On Wed, 28 Feb 2024 at 23:20, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>
> - take the lock exclusively if O_APPEND or if it *looks* like you
> might extend the file size.
>
> - otherwise, take the shared lock, and THEN RE-CHECK. The file size
> might have changed, so now you need to double-check that you're really
> not going to extend the size of the file, and if you are, you need to
> go back and take the inode lock exclusively after all.
Same goes for the suid/sgid checks. You need to take the inode lock
shared to even have the i_mode be stable, and at that point you might
decide "oh, I need to clear suid/sgid after all" and have to go drop
the lock and retake it for exclusive access after all.
(And yes, we *really* should have a rwsem_try_upgrade() operation and
at least avoid the "read_unlock -> write_lock" dance), but we don't.
See a comment in mmap_upgrade_trylock() about it. It should be
reasonably easy to add, but we've never really had enough reason to.
Maybe somebody decides it's worth the effort)
Linus
