Date: Tue, 30 Jul 2002 15:21:06 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Daniel Lundqvist <daniel@malarhojden.nu> Cc: freebsd-hackers@freebsd.org Subject: Re: select()/poll() i kernel. Message-ID: <3D471152.4347ABD1@mindspring.com> References: <20020730152514.GA66448@malarhojden.nu>
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Daniel Lundqvist wrote: > I was wondering if there is a select()/poll() for use in kernel by > kernel threads? I've been looking around in sys/kern/ but didn't find anything. > I'm currently developing under 4.6. > > Please CC me since I'm not a member of the list. 4.6 does not have kernel threads; it has kernel processes (kproc's), however, which are just standard processes which have been assumed to have entered the kernel. As such, they are suitable for use as a context for blocking operations (you can only sleep things that have the ability to have scheduler queue entries, and therefore be awakened at a later time). There is no explicit support for a "select" call; most operations on file descriptors assume a process context in user space from which parameters can be copied, and results copied out. Rather than correcting the FreeBSD model to internally add a space identifier parameter to the body of the call, and make the system calls call that, instead of being directly wired in, you have two real options: 1) Use "The mmap trick" to obtain memory in the user space portion of the kproc address space, so that when uiomove and/or copyin/copyout operate, they have someplace to get or put the data. 2) Write the moral equivalent of the select(2) (or poll(2)) code that can be called from kernel space, and call it from your kproc. Note that either of these will *only* work from a kproc, where the current process identfier is the kproc. So you can't use them in fault handlers or interrupt handlers, if that's what yu are thinking. Really, FreeBSD needs a better abstraction of the file I/O and other operations in kernel space, so that doing this type of work is much easier. On the other hand... it's likely that your expectations of how a kproc can be used as if it were a kernel thread are unreasonable, so maybe it's just as well that there is not explicit support for this kind of thing, like there is in Linux, AIX, and Solaris. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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