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Re: [dm-devel] [PATCH] block: add missing block_bio_complete() tracepoint


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


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.


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