Date: Fri, 17 Jul 2020 23:54:38 +0200 From: Marko Zec <zec@fer.hr> To: John-Mark Gurney <jmg@funkthat.com> Cc: freebsd-net@FreeBSD.org Subject: Re: IF_DRV_PREPEND unlocked? Message-ID: <20200717235438.1fee733b@x23> In-Reply-To: <20200717185609.GX4213@funkthat.com> References: <20200715232624.GR4213@funkthat.com> <20200716072622.5fa35ba2@x23> <20200716074917.04445daa@x23> <20200716185629.GT4213@funkthat.com> <20200717120311.59377e0d@x23> <20200717185609.GX4213@funkthat.com>
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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?
Marko
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