core dump: user_regset writeback

This makes the user_regset-based core dump code call user_regset writeback
hooks when available.  This is necessary groundwork to allow IA64 to set
CORE_DUMP_USE_REGSET.

Cc: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf.c

@@ -1424,6 +1424,18 @@ struct elf_note_info {
 	int thread_notes;
 };
 
+/*
+ * When a regset has a writeback hook, we call it on each thread before
+ * dumping user memory.  On register window machines, this makes sure the
+ * user memory backing the register data is up to date before we read it.
+ */
+static void do_thread_regset_writeback(struct task_struct *task,
+				       const struct user_regset *regset)
+{
+	if (regset->writeback)
+		regset->writeback(task, regset, 1);
+}
+
 static int fill_thread_core_info(struct elf_thread_core_info *t,
 				 const struct user_regset_view *view,
 				 long signr, size_t *total)
@@ -1445,13 +1457,16 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 		  sizeof(t->prstatus), &t->prstatus);
 	*total += notesize(&t->notes[0]);
 
+	do_thread_regset_writeback(t->task, &view->regsets[0]);
+
 	/*
 	 * Each other regset might generate a note too.  For each regset
 	 * that has no core_note_type or is inactive, we leave t->notes[i]
 	 * all zero and we'll know to skip writing it later.
 	 */
 	for (i = 1; i < view->n; ++i) {
 		const struct user_regset *regset = &view->regsets[i];
+		do_thread_regset_writeback(t->task, regset);
 		if (regset->core_note_type &&
 		    (!regset->active || regset->active(t->task, regset))) {
 			int ret;