netdev - net: Detect drivers that reschedule NAPI and exhaust budget

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Message-ID: <20141220003636.GA32187@gondor.apana.org.au>
Date:	Sat, 20 Dec 2014 11:36:36 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	netdev@...r.kernel.org, xen-devel@...ts.xenproject.org,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	edumazet@...gle.com, "David S. Miller" <davem@...emloft.net>
Subject: net: Detect drivers that reschedule NAPI and exhaust budget

On Sat, Dec 20, 2014 at 11:23:27AM +1100, Herbert Xu wrote:
>
> A similar bug exists in virtio_net.

In order to detect other drivers doing this we should add something
like this.

-- >8 --
The commit d75b1ade567ffab085e8adbbdacf0092d10cd09c (net: less
interrupt masking in NAPI) required drivers to leave poll_list
empty if the entire budget is consumed.

We have already had two broken drivers so let's add a check for
this.
 
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/net/core/dev.c b/net/core/dev.c
index f411c28..88f9725 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4620,7 +4620,11 @@ static void net_rx_action(struct softirq_action *h)
 					 */
 					napi_gro_flush(n, HZ >= 1000);
 				}
-				list_add_tail(&n->poll_list, &repoll);
+				/* Some drivers may have called napi_schedule
+				 * prior to exhausting their budget.
+				 */
+				if (!WARN_ON_ONCE(!list_empty(&n->poll_list)))
+					list_add_tail(&n->poll_list, &repoll);
 			}
 		}
 
Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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