s390: fix system call restart with multiple signals

glibc complained with "The futex facility returned an unexpected error
code.". It turned out that the futex syscall returned -ERESTARTSYS because
a signal is pending. arch_do_signal_or_restart() restored the syscall
parameters (nameley regs->gprs[2]) and set PIF_SYSCALL_RESTART. When
another signal is made pending later in the exit loop
arch_do_signal_or_restart() is called again. This function clears
PIF_SYSCALL_RESTART and checks the return code which is set in
regs->gprs[2]. However, regs->gprs[2] was restored in the previous run
and no longer contains -ERESTARTSYS, so PIF_SYSCALL_RESTART isn't set
again and the syscall is skipped.

Fix this by not clearing PIF_SYSCALL_RESTART - it is already cleared in
__do_syscall() when the syscall is restarted.

Reported-by: Bjoern Walk <[e-mail]>
Signed-off-by: Sven Schnelle <[e-mail]>
Reviewed-by: Heiko Carstens <[e-mail]>
Fixes: 56e62a737028 ("s390: convert to generic entry")
Cc: <[e-mail]> # 5.12
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/kernel/signal.c

···
 
 	/* No handlers present - check for system call restart */
 	clear_pt_regs_flag(regs, PIF_SYSCALL);
-	clear_pt_regs_flag(regs, PIF_SYSCALL_RESTART);
 	if (current->thread.system_call) {
 		regs->int_code = current->thread.system_call;
 		switch (regs->gprs[2]) {