diff mbox

[v2] netpoll: Close race condition between poll_one_napi and napi_disable

Message ID 1442952096-31430-1-git-send-email-nhorman@redhat.com
State Superseded, archived
Delegated to: David Miller
Headers show

Commit Message

Neil Horman Sept. 22, 2015, 8:01 p.m. UTC 
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

Change notes:
V2)
	Remove a trailing whtiespace
	Resubmit with proper subject prefix

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: jmaxwell@redhat.com
Tested-by: jmaxwell@redhat.com
---
 include/linux/netdevice.h |  2 ++
 net/core/dev.c            |  2 ++
 net/core/netpoll.c        | 12 ++++++++++--
 3 files changed, 14 insertions(+), 2 deletions(-)

Comments

Stephen Hemminger Sept. 22, 2015, 10:09 p.m. UTC | #1 
On Tue, 22 Sep 2015 16:01:36 -0400
Neil Horman <nhorman@redhat.com> wrote:

> +	clear_bit(NAPI_STATE_NPSVC, &n->state);
> +
>  }
why introduce extra line here?

> +	/*
> +	 * 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))

And why introduce line after comment before code.
My preference is to have comment as close to code as possible.
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Neil Horman Sept. 23, 2015, 10:39 a.m. UTC | #2 
On Tue, Sep 22, 2015 at 03:09:20PM -0700, Stephen Hemminger wrote:
> On Tue, 22 Sep 2015 16:01:36 -0400
> Neil Horman <nhorman@redhat.com> wrote:
> 
> > +	clear_bit(NAPI_STATE_NPSVC, &n->state);
> > +
> >  }
> why introduce extra line here?
> 
> > +	/*
> > +	 * 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))
> 
> And why introduce line after comment before code.
> My preference is to have comment as close to code as possible.

Mine is to add spaces, as I feel the code is more readable if its a bit more
spread out.

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David Miller Sept. 23, 2015, 6:03 p.m. UTC | #3 
From: Neil Horman <nhorman@redhat.com>
Date: Tue, 22 Sep 2015 16:01:36 -0400

> -	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;
>  

Networking comments should be styled:

	/* Like
	 * this.
	 */

Also you need a space after the "if" and before the openning parenthesis.

And like Stephen I think the empty line between the comment and the
if() statement is superfluous and eats up precious vertical space on
the screen :-)

Thanks.
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diff mbox

Patch

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..9312b66 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;
 }