RE: [PATCH] file: always lock position

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

From: Christian Brauner
> Sent: 26 July 2023 09:37
...
> Yes, and to summarize which I tried in my description for the commit.
> The getdents support patchset would have introduced a bug because the
> patchset copied the fdget_pos() file_count(file) > 1 optimization into
> io_uring.
> 
> That works fine as long as the original file descriptor used to register
> the fixed file is kept. The locking will work correctly as
> file_count(file) > 1 and no races are possible neither via getdent calls
> using the original file descriptor nor via io_uring using the fixed file
> or even mixing both.
> 
> But as soon as the original file descriptor is closed the f_count for
> the file drops back to 1 but continues to be usable from io_uring via
> the fixed file. Now the optimization that the patchset wanted to copy
> over would cause bugs as multiple racing getdent requests would be
> possible using the fixed file.

Could the io_uring code grab two references?
That would stop the optimisation without affecting any
normal code paths?

	David

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