1. A system for synchronous execution of a user space operation without a thread switch from a kernel context, the system comprising: a memory area storing an operating system having a kernel space and a user space; and a processor programmed to: initialize, from a thread executing in the user space, an upcall mode into the kernel space; perform, by the same thread from the kernel space via the initialized upcall mode, a function in the user space, wherein performing the function comprises filtering one or more input/output (I/O) requests from the thread in the user space; and upon completion of the function in the user space, return to the kernel space to perform another function from the same thread, wherein performing the another function comprises issuing the filtered one or more I/O requests further down storage layers in the kernel space, and wherein filtered I/O completions are completed further up the storage layers in the kernel space.