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Re: [PATCH 1/1] stp: rt: replace spin_lock with stp style lock and use STP_ALLOC_FLAGS
- From: David Smith <dsmith at redhat dot com>
- To: yzhu1 <Yanjun dot Zhu at windriver dot com>, systemtap at sourceware dot org
- Date: Mon, 26 Oct 2015 16:28:00 -0500
- Subject: Re: [PATCH 1/1] stp: rt: replace spin_lock with stp style lock and use STP_ALLOC_FLAGS
- Authentication-results: sourceware.org; auth=none
- References: <1445499965-23777-1-git-send-email-yanjun dot zhu at windriver dot com> <56290FFC dot 3030407 at redhat dot com> <562D9BAA dot 7040704 at windriver dot com>
On 10/25/2015 10:19 PM, yzhu1 wrote:
> Hi, David
>
> I do not have this backtrace. Because STP_ALLOC_SLEEP_FLAGS will cause
> kmalloc sleep,
> so I replaced it with STP_ALLOC_FLAGS.
Evidently I'm not explaining myself well, I'll try again in more detail.
We've got 2 sets of allocation flags in systemtap:
STP_ALLOC_FLAGS: These are the default flags and allocations using this
set of flags will not sleep.
STP_ALLOC_SLEEP_FLAGS: These flags are only supposed to be used from
contexts where it is safe to sleep (like begin probes).
Here's the big question - on the realtime kernel, is it ever safe to sleep?
If the answer to the previous question is "no", then your change is correct.
If the answer to the previous question is "yes", then your change isn't
correct. Instead, we need to look at uses of STP_ALLOC_SLEEP_FLAGS,
because we're using the wrong set of flags somewhere. We're using
STP_ALLOC_SLEEP_FLAGS in an allocation that should be using STP_ALLOC_FLAGS.
I see 6 uses of STP_ALLOC_SLEEP_FLAGS in the systemtap source. If we
can't get a backtrace, then I'll need you to change the 6 uses of
STP_ALLOC_SLEEP_FLAGS to STP_ALLOC_FLAGS one at a time to figure out
which use of STP_ALLOC_SLEEP_FLAGS isn't correct.
Thanks for continuing to work this problem.
>
> Zhu Yanjun
>
> On 10/23/2015 12:34 AM, David Smith wrote:
>> On 10/22/2015 02:46 AM, Zhu Yanjun wrote:
>>> -rt mode spin lock lead to __might_sleep calltrace.
>>> Replacing spin lock with stp type raw lock and
>>> changing STP_ALLOC_SLEEP_FLAGS to STP_ALLOC_FLAGS solves the problem.
>> In general, this patch looks fine. However, I'm not too sure about the
>> STP_ALLOC_SLEEP_FLAGS bit. Can show us a backtrace that happens with
>> your spinlock changes but without the alloc flags changes or explain why
>> the alloc flags change is necessary? It could be that we're using the
>> wrong set of flags in the caller.
>>
>> Thanks.
>>
>
--
David Smith
dsmith@redhat.com
Red Hat
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