vfs: add FAILIFEXISTS flag

[mjg]

Summary

Both FreeBSD and Linux mkdir -p walk the tree up ignoring any EEXIST on the way and both are used a lot when building respective kernels.

This poses a problem as spurious locking avoidably interferes with concurrent getdirentries and calls on affected directories.

Work around the problem by adding FAILIFEXISTS flag. In case of lockless lookup this manages to avoid any work to begin with, there is no speed up for the locked case but perhaps this can be augmented later on.

For simplicity the only supported semantics are as used by mkdir.

[kib]

sys/kern/vfs_cache.c

4235

Why is this line needed ?

sys/kern/vfs_lookup.c

1310

This is a duplicate condition. Why adding it ?

1312

() != 0

sys/kern/vfs_syscalls.c

3791

I think this comment should be moved into the herald of the bad_eexist block.

[mjg]

• feedback

[mjg]

sys/kern/vfs_cache.c

4235

So that the underlying can test that the entry is not found by the caller.

sys/kern/vfs_lookup.c

1310

duplicates what? hoisting the flag check above this would still require checking whether a vnode is found

[mjg]

sys/kern/vfs_lookup.c

1310

.. unless this was about the VDIR check, but it was already there and not added by me.

[kib]

sys/kern/vfs_lookup.c

1310

I am about v_type == VDIR. It is re-rechecked at the start of nameicap_tracker_add(). You can simple remove it there, you are re-arranging code anyway.