Date: Wed, 14 Jun 2017 08:05:13 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Jia-Ju Bai <baijiaju1990@163.com> Cc: freebsd-hackers@freebsd.org Subject: Re: A question about in lock usage in FreeBSD Message-ID: <20170614050513.GI2088@kib.kiev.ua> In-Reply-To: <59408D05.7070407@163.com> References: <59408D05.7070407@163.com>
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On Wed, Jun 14, 2017 at 09:10:29AM +0800, Jia-Ju Bai wrote: > Hi, > > I am a freshman in developing FreeBSD drivers, and I have a question in > lock usage in FreeBSD. > > The kernel provides some kinds of locks in developing drivers, such as > "mutex lock", "mutex spin lock", "rw lock" and "sx lock". I want to know > which locks should be held when the thread can sleep. Replace should with can in the question. > From my knowledge of FreeBSD document, I make the following points: > (1) "rw lock" and "sx lock" can be used in this situation; You cannot own rw lock while trying to sleep. > (2) "mutex lock" and "mutex spin lock" are forbidden in this situation. > > If my points are right, I will make another point: > mutex_lock will let the thread sleep when the lock is requested, so the > mutex_lock can not be called in nested style (namely it is unsafe that > mutex_lock is called again when a "mutex lock" is held). No, mutex does not put the thread which cannot obtain the lock immediately, to sleep. The thread is blocked on turnstile. > > Are my points are right? > I am looking forward to useful opinions and answers :) Read locking(9).
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