linux-kernel - Re: [PATCH v5 1/1] blktrace: switch trace spinlock to a raw spinlock

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Message-ID: <76333783-cb3a-d1b7-5e40-d07014c4e2c0@kernel.dk>
Date:   Mon, 20 Dec 2021 13:49:47 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Wander Costa <wcosta@...hat.com>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        open list <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v5 1/1] blktrace: switch trace spinlock to a raw spinlock

On 12/20/21 1:43 PM, Wander Costa wrote:
> On Mon, Dec 20, 2021 at 5:37 PM Jens Axboe <axboe@...nel.dk> wrote:
>>
>> On 12/20/21 1:34 PM, Wander Costa wrote:
>>> On Mon, Dec 20, 2021 at 5:24 PM Jens Axboe <axboe@...nel.dk> wrote:
>>>>
>>>> On 12/20/21 12:49 PM, Wander Costa wrote:
>>>>> On Mon, Dec 20, 2021 at 4:38 PM Jens Axboe <axboe@...nel.dk> wrote:
>>>>>>
>>>>>> On 12/20/21 12:28 PM, Wander Lairson Costa wrote:
>>>>>>> The running_trace_lock protects running_trace_list and is acquired
>>>>>>> within the tracepoint which implies disabled preemption. The spinlock_t
>>>>>>> typed lock can not be acquired with disabled preemption on PREEMPT_RT
>>>>>>> because it becomes a sleeping lock.
>>>>>>> The runtime of the tracepoint depends on the number of entries in
>>>>>>> running_trace_list and has no limit. The blk-tracer is considered debug
>>>>>>> code and higher latencies here are okay.
>>>>>>
>>>>>> You didn't put a changelog in here. Was this one actually compiled? Was
>>>>>> it runtime tested?
>>>>>
>>>>> It feels like the changelog reached the inboxes after patch (at least
>>>>> mine was so). Would you like that I send a v6 in the hope things
>>>>> arrive in order?
>>>>
>>>> Not sure how you are sending them, but they don't appear to thread
>>>> properly. But the changelog in the cover letter isn't really a
>>>> changelog, it doesn't say what changed.
>>>>
>>>
>>> Sorry, I think I was too brief in my explanation. I am backporting
>>> this patch to the RHEL 9 kernel (which runs kernel 5.14). I mistakenly
>>> generated the v4 patch from that tree, but it lacks this piece
>>>
>>> @@ -1608,9 +1608,9 @@ static int blk_trace_remove_queue(struct request_queue *q)
>>>
>>>         if (bt->trace_state == Blktrace_running) {
>>>                 bt->trace_state = Blktrace_stopped;
>>> -               spin_lock_irq(&running_trace_lock);
>>> +               raw_spin_lock_irq(&running_trace_lock);
>>>                 list_del_init(&bt->running_list);
>>> -               spin_unlock_irq(&running_trace_lock);
>>> +               raw_spin_unlock_irq(&running_trace_lock);
>>>                 relay_flush(bt->rchan);
>>>         }
>>>
>>> Causing the build error. v5 adds that. Sorry again for the confusion.
>>
>> Right, that's why I asked if a) you had even built this patch, and b) if
>> you had tested it as well.
>>
> 
> Yes, I had. But I had two versions of it. One for RHEL and one for
> torvalds/master. I just picked the wrong branch when generating it.
> I apologize for the mess once more.

Alright, fair enough, mistakes happen. I think the patch looks fine, my
main dislike is that it's Yet Another things that needs special RT
handling. But I guess that's how it is...

-- 
Jens Axboe