Date: Sat, 18 Jul 2020 10:52:43 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Marko Zec <zec@fer.hr> Cc: freebsd-net@FreeBSD.org Subject: Re: IF_DRV_PREPEND unlocked? Message-ID: <20200718175243.GZ4213@funkthat.com> In-Reply-To: <20200717235438.1fee733b@x23> References: <20200715232624.GR4213@funkthat.com> <20200716072622.5fa35ba2@x23> <20200716074917.04445daa@x23> <20200716185629.GT4213@funkthat.com> <20200717120311.59377e0d@x23> <20200717185609.GX4213@funkthat.com> <20200717235438.1fee733b@x23>
next in thread | previous in thread | raw e-mail | index | archive | help
Marko Zec wrote this message on Fri, Jul 17, 2020 at 23:54 +0200: > On Fri, 17 Jul 2020 11:56:09 -0700 > John-Mark Gurney <jmg@funkthat.com> wrote: > > Marko Zec wrote this message on Fri, Jul 17, 2020 at 12:03 +0200: > ... > > > #define IFQ_DRV_IS_EMPTY(ifq) \ > > > (((ifq)->ifq_drv_len == 0) && ((ifq)->ifq_len == 0)) > > > > > > So, if per altq(9) the contract is that with IFQ_DRV_* the ifq_drv_* > > > fields should be protected by some caller-provided mechanism, while > > > the other ifq_* fields will be implictly protected by ifq_mtx, how > > > can accessing ifw_len without holding ifq_mtx in the above example > > > be safe? > > > > Reading is safe when you aren't modifying it, and only using it to > > inform if you should recheck w/ a lock... > > > > This way a driver can do: > > if (!IFQ_DRV_IS_EMPTY(&ifp->if_snd)) { > > mtx_lock(sc->sc_mtx); > > for (;;) { > > IFQ_DRV_DEQUEUE(&ifp->if_snd, m); > > if (m == NULL) > > break; > > sendpkt(m); > > } > > mtx_unlock(sc->sc_mtx); > > } > > > > which saves an expensive lock/unlock op when there are no packets > > in the queue... > > The above snippet is fine even if IFQ_DRV_IS_EMPTY() returns 0 during a > race with another thread: per altq(9) the subsequent (then properly > locked) IFQ_DRV_DEQUEUE() might find out that the queue is actually > empty, and bail out if (m == NULL). > > But what if IFQ_DRV_IS_EMPTY() returns 1 due to a race with another > thread which has just executed IFQ_ENQUEUE() (invisible to the first > thread due to lack of synchronization), and therefore leaves the mbuf in > the queue, instead of dequeuing and processing it? This is where the _OACTIVE flag comes into play.. IFQ_HANDOFF_ADJ checks the flag, and calls if_start if it's not set.. (hmm, do we need a barrier here to ensure that enqueue completes before we check the _OACTIVE flag, as this check of the flag is unlocked?) As long the driver has not set the _OACTIVE flag, the if_start routine will be called for the enqueued packet... It's only a problem if the driver does an unlocked check for the queue and the driver has set the _OACTIVE flag, AND the driver will not do a future check of the queue, like it's suppose to when it finishes a packet TX and clears the _OACTIVE flag... -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20200718175243.GZ4213>