Does msodsfs_readdir() require a exclusively locked vnode

[John Baldwin]

On Tuesday, July 26, 2011 9:56:37 pm Attilio Rao wrote:
> 2011/7/26 Kostik Belousov <kostikbel at gmail.com>:
> > On Tue, Jul 26, 2011 at 10:07:28AM -0400, Rick Macklem wrote:
> >> Kostik Belousov wrote:
> >> > On Mon, Jul 25, 2011 at 07:22:40PM -0400, Rick Macklem wrote:
> >> > > Hi,
> >> > >
> >> > > Currently both NFS servers set the vnode lock LK_SHARED
> >> > > and so do the local syscalls (at least that's how it looks
> >> > > by inspection?).
> >> > >
> >> > > Peter Holm just posted me this panic, where a test for an
> >> > > exclusive vnode lock fails in msdosfs_readdir().
> >> > > KDB: stack backtrace:
> >> > > 
db_trace_self_wrapper(c0efa6f6,c71627f8,c79230b0,c0f2ef29,f19154b8,...)
> >> > > at db_trace_self_wrapper+0x26
> >> > > kdb_backtrace(c7f20b38,f19154fc,c0d586d5,f191550c,c7f20ae0,...) at
> >> > > kdb_backtrace+0x2a
> >> > > vfs_badlock(c101b180,f191550c,c1055580,c7f20ae0) at vfs_badlock+0x23
> >> > > assert_vop_elocked(c7f20ae0,c0ee5f4f,c09f3213,8,0,...) at
> >> > > assert_vop_elocked+0x55
> >> > > pcbmap(c7966e00,0,f191560c,f1915618,f191561c,...) at pcbmap+0x45
> >> > > msdosfs_readdir(f1915960,c0f4b343,c7f20ae0,f1915940,0,...) at
> >> > > msdosfs_readdir+0x528
> >> > > VOP_READDIR_APV(c101b180,f1915960,2,f1915a68,c7923000,...) at
> >> > > VOP_READDIR_APV+0xc5
> >> > > nfsrvd_readdir(f1915b64,0,c7f20ae0,c7923000,f1915a68,...) at
> >> > > nfsrvd_readdir+0x38e
> >> > > nfsrvd_dorpc(f1915b64,0,c7923000,c842a200,4,...) at
> >> > > nfsrvd_dorpc+0x1f79
> >> > > nfssvc_program(c7793800,c842a200,c0f24d67,492,0,...) at
> >> > > nfssvc_program+0x40f
> >> > > svc_run_internal(f1915d14,c09d9a98,c73dfa80,f1915d28,c0ef1130,...)
> >> > > at svc_run_internal+0x952
> >> > > svc_thread_start(c73dfa80,f1915d28,c0ef1130,3a5,c7e4b2c0,...) at
> >> > > svc_thread_start+0x10
> >> > > fork_exit(c0bed7d0,c73dfa80,f1915d28) at fork_exit+0xb8
> >> > > fork_trampoline() at fork_trampoline+0x8
> >> > > --- trap 0x804c12e, eip = 0xc, esp = 0x33, ebp = 0x1 ---
> >> > > pcbmap: 0xc7f20ae0 is not exclusive locked but should be
> >> > > KDB: enter: lock violation
> >> > >
> >> > > So, does anyone know if the msdosfs_readdir() really requires a
> >> > > LK_EXCLUSIVE
> >> > > locked vnode or is the ASSERT_VOP_ELOCKED() too strong in pcbmap()?
> >> >
> >> > Yes, msdosfs currently requires all vnode locks to be exclusive. One
> >> > of
> >> > the reasons is that each denode (the msdosfs-private vnode data)
> >> > carries
> >> > the fat entries cache, and this cache is updated even by the
> >> > operations
> >> > that do not modify vnode from the VFS POV.
> >> >
> >> > The locking regime is enforced by the getnewvnode() initializing the
> >> > vnode
> >> > lock with LK_NOSHARE flag, and msdosfs code not calling
> >> > VN_LOCK_ASHARE()
> >> > on the newly instantiated vnode.
> >> >
> >> > My question is, was the vnode in question locked at all ?
> >> I think the problem is that I do a LK_DOWNGRADE. From a quick
> >> look at __lockmgr_args(), it doesn't check LK_NOSHARE for a
> >> LK_DOWNGRADE.
> >>
> >> Maybe __lockmgr_args() should have something like:
> >>    if (op == LK_DOWNGRADE && (lk->lock_object.lo_flags & LK_NOSHARE))
> >>         return (0);   /* noop */
> >> after the
> >>    if (op == LK_SHARED && (lk->lock_object.lo_flags & LK_NOSHARE))
> >>         op = LK_EXCLUSIVE;
> >> lines?
> > The RELENG_7 lockmgr does not check the NOSHARE flag on downgrade,
> > but I agree with the essence of your proposal.
> 
> As long as the difference in semantic with the old lockmgr is
> correctly stressed out in the doc (and eventually comments) I'm fine
> with this change.

I think it is a bug in the LK_NOSHARE implementation if the old lockmgr()
didn't silently nop downgrade requests when LK_NOSHARE was set. :)  We
should definitely fix it to ignore downgrades for LK_NOSHARE.

-- 
John Baldwin