Date: Thu, 17 Oct 2002 11:54:49 -0400 (EDT) From: John Baldwin <jhb@FreeBSD.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-smp@freebsd.org Subject: RE: MP synchronization rules documentation?? Message-ID: <XFMail.20021017115449.jhb@FreeBSD.org> In-Reply-To: <15790.54204.265656.241333@grasshopper.cs.duke.edu>
next in thread | previous in thread | raw e-mail | index | archive | help
On 17-Oct-2002 Andrew Gallatin wrote: > > Are the synchronization rules for FreeBSD-current documented anywhere > other than mutex(9)? > http://people.FreeBSD.org/~jasone/smp/smp_synch_rules.html looks > promising, but it seems to be dead. > > I'm asking because I've just been porting my $day_job's device driver > to -current. At first, I thought the synchronization primitives would > be a direct port from Solaris, but we (FreeBSD) seem to make a few > assumptions that other OSes don't. > > Are the following assumptions currently true? > > 1) you cannot sleep holding a mutex, even if you know damned well its safe It's almost never safe. It would only be safe you are blocked and are guaranteed to be awoken in a short period of time. You might want to use the mutex to protect a flag or some such. > 2) you cannot hold any mutex other than Giant if you're holding Giant This is not true. The only rule with Giant is that if you are going to hold Giant, you have to acquire Giant before any other mutexes. Put another way: you cannot acquire Giant non-recursively while holding another mutex. You may acquire other mutexes while holding Giant, and you may acquire Giant recursively while holding other mutexes. > Are there other assumptions that are not documented in mutex(9)? Witness should enforce all of them. Hmm, one that it doesn't however: - Do not hold any mutexes (except for Giant) across copyin(), copyout(), uimove(), fuword(), etc. You shouldn't need locks when calling these functions anyhow. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-smp" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?XFMail.20021017115449.jhb>