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
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