On Fri, Apr 16, 2010 at 02:12:01PM -0700, Matthew Fleming wrote: > > -----Original Message----- > > From: Kostik Belousov [mailto:[email protected]] > > Sent: Friday, April 16, 2010 1:41 PM > > To: Matthew Fleming > > Cc: [email protected] > > Subject: Re: panic in vget() > > > > On Fri, Apr 16, 2010 at 01:23:17PM -0700, Matthew Fleming wrote: > > > I'm looking at this panic in vget() on stable/7: > > > > > > if (vp->v_iflag & VI_DOOMED && (flags & LK_RETRY) == 0) > > > panic("vget: vn_lock failed to return ENOENT\n"); > > > > > > It seems to me that this is not a correct assertion, because if the > > > caller passed in no lock flags (i.e. just checking the vnode for > > > validity) then there is a window between the VI_UNLOCK() in > > > _vn_lock(9) and the subsequent VI_LOCK() in vget() where another > > > thread could have set VI_DOOMED. > > > > > > This isn't a problem on CURRENT because the code has been changed to > > > not allow an empty lock flags. > > > > > > I believe the following is a potential fix is: > > > > > > vholdl(vp); > > > if ((error = vn_lock(vp, flags | LK_INTERLOCK, td)) != 0) { > > > vdrop(vp); > > > return (error); > > > } > > > VI_LOCK(vp); > > > + /* > > > + * Deal with a timing window when the interlock is not held > > > + * and VI_DOOMED can be set, since we only have a holdcnt, > > > + * not a usecount. > > > + */ > > > + if (vp->v_iflag & VI_DOOMED && (flags & LK_RETRY) == 0) { > > > + KASSERT((flags & LK_TYPE_MASK) == 0, ("Unexpected flags > > > %x", flags)); > > > + vdropl(vp); > > > + return (ENOENT); > > > + } > > > /* Upgrade our holdcnt to a usecount. */ > > > v_upgrade_usecount(vp); > > > - if (vp->v_iflag & VI_DOOMED && (flags & LK_RETRY) == 0) > > > - panic("vget: vn_lock failed to return ENOENT\n"); > > > if (oweinact) { > > > if (vp->v_iflag & VI_OWEINACT) > > > vinactive(vp, td); > > > VI_UNLOCK(vp); > > > if ((oldflags & LK_TYPE_MASK) == 0) > > > > Both the analysis and the patch look good. > > > > Did you considered locking the vnode even when no locking flags were > > given, as is done for VI_OWEINACT handling ? Your solution is better, > esp. > > for old lockmgr, but acquiring vnode lock might be safer. > > For our systems, the vnode lock is distributed across the entire > cluster, so we prefer not to take it unless required. The code path > that produced this panic is one such; it is using other mechanisms to > guarantee the data is correct. Ok.
> > (As a side note, splitting the vnode lock into a lock on the vnode > struct and a "file" lock would be really great, since the VOP_LOCK uses > seem split between serializing the file contents and serializing some of > the members of struct vnode itself, and we only need a distributed lock > for the file contents). Shameless plug: did you see http://wiki.freebsd.org/VM6 ?
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