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Date: Tue, 22 Sep 2015 12:46:13 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Neil Horman <nhorman@...driver.com>,
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 09/22/2015 12:01 PM, Neil Horman wrote:
> 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
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe netdev" in
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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?
It looks like this patch introduces some white-space errors as well.
The comment block has one trailing white space, and 4 spaces before
tabs. You might want to resubmit with that fixed.
- Alex
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