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

[Kostik Belousov]

On Fri, Oct 05, 2007 at 01:50:39PM -0400, John Baldwin wrote:
> 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.

Another is the vm/vnode_pager.c handling of the v_object.
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