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This patch fixes a race condition triggered by the thread test running on Linux inside VirtualBox. The 'test_stack_alloc' sporadically produced one of two errors: A segfault in the 'Thread::deinit_platform_thread' on the attempt to access the 'native_thread' of the to-be-destructed thread (this data structure is located on the thread's stack). Or, an error message about a region conflict within the stack area. The problem was that two instances of 'Region_map_mmap' issued a sequence of munmap and mmap each. Even though each instance locked the attach/detach operations, the lock was held per instance. In a situation where two instances performed attach/detach operations in parallel, the syscall sequences could interfere with each other. In the test scenario, the two region-map instances are the test's address space and the stack area. When creating a thread, the thread's trace-control dataspace is attached at an arbitrary place (picked by the Linux kernel) within the address space whereas the stack is attached at the stack area. The problem is the following sequence: Thread A wants to destruct a thread: 1. Remove stack from stack area (issue unmap syscall) 2. Preserve virtual address range that was occupied from the stack so that Linux won't use it (issue mmap syscall) Thread B wants to construct a thread: 1. Request trace-control dataspace from CPU session 2. Attach trace-control dataspace to address space at a location picked by the Linux kernel (issue mmap syscall) The problem occurs when thread B's second step is executed in between the steps 1 and 2 of thread A and the Linux kernel picks the just-unmapped address as the location for the new trace-control mapping. Now, the trace control dataspace is mapped at the virtual address that was designated for the stack of the to-be-created thread, and the attempt to map the real stack fails. The patch fixes the problem by replacing the former region-map-local locks by a component-global lock. Furthermore, it cleans up core's implementation of the support function for the region-map-mmap implementation, eliminating the temporary unlocking of the region-map lock during RPC. |
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This repository contains the Linux-specific implementation of Genode.