Implement POSIX Asynchronous IO

[Joel Sherrill]

RTEMS does not currently support POSIX Asynchronous IO as defined by The Open Group. These methods and associated constants are prototyped by the [​​http://www.opengroup.org/onlinepubs/009695399/basedefs/aio.h.html <aio.h>] header file which is cpukit/posix/include/aio.h in the RTEMS source tree. The following methods have already been implemented but should be reviewed against the POSIX specification to ensure they are as complete as possible:

• ​aio_read()

​- aio_write()
​- aio_cancel()
​- aio_error()
​- aio_return()

The implementation of the above AIO methods includes a server thread which performs the user's AIO requests. These AIO methods request a SINGLE operation.

The method ​lio_listio() is not implemented currently in RTEMS. The current implementation between the aio_ methods and the server thread will need to grow to support a list. The straightforward way to accomplish this is to modify the current aio_ infrastructure to treat a single aio_ request as a list of one. Then the lio_listio() should be a simple modification.

The current implementation leans to a long-ish function to process AIO requests. This processing should be refactored so while iterating the list, the server thread calls a helper method for each request type.

The implementation has a single server thread. The server thread characteristics such as stack size and priority should be application configurable.

There will need to be tests added for the new functionality. Coverage will need to be reviewed for the AIO functionality.

Possible Mentors: Joel Sherrill, Gedare Bloom, Chris Johns, Kinsey Moore

[Joel Sherrill]

My thoughts on an implementation plan. The functionality of lio_listio() can be implemented by a series of incremental changes. Much of the functionality is already present in the existing aio_* methods. Some of these can happen in parallel. Ensure all AIO tests pass as modifications are made and new tests are added as needed.

• Review existing AIO functions to gain understanding of the implementation and to check each POSIX function against the specification. If there is any missing functionality, this plan will have to be adjusted to account for that.

• Refactor AIO processing so a helper method exists for processing existing AIO operations. Eventually these will be called for each requested operation in a list. Refactoring helper methods will make the server thread easier to read. Change no functionality in this step.
  • This is likely a good spot to submit a patch.

• Modify the message structure between the aio_ functions and the server to include a number of messages. It is one for all aio_ operations and a user provided count for lio_listio. Included in this is modifying the server to process a list of requests.
  • This is likely a good spot to submit a patch.

• Implement lio_listio(). Messages to the server and it processing a list should already be implemented from the refactoring and adding a list length.

• Add generation of a signal at list completion. Verify this is or is not a requirement for aio_ functions.

• Ensure the implementation has good test coverage.