Deadlock between nfsd and snapshots.

[Tor Egge]

I wrote:

> The deadlock indicates that one or more of IN_CHANGE, IN_MODIFIED or
> IN_UPDATE was set on the inode, indicating a write operation
> (e.g. VOP_WRITE(), VOP_RENAME(), VOP_CREATE(), VOP_REMOVE(), VOP_LINK(),
> VOP_SYMLINK(), VOP_SETATTR(), VOP_MKDIR(), VOP_RMDIR(), VOP_MKNOD()) that was
> not protected by vn_start_write() or vn_start_secondary_write().

The most common "write" operation was probably VOP_GETATTR().

ufs_itimes(), called from ufs_getattr(), might set the IN_MODIFIED inode flag
if IN_ACCESS is set on the inode even if neither IN_CHANGE nor IN_UPDATE is
set, transitioning the inode flags into a state where ufs_inactive() calls the
blocking variant of vn_start_secondary_write().

calling ufs_itimes() with only a shared vnode lock might cause unsafe accesses
to the inode flags.  Setting of IN_ACCESS at the end of ffs_read() and
ffs_extread() might also be unsafe.  If DIRECTIO is enabled then O_DIRECT reads
might not even attempt to set the IN_ACCESS flag.

- Tor Egge