Date: Thu, 30 Aug 2001 23:57:42 -0700 (PDT) From: John Baldwin <jhb@FreeBSD.org> To: Mike Silbersack <silby@silby.com> Cc: cvs-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG, Matt Dillon <dillon@earth.backplane.com> Subject: Re: RE: cvs commit: src/sys/kern init_sysent.c sysv_msg.c sysv_s Message-ID: <XFMail.010830235742.jhb@FreeBSD.org> In-Reply-To: <Pine.BSF.4.30.0108302159470.19462-100000@niwun.pair.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On 31-Aug-01 Mike Silbersack wrote:
>
>
> On Thu, 30 Aug 2001, Matt Dillon wrote:
>
>> :You could probably minimize a lot of these diffs using a UGAR() macro
>> :similar
>> :ot BSD/OS's EGAR macro. It unlocks Giant and then returns the value passed
>> :in.
>> :Thus return(foo); becomes UGAR(foo);
>> :
>> :This would make the diffs much simpler, which also makes it easier to take
>> :Giant out of syscalls later on:
>> :
>> :#define UGAR(rval) do { \
>> : int _val = (rval); \
>> : mtx_unlock(&Giant); \
>> : return (_val); \
>> :} while (0)
>>
>> I think it's better to keep it explicit. I don't want to hide the
>> fact that I am manipulating Giant.
>>
>> -Matt
>
> There could be cases where you'd want to drop Giant earlier than at the
> end of the function too, I suppose. (Possibly in cases where you only
> need Giant to grab certain pieces of data, then can go on with a more
> local mutex.)
In these cases though you usually just grab the lock at the top and release at
the bottom (or for error returns).
> Actually, I guess now would be a good time to ask. Is grabbing Giant late
> / dropping it early a problem? I ask because I'm unclear if it's bad to
> have a function that's only part Giant-dependant.
It's not bad to be part Giant-dependent. You may want to avoid dropping and
picking up locks a lot though, it doesn't hurt to just hold the lock across a
few extra operations if it avoids a unlock/lock pair. However, the real
question is if dropping Giant breaks expected atomicity of the function in
question.
--
John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe cvs-all" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?XFMail.010830235742.jhb>