netdev - Re: [PATCH] net: stop endless flood about dst entry refcount underflow or overflow

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Message-ID: <55A4FD76.7050606@yandex-team.ru>
Date:	Tue, 14 Jul 2015 15:15:50 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] net: stop endless flood about dst entry refcount underflow
 or overflow

On 14.07.2015 15:04, Eric Dumazet wrote:
> On Tue, 2015-07-14 at 14:43 +0300, Konstantin Khlebnikov wrote:
>> Kernel generates a lot of warnings when dst entry reference counter
>> overflows and becomes negative. This patch prints address of dst entry,
>> its refcount and then resets reference counter to INT_MAX/2.
>>
>> That bug was seen several times at machines with outdated 3.10.y kernels.
>> Most like it's already fixed in upstream. Anyway flood of that warnings
>> completely kills machine and makes further debugging impossible.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>> ---
>>   net/core/dst.c |    3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/core/dst.c b/net/core/dst.c
>> index e956ce6d1378..2ed91082b3cf 100644
>> --- a/net/core/dst.c
>> +++ b/net/core/dst.c
>> @@ -284,7 +284,8 @@ void dst_release(struct dst_entry *dst)
>>   		int newrefcnt;
>>
>>   		newrefcnt = atomic_dec_return(&dst->__refcnt);
>> -		WARN_ON(newrefcnt < 0);
>> +		if (WARN(newrefcnt < 0, "dst: %p refcnt: %d\n", dst, newrefcnt))
>> +			atomic_set(&dst->__refcnt, INT_MAX / 2);
>>   		if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)
>>   			call_rcu(&dst->rcu_head, dst_destroy_rcu);
>>   	}
>
>
> WARN_ON_ONCE() if you want, but setting __refcnt like this is absolutely
> a dirty hack.

Simple warn-once will hide a lot of information which could be useful.
Also dst entry leak is better than freeing actually active entry.

>
>
>


-- 
Konstantin
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