[PATCH 1/1] s390: actually restart syscalls after sys_restart

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On x86, do_signal() leaves -516 in eax while it freezes, which
sys_restart() can use to detect that it should restart the
syscall which was interrupted by a signal (or the freezer).
On s390, gprs[2] gets tweaked to -EINTR (-4) instead, leaving
us no reliable way to tell whether should be restarted.

Define TIF_RESTARTBLOCK as a thread flag showing that the
thread expects to be frozen while kicked out of a restartable
syscall by a signal.

This is needed so that, if it is checkpointed and restarted,
the restarted task has a way to tell that, upon completion
of sys_restart(), it should restart the interrupted syscall.

Without this patch, restart of the program

	close(0); close(1); close(2);
	sleep(30);

immediately exits.  With the patch, it continues to sleep for
the remaining sleep time.

Signed-off-by: Serge E. Hallyn <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
---
 arch/s390/include/asm/checkpoint_hdr.h |    1 +
 arch/s390/include/asm/thread_info.h    |    2 ++
 arch/s390/kernel/signal.c              |    3 +++
 arch/s390/mm/checkpoint.c              |   16 ++++++++++++++++
 4 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/arch/s390/include/asm/checkpoint_hdr.h b/arch/s390/include/asm/checkpoint_hdr.h
index bc9f624..4ef14e8 100644
--- a/arch/s390/include/asm/checkpoint_hdr.h
+++ b/arch/s390/include/asm/checkpoint_hdr.h
@@ -73,6 +73,7 @@ struct ckpt_hdr_cpu {
 	__u8 access_id;
 	__u8 single_step;
 	__u8 instruction_fetch;
+	__u8 should_restart;
 };
 
 struct ckpt_hdr_mm_context {
diff --git a/arch/s390/include/asm/thread_info.h b/arch/s390/include/asm/thread_info.h
index 07eb61b..2fac866 100644
--- a/arch/s390/include/asm/thread_info.h
+++ b/arch/s390/include/asm/thread_info.h
@@ -100,6 +100,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_MEMDIE		19
 #define TIF_RESTORE_SIGMASK	20	/* restore signal mask in do_signal() */
 #define TIF_FREEZE		21	/* thread is freezing for suspend */
+#define TIF_RESTARTBLOCK	23	/* for checkpoint */
 
 #define _TIF_NOTIFY_RESUME	(1<<TIF_NOTIFY_RESUME)
 #define _TIF_RESTORE_SIGMASK	(1<<TIF_RESTORE_SIGMASK)
@@ -116,6 +117,7 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_POLLING_NRFLAG	(1<<TIF_POLLING_NRFLAG)
 #define _TIF_31BIT		(1<<TIF_31BIT)
 #define _TIF_FREEZE		(1<<TIF_FREEZE)
+#define _TIF_RESTARTBLOCK	(1<<TIF_RESTARTBLOCK)
 
 #endif /* __KERNEL__ */
 
diff --git a/arch/s390/kernel/signal.c b/arch/s390/kernel/signal.c
index 6b4fef8..1179b19 100644
--- a/arch/s390/kernel/signal.c
+++ b/arch/s390/kernel/signal.c
@@ -458,6 +458,7 @@ void do_signal(struct pt_regs *regs)
 			regs->psw.addr = restart_addr;
 			break;
 		case -ERESTART_RESTARTBLOCK:
+			set_thread_flag(TIF_RESTARTBLOCK);
 			regs->gprs[2] = -EINTR;
 		}
 		regs->svcnr = 0;	/* Don't deal with this again. */
@@ -467,6 +468,8 @@ void do_signal(struct pt_regs *regs)
 	   the debugger may change all our registers ... */
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 
+	clear_thread_flag(TIF_RESTARTBLOCK);
+
 	/* Depending on the signal settings we may need to revert the
 	   decision to restart the system call. */
 	if (signr > 0 && regs->psw.addr == restart_addr) {
diff --git a/arch/s390/mm/checkpoint.c b/arch/s390/mm/checkpoint.c
index 40dd417..d8d4b6b 100644
--- a/arch/s390/mm/checkpoint.c
+++ b/arch/s390/mm/checkpoint.c
@@ -65,6 +65,16 @@ static void s390_copy_regs(int op, struct ckpt_hdr_cpu *h,
 		BUG_ON(h->gprs[2] < 0);
 		h->gprs[2] = 0;
 	}
+	/*
+	 * if the checkpointed task was frozen in a syscall with
+	 * -ERESTART_RESTARTBLOCK (switched to -EINTR during do_signal()
+	 * before try_to_freeze() happened) * then after restart we need
+	 * to call __NR_restart_syscall to continue.  Fix up here.
+	 */
+	if (op == CKPT_RST && h->should_restart) {
+		regs->gprs[2] = __NR_restart_syscall;
+		set_thread_flag(TIF_RESTART_SVC);
+	}
 	CKPT_COPY_ARRAY(op, h->fprs, thr->fp_regs.fprs, NUM_FPRS);
 	CKPT_COPY_ARRAY(op, h->acrs, thr->acrs, NUM_ACRS);
 	CKPT_COPY_ARRAY(op, h->per_control_regs,
@@ -98,6 +108,12 @@ int checkpoint_cpu(struct ckpt_ctx *ctx, struct task_struct *t)
 
 	s390_copy_regs(CKPT_CPT, h, t);
 
+	/*
+	 * if t was frozen while in a restartable syscall, note that
+	 */
+	if (test_ti_thread_flag(task_thread_info(t), TIF_RESTARTBLOCK))
+		h->should_restart = 1;
+
 	ret = ckpt_write_obj(ctx, (struct ckpt_hdr *) h);
 	ckpt_hdr_put(ctx, h);
 
-- 
1.6.1