On Mon, May 06, 2024 at 02:47:23PM +0200, Daniel Vetter wrote: > On Sun, May 05, 2024 at 01:53:48PM -0700, Linus Torvalds wrote: > > On Sun, 5 May 2024 at 13:30, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > 0. special-cased ->f_count rule for ->poll() is a wart and it's > > > better to get rid of it. > > > > > > 1. fs/eventpoll.c is a steaming pile of shit and I'd be glad to see > > > git rm taken to it. Short of that, by all means, let's grab reference > > > in there around the call of vfs_poll() (see (0)). > > > > Agreed on 0/1. > > > > > 2. having ->poll() instances grab extra references to file passed > > > to them is not something that should be encouraged; there's a plenty > > > of potential problems, and "caller has it pinned, so we are fine with > > > grabbing extra refs" is nowhere near enough to eliminate those. > > > > So it's not clear why you hate it so much, since those extra > > references are totally normal in all the other VFS paths. > > > > I mean, they are perhaps not the *common* case, but we have a lot of > > random get_file() calls sprinkled around in various places when you > > end up passing a file descriptor off to some asynchronous operation > > thing. > > > > Yeah, I think most of them tend to be special operations (eg the tty > > TIOCCONS ioctl to redirect the console), but it's not like vfs_ioctl() > > is *that* different from vfs_poll. Different operation, not somehow > > "one is more special than the other". > > > > cachefiles and backing-file does it for regular IO, and drop it at IO > > completion - not that different from what dma-buf does. It's in > > ->read_iter() rather than ->poll(), but again: different operations, > > but not "one of them is somehow fundamentally different". > > > > > 3. dma-buf uses of get_file() are probably safe (epoll shite aside), > > > but they do look fishy. That has nothing to do with epoll. > > > > Now, what dma-buf basically seems to do is to avoid ref-counting its > > own fundamental data structure, and replaces that by refcounting the > > 'struct file' that *points* to it instead. > > > > And it is a bit odd, but it actually makes some amount of sense, > > because then what it passes around is that file pointer (and it allows > > passing it around from user space *as* that file). > > > > And honestly, if you look at why it then needs to add its refcount to > > it all, it actually makes sense. dma-bufs have this notion of > > "fences" that are basically completion points for the asynchronous > > DMA. Doing a "poll()" operation will add a note to the fence to get > > that wakeup when it's done. > > > > And yes, logically it takes a ref to the "struct dma_buf", but because > > of how the lifetime of the dma_buf is associated with the lifetime of > > the 'struct file', that then turns into taking a ref on the file. > > > > Unusual? Yes. But not illogical. Not obviously broken. Tying the > > lifetime of the dma_buf to the lifetime of a file that is passed along > > makes _sense_ for that use. > > > > I'm sure dma-bufs could add another level of refcounting on the > > 'struct dma_buf' itself, and not make it be 1:1 with the file, but > > it's not clear to me what the advantage would really be, or why it > > would be wrong to re-use a refcount that is already there. > > So there is generally another refcount, because dma_buf is just the > cross-driver interface to some kind of real underlying buffer object from > the various graphics related subsystems we have. > > And since it's a pure file based api thing that ceases to serve any > function once the fd/file is gone we tied all the dma_buf refcounting to > the refcount struct file already maintains. But the underlying buffer > object can easily outlive the dma_buf, and over the lifetime of an > underlying buffer object you might actually end up creating different > dma_buf api wrappers for it (but at least in drm we guarantee there's at > most one, hence why vmwgfx does the atomic_inc_unless_zero trick, which I > don't particularly like and isn't really needed). > > But we could add another refcount, it just means we have 3 of those then > when only really 2 are needed. Fwiw, the TTM thing described upthread and in the other thread really tries hard to work around the dma_buf == file lifetime choice by hooking into the dma-buf specific release function so it can access the dmabuf and then the file. All that seems like a pretty error prone thing to me. So a separate refcount for dma_buf wouldn't be the worst as that would allow that TTM thing to benefit and remove that nasty hacking into your generic dma_buf ops. But maybe I'm the only one who sees it that way and I'm certainly not familiar enough with dma-buf.
