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Message-ID: <20150922190154.GC31679@hmsreliant.think-freely.org>
Date: Tue, 22 Sep 2015 15:01:54 -0400
From: Neil Horman <nhorman@...driver.com>
To: Neil Horman <nhorman@...hat.com>
Cc: netdev@...r.kernel.org
Subject: Re: [RHEL6.6 PATCH] netpoll: Close race condition between
poll_one_napi and napi_disable
On Tue, Sep 22, 2015 at 02:00:23PM -0400, Neil Horman wrote:
> Drivers might call napi_disable while not holding the napi instance poll_lock.
> In those instances, its possible for a race condition to exist between
> poll_one_napi and napi_disable. That is to say, poll_one_napi only tests the
> NAPI_STATE_SCHED bit to see if there is work to do during a poll, and as such
> the following may happen:
>
> CPU0 CPU1
> ndo_tx_timeout napi_poll_dev
> napi_disable poll_one_napi
> test_and_set_bit (ret 0)
> test_bit (ret 1)
> reset adapter napi_poll_routine
>
> If the adapter gets a tx timeout without a napi instance scheduled, its possible
> for the adapter to think it has exclusive access to the hardware (as the napi
> instance is now scheduled via the napi_disable call), while the netpoll code
> thinks there is simply work to do. The result is parallel hardware access
> leading to corrupt data structures in the driver, and a crash.
>
> Additionaly, there is another, more critical race between netpoll and
> napi_disable. The disabled napi state is actually identical to the scheduled
> state for a given napi instance. The implication being that, if a napi instance
> is disabled, a netconsole instance would see the napi state of the device as
> having been scheduled, and poll it, likely while the driver was dong something
> requiring exclusive access. In the case above, its fairly clear that not having
> the rings in a state ready to be polled will cause any number of crashes.
>
> The fix should be pretty easy. netpoll uses its own bit to indicate that that
> the napi instance is in a state of being serviced by netpoll (NAPI_STATE_NPSVC).
> We can just gate disabling on that bit as well as the sched bit. That should
> prevent netpoll from conducting a napi poll if we convert its set bit to a
> test_and_set_bit operation to provide mutual exclusion
>
> Signed-off-by: Neil Horman <nhorman@...driver.com>
> CC: "David S. Miller" <davem@...emloft.net>
> CC: jmaxwell@...hat.com
> Tested-by: jmaxwell@...hat.com
> ---
> include/linux/netdevice.h | 2 ++
> net/core/dev.c | 2 ++
> net/core/netpoll.c | 12 ++++++++++--
> 3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index b791405..48becac 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -507,6 +507,8 @@ static inline void napi_enable(struct napi_struct *n)
> BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
> smp_mb__before_atomic();
> clear_bit(NAPI_STATE_SCHED, &n->state);
> + clear_bit(NAPI_STATE_NPSVC, &n->state);
> +
> }
>
> #ifdef CONFIG_SMP
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ee0d628..b6b01bf 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4723,6 +4723,8 @@ void napi_disable(struct napi_struct *n)
>
> while (test_and_set_bit(NAPI_STATE_SCHED, &n->state))
> msleep(1);
> + while (test_and_set_bit(NAPI_STATE_NPSVC, &n->state))
> + msleep(1);
>
> hrtimer_cancel(&n->timer);
>
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 6aa3db8..91cf217 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -142,7 +142,7 @@ static void queue_process(struct work_struct *work)
> */
> static int poll_one_napi(struct napi_struct *napi, int budget)
> {
> - int work;
> + int work = 0;
>
> /* net_rx_action's ->poll() invocations and our's are
> * synchronized by this test which is only made while
> @@ -151,7 +151,14 @@ static int poll_one_napi(struct napi_struct *napi, int budget)
> if (!test_bit(NAPI_STATE_SCHED, &napi->state))
> return budget;
>
> - set_bit(NAPI_STATE_NPSVC, &napi->state);
> + /*
> + * If we set this bit but see that it has already been set,
> + * that indicates that napi has been disabled and we need
> + * to abort this operation
> + */
> +
> + if(test_and_set_bit(NAPI_STATE_NPSVC, &napi->state))
> + goto out;
>
> work = napi->poll(napi, budget);
> WARN_ONCE(work > budget, "%pF exceeded budget in poll\n", napi->poll);
> @@ -159,6 +166,7 @@ static int poll_one_napi(struct napi_struct *napi, int budget)
>
> clear_bit(NAPI_STATE_NPSVC, &napi->state);
>
> +out:
> return budget - work;
> }
>
> --
> 2.1.0
>
> --
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>
Shoot, I forgot to change my subject prefix, sorry about that. Dave this applies
to net-next, shall I resubmit with a proper prefix, or are you good with it as
is?
Neil
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