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. Also maybe here two: dma_fence are bounded like other disk i/o (including the option of timeouts if things go very wrong), so it's very much not forever but at most a few seconds worst case (shit hw/driver excluded, as usual). -Sima -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
