cvs commit: src/sys/fs/devfs devfs_vnops.c src/sys/fs/fifofs fifo_vnops.c src/sys/kern uipc_usrreq.c vfs_vnops.c src/sys/vm vnode_pager.c

[John Baldwin]

On Thursday 04 October 2007 09:29:25 pm Jeff Roberson wrote:
> On Thu, 4 Oct 2007, John Baldwin wrote:
> 
> > On Wednesday 03 October 2007 07:48:00 pm Jeff Roberson wrote:
> >> On Wed, 3 Oct 2007, John Baldwin wrote:
> >>
> >>> jhb         2007-10-03 21:06:05 UTC
> >>>
> >>>  FreeBSD src repository
> >>>
> >>>  Modified files:        (Branch: RELENG_6)
> >>>    sys/fs/devfs         devfs_vnops.c
> >>>    sys/fs/fifofs        fifo_vnops.c
> >>>    sys/kern             uipc_usrreq.c vfs_vnops.c
> >>>    sys/vm               vnode_pager.c
> >>>  Log:
> >>>  MFC: Always use an exclusive lock on the leaf vnode during an open() 
when
> >>>  shared lookups are enabled.  This closes a few races including a race
> > where
> >>>  concurrent opens of a fifo could result in different v_fifoinfo
> > structures
> >>>  in different threads.
> >>
> >> Long term we should really look for a better solution to this problem.
> >> There are a number of was to improve snapshots in ffs by fixing shared
> >> locking.
> >
> > I don't disagree.  The fifo case can be fixed easily enough in the fifo 
code
> > by using fifo_mtx to protect v_fifoinfo perhaps (or doing an upgrade on 
the
> > vnode lock?), but for the MFC I didn't want to have to fix each of the 
races
> > with open(2).  Probably better to fix it more properly in HEAD first.
> 
> Definitely someting for head.  Were there any others that you ran into 
> besides v_fifoinfo?  We should audit this more closely anyhow.  I have 
> been reluctant to push too much shared locking into VFS because it's not 
> been so carefully studied.

I just saw v_fifoinfo, but Pawel's original commit referenced updates to
v_writecount, etc.  The v_writecount one is in vn_open() itself:

        if ((error = VOP_OPEN(vp, fmode, cred, td, fp)) != 0)
                goto bad;

        if (fmode & FWRITE)
                vp->v_writecount++;
        *flagp = fmode;
        ASSERT_VOP_ELOCKED(vp, "vn_open_cred");
        if (!mpsafe)
                VFS_UNLOCK_GIANT(vfslocked);
        return (0);

If you just held a shared lock there, you could use atomic ops for 
vp->v_writecount (and still hold at least a shared vnode lock everywhere 
v_writecount is updated) and still be able to read vp->v_writecount safely 
while holding an exclusive lock on the vnode.

-- 
John Baldwin