Date: Tue, 6 Jun 2006 01:13:53 +0800 From: MingyanGuo <guomingyan@gmail.com> To: "Robert Watson" <rwatson@freebsd.org> Cc: delphij@gmail.com, freebsd-arch@freebsd.org Subject: Re: Why use `thread' as an argument of Syscalls? Message-ID: <1fa17f810606051013m38cf08d7i82b6b11c7b119686@mail.gmail.com> In-Reply-To: <20060605165355.L50057@fledge.watson.org> References: <1fa17f810606050044k2847e4a2i150eb934ed84006f@mail.gmail.com> <Pine.GSO.4.64.0606050744190.13542@sea.ntplx.net> <1fa17f810606050608l5bd2ec5ch37663375f6fa5b64@mail.gmail.com> <Pine.GSO.4.64.0606051118180.14745@sea.ntplx.net> <20060605163559.N50057@fledge.watson.org> <448450FD.4030709@FreeBSD.org> <20060605165355.L50057@fledge.watson.org>
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On 6/5/06, Robert Watson <rwatson@freebsd.org> wrote:
>
>
> On Mon, 5 Jun 2006, Suleiman Souhlal wrote:
>
> > Robert Watson wrote:
> >>
> >> On Mon, 5 Jun 2006, Daniel Eischen wrote:
> >>
> >>>> They are the same questions, I think ;-). Now would you please
> explain
> >>>> "why use `proc' as an argument of Syscalls" to me :)? I've read
> some
> >>>> source code of the kernel, but no comments about it found.
> >>>
> >>> I don't know. Convention? It makes sense to me.
> >>
> >> Certainly consistency. Most system calls do actually use the argument
> at
> >> some point -- be it to look up a file descriptor, access control, or
> the
> >> like, and the calling context has it for free and in-hand anyway.
> >
> > But couldn't they just use curthread/curproc?
>
> In the past, in micro-benchmarking, I've measured a small performance hit
> from
> using per-cpu variables over variables already in the stack. However,
> that
> was quite a while ago, and I'm not entirely convinced the test results
> were
> valid. In the general case, it's pretty helpful to be able to pass in,
> for
> example, explicit credential references, as it means you can do acess
> control
> checks, auditing, accounting, etc, against arbitrary credentials rather
> than
> always against curthread->td_ucred. In a number of places, we pass
> threads
> down the stack where we mean to pass credentials, such as at several spots
> in
> the network stack. There are also places where the process is passed
> around
> so it can become a later argument to lockmgr() locking primitives, and
> since
> those are decreasingly used, the references are increasingly unnecessary.
>
> Robert N M Watson
>
Thanks.
MingyanGuo
--
Three passions, simple but overwhelmingly strong, have governed my life:
the longing for love, the search for knowledge, and unbearable pity for
the suffering of mankind.
---------Bertrand Russell
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