I have a user level process which is sleeping currently, by using sleep() function. I am trying to write a kernel module which can first extract the task_struct of the user process from its PID, and then can wake the process. Till date I have implemented the code for getting the task_struct from PID. But, I dont know of any function which can wake up that process. I tried wake_up_process(task_struct), though its returning 1, i.e, success in waking up the process, but the the printf() statement just after the sleep() statement of the user process is not getting executed. Will changing the state of the task_struct help? Or there's some another approach for doing the same? Please guide me further.
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1 Answers
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It is possible, but you might be going about it the wrong way. sleep() waits on a delay, and even though you could signal the process from within the kernel (essentially like kill(2) in user mode, with some non harmful signal, but something that will "kick you out" of the system call, the correct way of doing so is having the sleeping process block on a device which your kernel module exports. This way, the kernel module will have control - the process will be stuck in a read(2) call, and until your read implmentation in the module returns, the process will be stuck.
This is preferable, because the whole idea of sleeping is when you are waiting for something. When you simple sleep(xxxx) indefinitely, you're basically waiting on a time out. What more, using the device approach, you can add the file descriptor to a select(2)/poll(2) loop, as well, which makes for very elegant synchronization with other input/output descriptors.
