rst: Formalize the shared resource restore order

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files.c

@@ -443,10 +443,9 @@ static int collect_fd(int pid, FdinfoEntry *e, struct rst_info *rst_info)
 		return -1;
 	}
 
-	list_for_each_entry(le, &fdesc->fd_info_head, desc_list) {
-		if (le->pid > new_le->pid)
+	list_for_each_entry(le, &fdesc->fd_info_head, desc_list)
+		if (pid_rst_prio(new_le->pid, le->pid))
 			break;
-	}
 
 	list_add_tail(&new_le->desc_list, &le->desc_list);
 	new_le->desc = fdesc;
@@ -972,7 +971,7 @@ int shared_fdt_prepare(struct pstree_item *item)
 	item->rst->fdt = fdt;
 	item->rst->service_fd_id = fdt->nr;
 	fdt->nr++;
-	if (fdt->pid > item->pid.virt)
+	if (pid_rst_prio(item->pid.virt, fdt->pid))
 		fdt->pid = item->pid.virt;
 
 	return 0;

include/crtools.h

@@ -189,4 +189,14 @@ static inline int in_vma_area(struct vma_area *vma, unsigned long addr)
 		addr < (unsigned long)vma->vma.end;
 }
 
+/*
+ * When we have to restore a shared resource, we mush select which
+ * task should do it, and make other(s) wait for it. In order to
+ * avoid deadlocks, always make task with lower pid be the restorer.
+ */
+static inline bool pid_rst_prio(unsigned pid_a, unsigned pid_b)
+{
+	return pid_a < pid_b;
+}
+
 #endif /* __CR_CRTOOLS_H__ */

include/files.h

@@ -6,6 +6,7 @@
 #include "lock.h"
 #include "list.h"
 #include "image.h"
+#include "crtools.h"
 
 #include "protobuf/fdinfo.pb-c.h"
 #include "protobuf/fown.pb-c.h"
@@ -69,7 +70,8 @@ struct fdinfo_list_entry {
 /* reports whether fd_a takes prio over fd_b */
 static inline int fdinfo_rst_prio(struct fdinfo_list_entry *fd_a, struct fdinfo_list_entry *fd_b)
 {
-	return (fd_a->pid < fd_b->pid) || ((fd_a->pid == fd_b->pid) && (fd_a->fe->fd < fd_b->fe->fd));
+	return pid_rst_prio(fd_a->pid, fd_b->pid) ||
+		((fd_a->pid == fd_b->pid) && (fd_a->fe->fd < fd_b->fe->fd));
 }
 
 struct file_desc_ops {

shmem.c

@@ -44,7 +44,7 @@ static int collect_shmem(int pid, VmaEntry *vi)
 		 * will wait until the kernel propagate this mapping
 		 * into /proc
 		 */
-		if (si->pid <= pid)
+		if (!pid_rst_prio(pid, si->pid))
 			return 0;
 
 		si->pid	 = pid;