Does msodsfs_readdir() require a exclusively locked vnode

[Rick Macklem]

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?

Anyhow, I'll get pho@ to test a patch without the LK_DOWNGRADE in
it. (It was pretty useless and would go away soon anyhow, once the
lkflags argument to VFS_FHTOVP() gets used.)

Thanks for the info, rick