Date: Wed, 29 May 2002 00:20:20 -0700 From: Peter Wemm <peter@wemm.org> To: Terry Lambert <tlambert2@mindspring.com> Cc: Richard Wenninger <richard@richardw.net>, current@FreeBSD.ORG Subject: Re: UMA lock Message-ID: <20020529072020.89F16380A@overcee.wemm.org> In-Reply-To: <3CF475E3.8925781A@mindspring.com>
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Terry Lambert wrote:
> Peter Wemm wrote:
> > > I think _sleeping_ is a problem, but allocation with M_WAITOK
> > > shouldn't be, given it's strange definition of "waiting". This
> > > is one of those hacks that John Baldwin was talking about earlier...
> >
> > As you said, _sleeping_ is the problem. M_WAITOK means "you may sleep if
> > you like". ie: it is a time bomb waiting for the right low memory conditi
on
> > which will then explode with a 100% authentic crash or lock up.
> >
> > Pretend it said M_SLEEPOK instead of M_WAITOK.
>
> M_WAIT became M_WAITOK. Unless M_SLEEPOK becomes M_SLEEP,
> I don't think it really matters: it's not going to wait
> indefinitely, like you'd want it to, so eventually, it's
> going to time out.
>
> You might get some big latencies along with some kernel
> printf's about inverted locks, but it shouldn't end up
> being fatal, like a real blocking wait would be, right?
>
> Or has M_WAITOK gone back to meaning M_WAIT, instead of
> M_WAIT_IF_YOU_FEEL_LIKE_IT_OTHERWISE_MAKE_ME_CHECK_FOR_NULL
> again?
>
> Can we get rid of the NULL tests we had to put in when M_WAIT
> turned into M_WAITOK?
Umm, Terry, have you looked at the code? We *only* have two wait states -
M_NOWAIT and M_WAITOK. I dont care what it was ages ago, but in recent
history we have a single flag boolean flag.
M_WAITOK = 0x00, M_NOWAIT = 0x01. In spite of having two names, it is
really a single bit where M_WAITOK != M_NOWAIT.
M_WAIT for mbufs (not malloc) was an alias for M_WAITOK, and M_DONTWAIT
(also just for mbufs) was an alias for M_NOWAIT.
You call things and either permit them to tsleep() or you do not.
M_NOWAIT to the mbuf m_get*, malloc*, contigmalloc*, uma_* etc means
"you must not tsleep!". M_WAITOK conversely means that tsleep should be
called as needed. Things like malloc still can return NULL even with M_WAITOK
for non-recoverable scenarios.
Cheers,
-Peter
--
Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5
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