Date:	Tue, 22 Sep 2015 15:57:49 -0400
From:	Neil Horman <nhorman@...hat.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Neil Horman <nhorman@...driver.com>, 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 12:46:13PM -0700, Alexander Duyck wrote:
> 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
> >>the body of a message to majordomo@...r.kernel.org
> >>More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>
> >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.
> 
 I don't think so.  Its got one trailing space that I see, you're right about
that, but the only leading white space is directly in front of the astersks,
which is necessecary for alignment, its identical to other simmilar comments in
the file. 

No matter though, I'll just repost it at this point
Neil

> - Alex
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