Re: acquiring duplicate lock of same type: "vnode interlock"

From: Bruce Evans (bde_at_zeta.org.au)
Date: 02/12/04

  • Next message: Andre Guibert de Bruet: "Re: nfs lockup" 
    Date: Fri, 13 Feb 2004 06:16:21 +1100 (EST)
To: Jun Kuriyama <kuriyama@imgsrc.co.jp>

    On Thu, 12 Feb 2004, Jun Kuriyama wrote:

    > Is this patch safe for locking? This may remove warnings below:

    Perhaps, but it has some style bugs.

    > Index: ffs_snapshot.c
    > ===================================================================
    > RCS file: /home/ncvs/src/sys/ufs/ffs/ffs_snapshot.c,v
    > retrieving revision 1.77
    > diff -u -r1.77 ffs_snapshot.c
    > --- ffs_snapshot.c 4 Jan 2004 04:08:34 -0000 1.77
    > +++ ffs_snapshot.c 12 Feb 2004 01:08:31 -0000
    > @@ -488,9 +488,10 @@
    > VI_LOCK(devvp);
    > snaphead = &devvp->v_rdev->si_snapshots;
    > if ((xp = TAILQ_FIRST(snaphead)) != NULL) {
    > - VI_LOCK(vp);
    > - vp->v_vnlock = ITOV(xp)->v_vnlock;
    > + struct lock *lkp = ITOV(xp)->v_vnlock;

    (1) Nested declaration.
    (2) Initialization in declaration.
    (3) No blank line after declaration.

    > VI_UNLOCK(devvp);
    > + VI_LOCK(vp);
    > + vp->v_vnlock = lkp;
    > } else {
    > struct lock *lkp;
    >

    However, (1) seems to be a normal style in this file. It is used here in
    similar code. But (2) and (3) are not used here.

    > @@ -1793,9 +1794,10 @@
    > */
    > VI_LOCK(devvp);
    > if ((xp = TAILQ_FIRST(snaphead)) != NULL) {
    > - VI_LOCK(vp);
    > - vp->v_vnlock = ITOV(xp)->v_vnlock;
    > + struct lock *lkp = ITOV(xp)->v_vnlock;
    > VI_UNLOCK(devvp);
    > + VI_LOCK(vp);
    > + vp->v_vnlock = lkp;

    As above.

    > } else {
    > struct lock *lkp;
    >

    As above.

    The lkp local is now defined nested twice, so (1) is a larger style bug
    than before; however, the functions are so large that the style bug is
    more in the other direction -- they begin with a large list of declarations
    and might benefit from more nested ones. Anyway, following ther nearby
    style is never wrong.

    Bruce
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