Did this issue get resolved? On 6/8/2018 11:37 AM, Konstantin Belousov wrote: > 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. >
-- Regards, Bryan Drewery
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