[Bug 1670] New: IO driver unregister is problematic

[bugzilla-daemon at rtems.org]

https://www.rtems.org/bugzilla/show_bug.cgi?id=1670

           Summary: IO driver unregister is problematic
           Product: RTEMS
           Version: HEAD
          Platform: All
        OS/Version: RTEMS
            Status: NEW
          Severity: normal
          Priority: P3
         Component: cpukit
        AssignedTo: joel.sherrill at oarcorp.com
        ReportedBy: sebastian.huber at embedded-brains.de


Created an attachment (id=999)
 --> (https://www.rtems.org/bugzilla/attachment.cgi?id=999)
IO driver with reference counting.

IO drivers will be registered with rtems_io_register_driver() and unregistered
with rtems_io_unregister_driver().  User typically access the drivers via the
file system.  Some drivers have other users e.g. the block device cache
(bdbuf).  Now we have the problem that nobody knows if a driver is in use by
someone.  This is problematic if drivers come and go during the system lifetime
for example in case of removable media (USB sticks).  Currently it is possible
that the driver changes during two read() or write() calls.  We have to
introduce some sort of reference counting.  A simple approach is provided by
the attached patch.

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