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Date:      Mon, 30 Jul 2012 15:56:34 +0100
From:      Attilio Rao <attilio@freebsd.org>
To:        Konstantin Belousov <kostikbel@gmail.com>
Cc:        Davide Italiano <davide@freebsd.org>, svn-src-projects@freebsd.org, src-committers@freebsd.org
Subject:   Re: svn commit: r238907 - projects/calloutng/sys/kern
Message-ID:  <CAJ-FndCQ6HGAfFdjofNfJ%2BHeNaE7uqoNhJB9GH4pGFxyZ_1yLg@mail.gmail.com>
In-Reply-To: <CAJ-FndByYcZ%2BUhnkFT_n2=W=UheqUCi0%2BUAX%2BF07EqbVU=6iDQ@mail.gmail.com>
References:  <201207301350.q6UDobCI099069@svn.freebsd.org> <CAJ-FndBJNNBNDUEDsDBUvwoVExZpnXmoJmpY58gE3QQbw3hRGA@mail.gmail.com> <CACYV=-HmOwZ=E8Pw3-mUw0994SbvZaA3eMfcwM0fDTu_zykBJg@mail.gmail.com> <CAJ-FndBmXkyJJ=fCkEpVm84E56A2_EoM6kbch03e4RMEM6WCGQ@mail.gmail.com> <20120730143943.GY2676@deviant.kiev.zoral.com.ua> <CAJ-FndByYcZ%2BUhnkFT_n2=W=UheqUCi0%2BUAX%2BF07EqbVU=6iDQ@mail.gmail.com>

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On 7/30/12, Attilio Rao <attilio@freebsd.org> wrote:
> On 7/30/12, Konstantin Belousov <kostikbel@gmail.com> wrote:
>> On Mon, Jul 30, 2012 at 03:24:26PM +0100, Attilio Rao wrote:
>>> On 7/30/12, Davide Italiano <davide@freebsd.org> wrote:
>>> > On Mon, Jul 30, 2012 at 4:02 PM, Attilio Rao <attilio@freebsd.org>
>>> > wrote:
>>> > Thanks for the comment, Attilio.
>>> > Yes, it's exactly what you thought. If direct flag is equal to one
>>> > you're sure you're processing a callout which runs directly from
>>> > hardware interrupt context. In this case, the running thread cannot
>>> > sleep and it's likely you have TDP_NOSLEEPING flags set, failing the
>>> > KASSERT() in THREAD_NO_SLEEPING() and leading to panic if kernel is
>>> > compiled with INVARIANTS.
>>> > In case you're running from SWI context (direct equals to zero) code
>>> > remains the same as before.
>>> > I think what I'm doing works due the assumption thread running never
>>> > sleeps. Do you suggest some other way to handle this?
>>>
>>> Possibly the quicker way to do this is to have a way to deal with the
>>> TDP_NOSLEEPING flag in recursed way, thus implement the same logic as
>>> VFS_LOCK_GIANT() does, for example.
>>> You will need to change the few callers of THREAD_NO_SLEEPING(), but
>>> the patch should be no longer than 10/15 lines.
>>
>> There are already curthread_pflags_set/restore KPI designed exactly to
>> handle
>> nested private thread flags.
>
> Yes, however I would use curthread_pflags* KPI within
> THREAD_NO_SLEEPING() as this name is much more explicit.
>
>> Also, I wonder, should you assert somehow that direct dispatch cannot
>> block
>> as well ?
>
> Yes, it would be optimal, but I don't think we have a flag for that
> right now, do we?

More explicitly, I think such combination TDP_NOSLEEPING +
TDP_NOBLOCKING (name invented) should be set on entering the interrupt
context, not only related to this part of callouts. This would be a
very good help for catching buggy situations.

Attilio


-- 
Peace can only be achieved by understanding - A. Einstein



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