[PATCH] user-cr: invoke exit system call directly from ckpt_do_feeder

[Nathan Lynch <ntl-e+AXbWqSrlAAvxtiuMwx3w@public.gmane.org>]

The feeder thread can cause the restart process to fail by indirectly
calling exit_group, which sends SIGKILL to all other threads in the
process.  If the feeder thread "wins" the race, the restart is
disrupted.  A common symptom of this race is the coordinator task
returning from the wait_for_completion_interruptible in
wait_all_tasks_finish with a signal (the SIGKILL) pending.

Calling _exit isn't enough; see
http://www.kernel.org/doc/man-pages/online/pages/man2/exit.2.html#NOTES

Exit the feeder thread by using the syscall() macro.

Signed-off-by: Nathan Lynch <ntl-e+AXbWqSrlAAvxtiuMwx3w@public.gmane.org>
---
 restart.c |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/restart.c b/restart.c
index d5d069a..ed4268c 100644
--- a/restart.c
+++ b/restart.c
@@ -2079,8 +2079,16 @@ static int ckpt_do_feeder(void *data)
 		ckpt_read_write_inspect(ctx);
 	else
 		ckpt_read_write_blind(ctx);
-		
-	/* all is well: feeder thread is done */
+
+	/* All is well: feeder thread is done.  However, we must
+	 * invoke the exit system call directly. Otherwise, upon
+	 * return from this function, glibc's clone wrapper will call
+	 * _exit, which calls exit_group, which will terminate the
+	 * whole process, which is not what we want.
+	 */
+	syscall(SYS_exit, 0);
+
+	/* not reached */
 	return 0;
 }
 
-- 
1.6.0.6