svn commit: r292621 - head/sys/fs/devfs

[Bruce Evans]

On Tue, 22 Dec 2015, Konstantin Belousov wrote:

> Log:
>  Keep devfs mount locked for the whole duration of the devfs_setattr(),
>  and ensure that our dirent is instantiated.
>
>  Reported and tested by:	bde
>  Sponsored by:	The FreeBSD Foundation
>  MFC after:	1 week

Thanks.

This is part of fixing revoke(2).  Even stat() doesn't work right when
it races revoke().  setattr() is used surprisingly often since it is used
for opening with O_TRUNC.  open() racing with revoke() caused problems
doing the truncation even though truncation is a no-op for devfs.
Truncation is not atomic for opening with O_TRUNC.

Bruce