Thanks Robert and Dr. Michael for answering my question :) This [copying to-user-space from kernel threads] came up in my mind, because I was wondering about the working of Async I/O library (libaio) from RedHat. How does it handle async reads from the kernel. I guessed that when a poll() is done it returns immediately and a kernel thread is created to do the async copy-to-user by the kernel. But that can't be done! I think I need to read the sources of libaio. They must be doing something different. Does anybody know how its done there? Thanks once again, A. --- Robert Love <rml@tech9.net> wrote: > On Fri, 2002-12-27 at 11:23, zw wrote: > > > > But I thought while doing copy_to_user() the task should have > > > a valid task->mm. no? > > > > You mean the kernel thread, or the user task? I think you don't > have > > to do anything special on your kernel thread. You kernel thread, I > > think, basically waiting there for a pointer from user space. Then > > you call copy_to_user() in your kernel thread. That's it. I think. > > Think about what devnetfs is asking. > > The function is prototyped as copy_to_user(to, from, len) where to > and > from are pointers. > > If current->mm is invalid, then exactly whose "from" are you copying > to? Remember, Linux is a virtual memory operating system: there can > be > many mappings at the same address. > > Likewise, with copy_from_user(), without a valid ->mm from where > exactly > are you copying from? > > The functions pull the to/from user addresses from the loaded user > address space. Without a user address space, these functions do not > work. Nor could they. > > In short, to answer the original question, kernel threads cannot copy > from user-space since they have no user-space. If you want to get > data > into and out of a kernel thread, expose an interface (procfs, sysfs, > syscall, device file, etc.) > > Robert Love > __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
