l4-hackers February 2021

l4-hackers@os.inf.tu-dresden.de

3 participants
1 discussions

L4Re: region mapper/manager concurrency considerations and resource limits
by Paul Boddie 02 Mar '21

02 Mar '21

Hello, Apologies for introducing two potentially separate topics, but they both seem to present themselves in my most recent endeavour. In short, I am wondering about the following... What considerations need to be given to concurrency with regard to the use of the l4re_rm_attach function and similar functionality? Is access to the region mapper/manager from different threads in the same task advisable, are there thread safety issues, and if so, can these be sensibly worked around? (I … [View More]have observed what seems to be the same address being provided for two different dataspaces at the same time, which does not appear to happen when I introduce locking around calls to l4re_rm_attach. Are such problems expected or could this be some other problem manifesting itself?) What kind of resource limits apply to L4Re tasks? In my work, I am allocating capabilities for objects acting as dataspaces. Things seem to work fairly well up to a certain point, and then capability allocation seems to fail, and program failure is then not particularly graceful. (I currently observe that creating 160 threads, each with its own IPC gate capability seems to be unproblematic, but more than this will produce errors that manifest themselves either in pthread library code or in my own code where a critical section is supposed to be enforced.) I suppose some contextual remarks might be useful. My previous work involved providing some simple filesystem-related functionality through filesystem servers, where a new server object (exposed via an IPC gate and having its own thread) is created for each open file. Some stress testing in a multiprocessor environment indicated that various concurrency issues were highly likely to be present. (I also probably didn't provide general enough support for flexpages, but since all my "send" flexpages were single machine pages, I don't think this would have caused additional problems or confusion.) Consequently, I migrated the principal mechanisms to my Linux environment and wrote a simulation to explore the concurrency situation, identifying and hopefully remedying various deficiencies (including more general flexpage support). This new endeavour involves bringing this tested functionality back into L4Re, wiring it up with the IPC system, and testing using multithreaded clients. I might also ask about support in L4Re for C++ threading constructs. When developing my simulation, I decided to use the standard C++ support for threading, mutexes, condition variables, and so on, as opposed to using pthread functionality. I imagine that C++ library support for these things just wraps the pthread functionality, but I wonder if there are not other considerations that I might need to be aware of. Sorry if this is in any way vague, but since there isn't really any discussion about system architecture or implementation on this list with respect to these technologies (or really any technologies), I find myself experimenting with the available software to determine techniques and approaches that I hope will be viable. Nevertheless, any constructive remarks or advice would be greatly appreciated. Thanks in advance, Paul [View Less]

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l4-hackers February 2021