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From: "Christian König" <[email protected]>
To: Linus Torvalds <[email protected]>,
	Al Viro <[email protected]>
Cc: Christian Brauner <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected], [email protected], [email protected],
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected],
	[email protected],
	[email protected]
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes
Date: Mon, 6 May 2024 18:15:06 +0200	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAHk-=whFg8-WyMbVUGW5c0baurGzqmRtzFLoU-gxtRXq2nVZ+w@mail.gmail.com>

Am 05.05.24 um 22:53 schrieb Linus Torvalds:
> On Sun, 5 May 2024 at 13:30, Al Viro <[email protected]> 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.

Sorry to maybe jumping into the middle of the discussion, but for 
DMA-buf the behavior Al doesn't want is actually desired.

And I totally understand why Al is against it for file system based 
files, but for this case it's completely intentional.

Removing the callback on close is what we used to do a long time ago, 
but that turned out into a locking nightmare because it meant that we 
need to be able to wait for driver specific locks from whatever non 
interrupt context fput() is called from.

Regards,
Christian.

>
> 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.
>
>                   Linus


  parent reply	other threads:[~2024-05-06 16:15 UTC|newest]

Thread overview: 87+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-08  8:26 [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove syzbot
2024-04-15 14:31 ` Jens Axboe
2024-04-15 14:57   ` Pavel Begunkov
2024-05-03 11:54 ` Bui Quang Minh
2024-05-03 18:26   ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Kees Cook
2024-05-03 18:49     ` Jens Axboe
2024-05-03 19:22       ` Kees Cook
2024-05-03 19:35         ` Jens Axboe
2024-05-03 19:59           ` Kees Cook
2024-05-03 20:28             ` Kees Cook
2024-05-03 21:11               ` Al Viro
2024-05-03 21:24                 ` Linus Torvalds
2024-05-03 21:30                   ` Al Viro
2024-05-06 17:46                   ` Stefan Metzmacher
2024-05-06 18:17                     ` Linus Torvalds
2024-05-08  8:47                       ` David Laight
2024-05-03 21:36                 ` Al Viro
2024-05-03 21:42                   ` Linus Torvalds
2024-05-03 21:53                     ` Al Viro
2024-05-06 12:23                       ` Daniel Vetter
2024-05-04  9:59             ` Christian Brauner
2024-05-03 21:11     ` [PATCH] epoll: try to be a _bit_ better about file lifetimes Linus Torvalds
2024-05-03 21:24       ` Al Viro
2024-05-03 21:33         ` Linus Torvalds
2024-05-03 21:45           ` Al Viro
2024-05-03 21:52             ` Linus Torvalds
2024-05-03 22:01               ` Al Viro
2024-05-03 22:07                 ` Al Viro
2024-05-03 23:16                   ` Linus Torvalds
2024-05-03 23:39                     ` Al Viro
2024-05-03 23:54                       ` Linus Torvalds
2024-05-04 10:44                       ` Christian Brauner
2024-05-03 22:46               ` Kees Cook
2024-05-03 23:03                 ` Al Viro
2024-05-03 23:23                   ` Kees Cook
2024-05-03 23:41                     ` Linus Torvalds
2024-05-04  9:19                       ` Christian Brauner
2024-05-06 12:37                       ` Daniel Vetter
2024-05-04  9:37           ` Christian Brauner
2024-05-04 15:32             ` Linus Torvalds
2024-05-04 15:40               ` Linus Torvalds
2024-05-04 15:53                 ` Linus Torvalds
2024-05-05 19:46                   ` Al Viro
2024-05-05 20:03                     ` Linus Torvalds
2024-05-05 20:30                       ` Al Viro
2024-05-05 20:53                         ` Linus Torvalds
2024-05-06 12:47                           ` Daniel Vetter
2024-05-06 14:46                             ` Christian Brauner
2024-05-07 10:58                               ` Daniel Vetter
2024-05-06 16:15                           ` Christian König [this message]
2024-05-05 10:50                 ` Christian Brauner
2024-05-05 16:46                   ` Linus Torvalds
2024-05-05 17:55                     ` [PATCH v2] epoll: be " Linus Torvalds
2024-05-05 18:04                       ` Jens Axboe
2024-05-05 20:01                       ` David Laight
2024-05-05 20:16                         ` Linus Torvalds
2024-05-05 20:12                     ` [PATCH] epoll: try to be a _bit_ " Al Viro
2024-05-06  8:45                     ` Christian Brauner
2024-05-06  9:26                       ` Christian Brauner
2024-05-06 14:19                         ` Christian Brauner
2024-05-07 21:02                       ` David Laight
2024-05-04 18:20               ` Linus Torvalds
2024-05-06 14:29                 ` [Linaro-mm-sig] " Christian König
2024-05-07 11:02                   ` Daniel Vetter
2024-05-07 16:46                     ` Linus Torvalds
2024-05-07 17:45                       ` Christian König
2024-05-08  7:51                         ` Michel Dänzer
2024-05-08  7:59                           ` Christian König
2024-05-08  8:23                         ` Christian Brauner
2024-05-08  9:10                           ` Christian König
2024-05-07 18:04                       ` Daniel Vetter
2024-05-07 19:07                         ` Linus Torvalds
2024-05-08  5:55                           ` Christian König
2024-05-08  8:32                             ` Daniel Vetter
2024-05-08 10:16                               ` Christian Brauner
2024-05-08  8:05                           ` Christian Brauner
2024-05-08 16:19                           ` Linus Torvalds
2024-05-08 17:14                             ` Linus Torvalds
2024-05-09 11:38                               ` Christian Brauner
2024-05-09 15:48                                 ` Linus Torvalds
2024-05-10  6:33                                   ` Christian Brauner
2024-05-08 10:08                   ` Christian Brauner
2024-05-08 15:45                     ` Daniel Vetter
2024-05-10 10:55                       ` Christian Brauner
2024-05-11 18:25                         ` David Laight
2024-05-05 17:31       ` Jens Axboe
2024-05-04  9:45     ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Christian Brauner

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