RE: [PATCH v2] epoll: be better about file lifetimes

[David Laight <David.Laight@ACULAB.COM>]

From: Linus Torvalds
> Sent: 05 May 2024 18:56
> 
> epoll can call out to vfs_poll() with a file pointer that may race with
> the last 'fput()'. That would make f_count go down to zero, and while
> the ep->mtx locking means that the resulting file pointer tear-down will
> be blocked until the poll returns, it means that f_count is already
> dead, and any use of it won't actually get a reference to the file any
> more: it's dead regardless.
> 
> Make sure we have a valid ref on the file pointer before we call down to
> vfs_poll() from the epoll routines.

How much is the extra pair of atomics going to hurt performance?
IIRC a lot of work was done to (try to) make epoll lockless.

Perhaps the 'hook' into epoll (usually) from sys_close should be
done before any of the references are removed?
(Which is different from Q6/A6 in man epoll - but that seems to be
documenting a bug!)
Then the ->poll() callback can't happen (assuming it is properly
locked) after the ->release() one.

It seems better to add extra atomics to the close/final-fput path
rather than ever ->poll() call epoll makes.

I can get extra references to a driver by dup() open("/dev/fd/n")
and mmap() - but epoll is definitely fd based.
(Which may be why it has the fd number in its data.)

Is there another race between EPOLL_CTL_ADD and close() (from a
different thread)?

	David

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