On Fri, Jun 08, 2018 at 02:30:10PM -0400, Mark Johnston wrote: > On Fri, Jun 08, 2018 at 08:37:55PM +0300, Konstantin Belousov wrote: > > On Thu, Jun 07, 2018 at 11:02:29PM -0700, Ryan Libby wrote: > > > On Thu, Jun 7, 2018 at 10:03 PM, Mateusz Guzik <mjgu...@gmail.com> wrote: > > > > Checking it without any locks is perfectly valid in this case. It is > > > > done > > > > after v_holdcnt gets bumped from a non-zero value. So at that time it > > > > is at least two. Of course that result is stale as an arbitrary number > > > > of > > > > other threads could have bumped and dropped the ref past that point. > > > > The minimum value is 1 since we hold the ref. But this means the > > > > vnode must not be on the free list and that's what the assertion is > > > > verifying. > > > > > > > > The problem is indeed lack of ordering against the code clearing the > > > > flag for the case where 2 threads to vhold and one does the 0->1 > > > > transition. > > > > > > > > That said, the fence is required for the assertion to work. > > > > > > > > > > Yeah, I agree with this logic. What I mean is that reordering between > > > v_holdcnt 0->1 and v_iflag is normally settled by the release and > > > acquisition of the vnode interlock, which we are supposed to hold for > > > v_*i*flag. A quick scan seems to show all of the checks of VI_FREE that > > > are not asserts do hold the vnode interlock. So, I'm just saying that I > > > don't think the possible reordering affects them. > > But do we know that only VI_FREE checks are affected ? > > > > My concern is that users of _vhold() rely on seeing up to date state of the > > vnode, and VI_FREE is only an example of the problem. Most likely, the > > code which fetched the vnode pointer before _vhold() call, should guarantee > > visibility. > > Wouldn't this be a problem only if we permit lockless accesses of vnode > state outside of _vhold() and other vnode subroutines? The current > protocol requires that the interlock be held, and this synchronizes with > code which performs 0->1 and 1->0 transitions of the hold count. If this > requirement is relaxed in the future, then fences would indeed be > needed.
I do not claim that my concern is a real problem. I stated it as a thing to look at when deciding whether the fences should be added (unconditionally ?). If you argument is that the only current lock-less protocol for the struct vnode state is the v_holdcnt transitions for > 1, then I can agree with it. _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
