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

[Daniel Vetter <daniel@ffwll.ch>]

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@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=5d4cb6b4409edfd18646
> > > Tested-by: Dmitry Antipov <dmantipov@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.

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 ...
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch