linux-kernel - Re: [PATCH] lib/dynamic_queue_limits.c: relax BUG_ON to WARN_ON in dql

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Date:   Wed, 18 Oct 2017 18:57:26 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] lib/dynamic_queue_limits.c: relax BUG_ON to WARN_ON in dql_complete()

On 18 October 2017 at 17:29, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-10-18 at 16:45 +0100, Ard Biesheuvel wrote:
>> Even though calling dql_completed() with a count that exceeds the
>> queued count is a serious error, it still does not justify bringing
>> down the entire kernel with a BUG_ON(). So relax it to a WARN_ON()
>> instead.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> ---
>>  lib/dynamic_queue_limits.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/dynamic_queue_limits.c b/lib/dynamic_queue_limits.c
>> index f346715e2255..24ce495d78f3 100644
>> --- a/lib/dynamic_queue_limits.c
>> +++ b/lib/dynamic_queue_limits.c
>> @@ -23,7 +23,7 @@ void dql_completed(struct dql *dql, unsigned int count)
>>       num_queued = ACCESS_ONCE(dql->num_queued);
>>
>>       /* Can't complete more than what's in queue */
>> -     BUG_ON(count > num_queued - dql->num_completed);
>> +     WARN_ON(count > num_queued - dql->num_completed);
>>
>>       completed = dql->num_completed + count;
>>       limit = dql->limit;
>
> So instead fixing the faulty driver, you'll have strange lockups, and
> force your users to reboot anyway, after annoying periods where
> "Internet does not work"
>
> These kinds of errors should be found when testing a new device driver
> or new kernel.
>
> Have you found the root cause ?
>

Not yet, and I don't intend to send out any patches for this
particular hardware until this is fixed.

But that still doesn't mean you should crash hard. As Linus puts it,
it is better to 'limp on' if you can (unless we're likely to corrupt
any non-volatile data, e.g., files on disk etc)