Date: Fri, 27 Apr 2007 14:36:21 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Hans Petter Selasky <hselasky@freebsd.org> Cc: Attilio Rao <attilio@freebsd.org>, freebsd-hackers@freebsd.org, Julian Elischer <julian@elischer.org> Subject: Re: msleep() on recursivly locked mutexes Message-ID: <Pine.GSO.4.64.0704271426080.9587@sea.ntplx.net> In-Reply-To: <200704271917.29939.hselasky@freebsd.org> References: <200704262136.33196.hselasky@c2i.net> <200704270748.49404.hselasky@c2i.net> <Pine.GSO.4.64.0704270906001.8536@sea.ntplx.net> <200704271917.29939.hselasky@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, 27 Apr 2007, Hans Petter Selasky wrote: > On Friday 27 April 2007 15:14, Daniel Eischen wrote: >> >> When you hold a mutex, it should be for a very short time. And >> I agree with the other comment that all drivers should be multi-thread >> safe, so we shouldn't add cruft to allow for non MT-safe drivers. > > Yes, and no. > > Mutexes are used to get the CPU out of the code. Therefore you should not > lock/unlock all the time, to ensure that the locked time is as short as > possible. Because then you get double work checking the state after that you > lock a mutex again. Surely, in a "static" environment there is nothing to > check. But in a dynamic environment you need to check that "descriptors" of > all kinds are still present, after that you lock a mutex. Unlocking a mutex > allows "anything" to happen. Keeping a mutex locked prevents certain things > from happening. If you need to prevent "things" from happening, and it is at more of a macro level than micro level, then you probably want a condvar or barrier sort of synchroninzation, or possibly a rwlock. When the thread currently in the guts of your driver exits, he releases the CV or rwlock and allows another thread to enter (which possibly causes another "anything" to happen). -- DE
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.GSO.4.64.0704271426080.9587>