Date: Fri, 07 Aug 2015 12:41:55 -0700 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Hans Petter Selasky <hps@selasky.org>, Garrett Cooper <yaneurabeya@gmail.com> Subject: Re: When and when not to use CTLFLAG_MPSAFE with the SYSCTL macros..? Message-ID: <2324251.0KSjbGaFFg@ralph.baldwin.cx> In-Reply-To: <55C1AF5A.1080601@selasky.org> References: <F4A56F9C-058B-4D91-B75D-92DEC16E8B2E@gmail.com> <55C1AF5A.1080601@selasky.org>
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On Wednesday, August 05, 2015 08:38:18 AM Hans Petter Selasky wrote: > On 08/05/15 00:47, Garrett Cooper wrote: > > Hi, > > =09I=E2=80=99ve been trying to figure out (because sysctl(9) is lac= king) when to use CTLFLAG_MPSAFE. Is it strictly when dealing with SYSC= TL_PROC handlers that do proper locking of shared resources, or are the= re other nuances that need to be handled? > > =09I=E2=80=99m also asking because SYSCTL_UQUAD, for instance, expl= icitly uses CTLFLAG_MPSAFE in the handler, which is a bit confusing. > > Thanks! > > -NGie >=20 > Hi, >=20 > This flag decides if you have Giant automatically locked or not aroun= d=20 > the sysctl proc. Your functions should have their own locks basically= . I=20 > believe it is a leftover from many years ago, when the FreeBSD kernel= =20 > was going multi threaded. It's only about 2-3 years old actually. To answer your question Garrett: yes it is really only for SYSCTL_PROC handlers. The existing "simple" handlers like sysctl_handle_int are as atomic as they can be regardless of Giant, so they don't need Giant.= If you have a variable that you want to control access to via locking y= ou need to use a custom handler, even if it is a simple int. --=20 John Baldwin
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