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SubjectRe: [PATCH] uio:uio_pci_generic:Don't clear master bit when the process does not exit
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On 2023/2/14 21:17, Greg KH wrote:
> On Tue, Feb 14, 2023 at 09:21:57PM +0800, Su Weifeng wrote:
>> From: Weifeng Su <suweifeng1@huawei.com>
>>
>> The /dev/uioX device is used by multiple processes. The current behavior
>> is to clear the master bit when a process exits. This affects other
>> processes that use the device, resulting in command suspension and
>> timeout. This behavior cannot be sensed by the process itself.
>> The solution is to add the reference counting. The reference count is
>> self-incremented and self-decremented each time when the device open and
>> close. The master bit is cleared only when the last process exited.
>>
>> Signed-off-by: Weifeng Su <suweifeng1@huawei.com>
>> ---
>> drivers/uio/uio_pci_generic.c | 18 +++++++++++++++++-
>> 1 file changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c
>> index e03f9b532..d36d3e08e 100644
>> --- a/drivers/uio/uio_pci_generic.c
>> +++ b/drivers/uio/uio_pci_generic.c
>> @@ -31,6 +31,7 @@
>> struct uio_pci_generic_dev {
>> struct uio_info info;
>> struct pci_dev *pdev;
>> + refcount_t dev_refc;
>> };
>>
>> static inline struct uio_pci_generic_dev *
>> @@ -39,10 +40,22 @@ to_uio_pci_generic_dev(struct uio_info *info)
>> return container_of(info, struct uio_pci_generic_dev, info);
>> }
>>
>> +static int open(struct uio_info *info, struct inode *inode)
>> +{
>> + struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
>> +
>> + if (gdev)
>> + refcount_inc(&gdev->dev_refc);
>
> This flat out does not work, sorry.
>
> You should never rely on trying to count open/release calls, just let
> the vfs layer handle that for us as it currently does so.
>
> Think about what happens if you call dup() in userspace on a
> filehandle...
>
>> + return 0;
>> +}
>> +
>> static int release(struct uio_info *info, struct inode *inode)
>> {
>> struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
>>
>> + if (gdev && refcount_dec_not_one(&gdev->dev_refc))
>
> I don't think you actually tested this as it is impossible for gdev to
> ever be NULL.
>
> sorry, but this patch is not correct.
>
> greg k-h

First of all, thank you for taking the time to read this patch, your
comments are very enlightening, but I do have a strange problem here, I
test such programs on kernels 5.10 and 6.2.
fd = open("/dev/uio0". O_RDWR);
while (true)
sleep(1);
This program only opens the uio device. After starting multiple such
processes, I close one of them. From the added print, it can be seen
that the "uio_pci_generic.c:release" function is called and the master
bit is cleared, instead of being called when the last process exits as
expected. I think the vfs is not protected as it should be. Such a
problem cannot be handled in the user-mode program. We have to clear the
master bit when the last process exits. Otherwise, user-mode programs
(for example, the DPDK process that uses uio_pci_generic) cannot work
properly.

Best regards,
Weifeng Su

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