[lvc-project] [PATCH] [RFC] dma-buf: fix race condition between poll and close

[Christian König]

Am 07.05.24 um 12:40 schrieb Daniel Vetter:
> On Tue, May 07, 2024 at 11:58:33AM +0200, Christian König wrote:
>> Am 06.05.24 um 08:52 schrieb Fedor Pchelkin:
>>> On Fri, 03. May 14:08, Dmitry Antipov wrote:
>>>> On 5/3/24 11:18 AM, Christian König wrote:
>>>>
>>>>> Attached is a compile only tested patch, please verify if it fixes your problem.
>>>> LGTM, and this is similar to get_file() in __pollwait() and fput() in
>>>> free_poll_entry() used in implementation of poll(). Please resubmit to
>>>> linux-fsdevel@ including the following:
>>>>
>>>> Reported-by: syzbot+5d4cb6b4409edfd18646 at syzkaller.appspotmail.com
>>>> Closes: https://syzkaller.appspot.com/bug?extid=5d4cb6b4409edfd18646
>>>> Tested-by: Dmitry Antipov <dmantipov at yandex.ru>
>>> I guess the problem is addressed by commit 4efaa5acf0a1 ("epoll: be better
>>> about file lifetimes") which was pushed upstream just before v6.9-rc7.
>>>
>>> Link: https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/
>> Yeah, Linus took care of that after convincing Al that this is really a bug.
>>
>> They key missing information was that we have a mutex which makes sure that
>> fput() blocks for epoll to stop the polling.
>>
>> It also means that you should probably re-consider using epoll together with
>> shared DMA-bufs. Background is that when both client and display server try
>> to use epoll the kernel will return an error because there can only be one
>> user of epoll.
> I think for dma-buf implicit sync the best is to use the new fence export
> ioctl, which has the added benefit that you get a snapshot and so no funny
> livelock issues if someone keeps submitting rendering to a shared buffer.

+1

>
> That aside, why can you not use the same file with multiple epoll files in
> different processes? Afaik from a quick look, all the tracking structures
> are per epoll file, so both client and compositor using it should work?
>
> I haven't tried, so I just might be extremely blind ...

I've misunderstood one of the comments in the discussion.

You can't add the same file with the same file descriptor number to the 
same epoll file descriptor, but you can add it to different epoll file 
descriptors.

So using epoll in both client and server should actually work.

Sorry for the noise,
Christian.

> -Sima