On Wed, 18 Nov 2020 09:57:30 +0800 Yunsheng Lin wrote:

On 2020/11/3 3:41, Jakub Kicinski wrote:

quoted

On Mon, 2 Nov 2020 11:14:32 +0800 Yunsheng Lin wrote:  

quoted

On 2020/11/1 6:38, Jakub Kicinski wrote:  

quoted

On Thu, 29 Oct 2020 19:34:48 +0800 Yunsheng Lin wrote:    

quoted

The current semantic for napi_consume_skb() is that caller need
to provide non-zero budget when calling from NAPI context, and
breaking this semantic will cause hard to debug problem, because
_kfree_skb_defer() need to run in atomic context in order to push
the skb to the particular cpu' napi_alloc_cache atomically.

So add a in_softirq() debug checking in napi_consume_skb() to catch
this kind of error.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yunsheng Lin <redacted>    

quoted

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1ba8f01..1834007 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -897,6 +897,10 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
 		return;
 	}
 
+	DEBUG_NET_WARN(!in_softirq(),
+		       "%s is called with non-zero budget outside softirq context.\n",
+		       __func__);    

Can't we use lockdep instead of defining our own knobs?    

From the first look, using the below seems better than defining our
own knobs, because there is similar lockdep_assert_in_irq() checking
already and lockdep_assert_in_*() is NULL-OP when CONFIG_PROVE_LOCKING
is not defined.
 

quoted

Like this maybe?

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index f5594879175a..5253a167d00c 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h

@@ -594,6 +594,14 @@ do {                                                                       \
                      this_cpu_read(hardirqs_enabled)));                \
 } while (0)
 
+#define lockdep_assert_in_softirq()                                    \
+do {                                                                   \
+       WARN_ON_ONCE(__lockdep_enabled                  &&              \
+                    (softirq_count() == 0              ||              \
+                     this_cpu_read(hardirq_context)));                 \    

Using in_softirq() seems more obvious then using softirq_count()?
And there is below comment above avoiding the using of in_softirq(), maybe
that is why you use softirq_count() directly here?
"softirq_count() == 0" still mean we are not in the SoftIRQ context and
BH is not disabled. right? Perhap lockdep_assert_in_softirq_or_bh_disabled()
is more obvious?  

Let's add Peter to the recipients to get his opinion.

We have a per-cpu resource which is also accessed from BH (see
_kfree_skb_defer()).

We're trying to come up with the correct lockdep incantation for it.  

Hi, Peter
	Any suggestion?

Let's just try lockdep_assert_in_softirq() and see if anyone complains.
People are more likely to respond to a patch than a discussion.

quoted

quoted

/*
 * Are we doing bottom half or hardware interrupt processing?
 *
 * in_irq()       - We're in (hard) IRQ context
 * in_softirq()   - We have BH disabled, or are processing softirqs
 * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
 * in_serving_softirq() - We're in softirq context
 * in_nmi()       - We're in NMI context
 * in_task()	  - We're in task context
 *
 * Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really
 *       should not be used in new code.
 */


Also, is there any particular reason we do the "this_cpu_read(hardirq_context)"
checking?  

Accessing BH resources from a hard IRQ context would be a bug, we may
have interrupted a BH, so AFAIU softirq_count() != 0, but we can nest
calls to _kfree_skb_defer().  

In that case, maybe just call lockdep_assert_in_irq() is enough?

TBH the last sentence I wrote isn't clear even to me at this point ;D

Maybe using just the macros from preempt.h - like this?

#define lockdep_assert_in_softirq()                                    \
do {                                                                   \
       WARN_ON_ONCE(__lockdep_enabled                  &&              \
                    (!in_softirq() || in_irq() || in_nmi())	\
} while (0)

We know what we're doing so in_softirq() should be fine (famous last
words).