Computer system, method and program for defining first and second virtual machines and a memory shared by the first and second virtual machines. A filesystem cache resides in the shared memory. A lock structure resides in the shared memory to record which virtual machine, if any, currently has an exclusive lock for writing to the cache. The first virtual machine includes a first program function to acquire the exclusive lock when available by manipulation of the lock structure, and a second program function active after the first virtual machine acquires the exclusive lock, to write to the cache. The lock structure is directly accessible by the first program function. The cache is directly accessible by the second program function. The second virtual machine includes a third program function to acquire the exclusive lock when available by manipulation of the lock structure, and a fourth program function active after the second virtual machine acquires the exclusive lock, to write to the cache. The lock structure is directly accessible by the third program function. The cache is directly accessible by the fourth program function. Another computer system, method and program is embodied in logical partitions of a real computer, instead of virtual machines.