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

[Namhyung Kim]

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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