>    		if ((iter->raddr == req.vaddrout) && (iter->size == req.size)) {
>>>> +			list_del(&iter->node);
>>>>    			buf = iter;
>>>>    			break;
>>>>    		}
>>>> @@ -1900,7 +1902,14 @@ 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(buf->list_lock);
>>>> +		list_add_tail(&buf->node, &fl->mmaps);
>>>> +		spin_unlock(buf->list_lock);
>>>> +	}
>>>> +
>>>> +	return err;
>>>>    }
>>>>    static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
>>>> @@ -1985,20 +1994,23 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
>>>>    		}
>>>>    	}
>>>> -	spin_lock(&fl->lock);
>>>> +	spin_lock(buf->list_lock);
>>>>    	list_add_tail(&buf->node, &fl->mmaps);
>>>> -	spin_unlock(&fl->lock);
>>>> +	spin_unlock(buf->list_lock);
>>>>    	if (copy_to_user((void __user *)argp, &req, sizeof(req))) {
>>>>    		err = -EFAULT;
>>>> -		goto err_assign;
>>>> +		goto err_copy;
>>>>    	}
>>>>    	dev_dbg(dev, "mmap\t\tpt 0x%09lx OK [len 0x%08llx]\n",
>>>>    		buf->raddr, buf->size);
>>>>    	return 0;
>>>> -
>>>> +err_copy:
>>>> +	spin_lock(buf->list_lock);
>>>> +	list_del(&buf->node);
>>>> +	spin_unlock(buf->list_lock);
>>>>    err_assign:
>>>>    	fastrpc_req_munmap_impl(fl, buf);
>>>> -- 
>>>> 2.43.0
>>>>
>>>
>>
>