Date: Thu, 19 Jul 2018 12:09:21 -0700 From: Bryan Drewery <bdrewery@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com>, Mark Johnston <markj@freebsd.org>, Ryan Libby <rlibby@freebsd.org>, Mateusz Guzik <mjguzik@gmail.com>, Justin Hibbits <jhibbits@freebsd.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r334708 - head/sys/kern Message-ID: <0b128417-7107-5090-e65a-afa94fd1aed6@FreeBSD.org> In-Reply-To: <20180608183732.GK2450@kib.kiev.ua> References: <201806061257.w56CvCwq089369@repo.freebsd.org> <20180606140311.GU2450@kib.kiev.ua> <20180608033242.GA54099@pesky> <CAHgpiFyOQf6B3oGFGMz3avXqrrP0i6Puy9HqLER1XG5xE67BeQ@mail.gmail.com> <CAGudoHENGpCxn_omxfaRLOAH5fP6qdFcmmqZ7He%2BpcC=-1HFKQ@mail.gmail.com> <CAHgpiFzMQjHfnQLQrWc86FSxB2veHZeAc44qmROkaJugpGoU=g@mail.gmail.com> <20180608173755.GJ2450@kib.kiev.ua> <20180608183010.GC65388@pesky> <20180608183732.GK2450@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --Ihlwrz2hwW8P4i4TIUbx990WlXF6P2iuy Content-Type: multipart/mixed; boundary="gSEzCqGhF3w2oEwSXkvyFr5cqiusmgFTR"; protected-headers="v1" From: Bryan Drewery <bdrewery@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com>, Mark Johnston <markj@freebsd.org>, Ryan Libby <rlibby@freebsd.org>, Mateusz Guzik <mjguzik@gmail.com>, Justin Hibbits <jhibbits@freebsd.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Message-ID: <0b128417-7107-5090-e65a-afa94fd1aed6@FreeBSD.org> Subject: Re: svn commit: r334708 - head/sys/kern References: <201806061257.w56CvCwq089369@repo.freebsd.org> <20180606140311.GU2450@kib.kiev.ua> <20180608033242.GA54099@pesky> <CAHgpiFyOQf6B3oGFGMz3avXqrrP0i6Puy9HqLER1XG5xE67BeQ@mail.gmail.com> <CAGudoHENGpCxn_omxfaRLOAH5fP6qdFcmmqZ7He+pcC=-1HFKQ@mail.gmail.com> <CAHgpiFzMQjHfnQLQrWc86FSxB2veHZeAc44qmROkaJugpGoU=g@mail.gmail.com> <20180608173755.GJ2450@kib.kiev.ua> <20180608183010.GC65388@pesky> <20180608183732.GK2450@kib.kiev.ua> In-Reply-To: <20180608183732.GK2450@kib.kiev.ua> --gSEzCqGhF3w2oEwSXkvyFr5cqiusmgFTR Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable 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 <mjguzik@gmail.com> w= rote: >>>>> Checking it without any locks is perfectly valid in this case. It i= s 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 num= ber of >>>>> other threads could have bumped and dropped the ref past that point= =2E >>>>> 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 th= e >>>>> 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 betwe= en >>>> 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 fo= r >>>> 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 th= at 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, t= he >>> code which fetched the vnode pointer before _vhold() call, should gua= rantee >>> visibility. >> >> Wouldn't this be a problem only if we permit lockless accesses of vnod= e >> state outside of _vhold() and other vnode subroutines? The current >> protocol requires that the interlock be held, and this synchronizes wi= th >> code which performs 0->1 and 1->0 transitions of the hold count. If th= is >> requirement is relaxed in the future, then fences would indeed be >> needed. >=20 > 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 ?). >=20 > 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. >=20 --=20 Regards, Bryan Drewery --gSEzCqGhF3w2oEwSXkvyFr5cqiusmgFTR-- --Ihlwrz2hwW8P4i4TIUbx990WlXF6P2iuy Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQGTBAEBCgB9FiEE+Rc8ssOq6npcih8JNddxu25Gl88FAltQ4eFfFIAAAAAALgAo aXNzdWVyLWZwckBub3RhdGlvbnMub3BlbnBncC5maWZ0aGhvcnNlbWFuLm5ldEY5 MTczQ0IyQzNBQUVBN0E1QzhBMUYwOTM1RDc3MUJCNkU0Njk3Q0YACgkQNddxu25G l8/FSwgAiaerDXHKcN3mkQPl+OjXxVlRPv0CDTsCivMjkB7bF4VXA9+SOHxK0vWt WTF65/+ypXSc1yNe66/rTd3WChArt2wIXNHRWIUcuGgJVUmClAOeYxKEJcCsm7+8 Je7Lku9NjHPF+C+LUmqGQZp3UTIgtl98nSExG6NnYH7P33mnnXpe+7lNVgmPaHxR 4XhNeklecwwLNU3HrZ9xW40GHrsF42taRPHdnRFGGhC3R9cRVafKSRpmZH4BAiMh p8eNaWf+T7PS7oUrLNtnEashbECUWHhYttKWT27SrYtfvEUGFzRPuoWh0Cp68CMB gHoxu0eTF9DXPEm6+rXh5Ub7hQVtOA== =Q/A4 -----END PGP SIGNATURE----- --Ihlwrz2hwW8P4i4TIUbx990WlXF6P2iuy--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?0b128417-7107-5090-e65a-afa94fd1aed6>