restart_syscall() is used for restarting only those system calls that, when restarted, should adjust their time-related parameters---namely poll(2) (since Linux 2.6.24), nanosleep(2) (since Linux 2.6), clock_nanosleep(2) (since Linux 2.6), and futex(2), when employed with the FUTEX_WAIT (since Linux 2.6.22) and FUTEX_WAIT_BITSET (since Linux 2.6.31) operations. restart_syscall() restarts the interrupted system call with a time argument that is suitably adjusted to account for the time that has already elapsed (including the time where the process was stopped by a signal). Without the restart_syscall() mechanism, restarting these system calls would not correctly deduct the already elapsed time when the process continued execution.
The kernel uses restart_syscall() to ensure that when a system call is restarted after a process has been stopped by a signal and then resumed by SIGCONT, then the time that the process spent in the stopped state is counted against the timeout interval specified in the original system call. In the case of system calls that take a timeout argument and automatically restart after a stop signal plus SIGCONT, but which do not have the restart_syscall() mechanism built in, then, after the process resumes execution, the time that the process spent in the stop state is not counted against the timeout value. Notable examples of system calls that suffer this problem are ppoll(2), select(2), and pselect(2).
From user space, the operation of restart_syscall() is largely invisible: to the process that made the system call that is restarted, it appears as though that system call executed and returned in the usual fashion.