cr-restore: set cr_error to EEXIST if such pid already exists, v3

This is a very common error when using criu.

The problem here is that we need to somehow transfer cr_errno
from one process to another. I suggest using pipe to give
one end to children and read cr_errno on other after restore
is finished.

v2, Pavel suggested putting errno into shared task_entries.
v3. and he also suggested using cmpxchg
Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -84,6 +84,9 @@
 #include "protobuf/siginfo.pb-c.h"
 
 #include "asm/restore.h"
+#include "asm/atomic.h"
+
+#include "cr-errno.h"
 
 static struct pstree_item *current;
 
@@ -858,6 +861,9 @@ static inline int sig_fatal(int sig)
 struct task_entries *task_entries;
 static unsigned long task_entries_pos;
 
+#define set_task_cr_err(new_err)	atomic_cmpxchg(&task_entries->cr_err, 0, new_err)
+#define get_task_cr_err()		atomic_read(&task_entries->cr_err)
+
 static int restore_one_zombie(int pid, CoreEntry *core)
 {
 	int exit_code = core->tc->exit_code;
@@ -1418,6 +1424,7 @@ static int restore_task_with_children(void *_arg)
 	pid = getpid();
 	if (current->pid.virt != pid) {
 		pr_err("Pid %d do not match expected %d\n", pid, current->pid.virt);
+		set_task_cr_err(EEXIST);
 		goto err;
 	}
 
@@ -1541,8 +1548,10 @@ static int restore_wait_inprogress_tasks()
 
 	futex_wait_while_gt(np, 0);
 	ret = (int)futex_get(np);
-	if (ret < 0)
+	if (ret < 0) {
+		set_cr_errno(get_task_cr_err());
 		return ret;
+	}
 
 	return 0;
 }

include/cr-errno.h

@@ -7,6 +7,7 @@ int get_cr_errno(void);
 /*
  * List of symbolic error names:
  * ESRCH	- no process can be found corresponding to that specified by pid
+ * EEXIST	- process with such pid already exists
  */
 
 #endif /* __CR_ERRNO_H__ */

include/rst_info.h

@@ -10,6 +10,7 @@ struct task_entries {
 	futex_t nr_in_progress;
 	futex_t start;
 	mutex_t	zombie_lock;
+	atomic_t cr_err;
 };
 
 struct fdt {