On 01/10/2012 04:04 PM, Jan-Erik Lange wrote:


I have a question about basics of the synchronization of shared memory
with mutexes.

The situation: The Sender is a RT task (primary domain) and the
recipient is a non-RT task (usually in the secondary domain). Namely,
the receiver is used to interact with a Web server. He calls to syscalls
and stuff and because of that he's usually in the secondary mode.

Suppose the sender has written something to the shared memory: He uses
mutex for synchronization, so he calls the rt_mutex_release() function.

The receiver will now get time to work from the scheduler. He calls
rt_mutex_acquire() function to lock the shared memory. Then a context
switch occurs from the secondary mode in the primary mode. He has now
the resource for himself.

Now the scheduler lets sender-task to work and it wants to write
something. So it calls rt_mutex_acquire() function. And now comes my
question: Provides rt_mutex_acquire() a mechanism to signal the cheduler
to immediately continue with the recipient-task? If so, how does the
rt_mutex_acquire() function tells the scheduler that?

There are two tasks controlled by the same (Xenomai) scheduler. One is trying to grab a mutex the other one holds, so it is put to sleep on that mutex. The scheduler will simply switch to the next ready-to-run task since the sender task cannot run anymore, and that next task may be the receiver task. There is no special signaling magic required.

I came out because I in the documention I read the term "Rescheduling:

The documentation for rt_mutex_acquire is "Rescheduling: always unless the request is immediately satisfied or
timeout specifies a non-blocking operation.".

Best regards

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