linux-kernel - Re: [PATCH] block: add missing block_bio

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Date:	Tue, 31 Jan 2012 15:30:16 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	dm-devel@...hat.com
Subject: Re: [PATCH] block: add missing block_bio_complete() tracepoint

Hi,

2012-01-31 2:05 AM, Tejun Heo wrote:
> On Mon, Jan 30, 2012 at 03:38:57PM +0900, Namhyung Kim wrote:
>> 2012-01-29 6:41 PM, Namhyung Kim wrote:
>>> The block_bio_complete() TP has been missed so long, so that bio-based
>>> drivers haven't been able to trace its IO behavior. Add it.
>>>
>>> In some rare cases, such as loop_switch, @bio->bi_bdev can be NULL.
>>> Thus convert it to TRACE_EVENT_CONDITION() as Steven suggested.
>>>
>>
>> Now I see that it seems TRACE_EVENT_CONDITION() can protect event
>> tracing from such condition, but what about other users of the TP
>> like blktrace? I think it'll still get NULL pointer dereference on
>> bdev_get_queue() after the change, right? If so, convert to T_E_C()
>> looks meaningless IMHO. Do I miss something?
>
> Not really following, but the whole point of using
> TRACE_EVENT_CONDITION() is avoiding the conditional jump when the TP
> is disabled.  Whether the TP users need to test again / more isn't too
> important.
>
> Thanks.
>

Right, but the point is it could make a NULL pointer dereference during 
evaluation of the argument of the TP AFAICS. I'm not sure about the TP 
implementation though, I think I was wrong - T_E_C() cannot protect us 
from it because it happens just before jumping to the TP, right?

So I think we need a conditional jump (with the "likely" annotation) for 
this even when the TP is disabled.

Thanks,
Namhyung

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