Revert "vfs: remove unnecessary d_unhashed() check from __d_lookup_rcu"
This reverts commit 8c01a529. It turns out the d_unhashed() check isn't unnecessary after all: while it's true that unhashing will increment the sequence numbers, that does not necessarily invalidate the RCU lookup, because it might have seen the dentry pointer (before it got unhashed), but by the time it loaded the sequence number, it could have seen the *new* sequence number (after it got unhashed). End result: we might look up an unhashed dentry that is about to be freed, with the sequence number never indicating anything bad about it. So checking that the dentry is still hashed (*after* reading the sequence number) is indeed the proper fix, and was never unnecessary. Reported-by: Dave Jones <email@example.com> Signed-off-by: Linus Torvalds <firstname.lastname@example.org>
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