coredump: format_corename: fix the "core_uses_pid" logic

I don't understand why the multi-thread coredump implies the core_uses_pid
behaviour, but we shouldn't use mm->mm_users for that.  This counter can
be incremented by get_task_mm().  Use the valued returned by
coredump_wait() instead.

Also, remove the "const char *pattern" argument, format_corename() can use
core_pattern directly.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1379,17 +1379,14 @@ EXPORT_SYMBOL(set_binfmt);
  * name into corename, which must have space for at least
  * CORENAME_MAX_SIZE bytes plus one byte for the zero terminator.
  */
-static int format_corename(char *corename, const char *pattern, long signr)
+static int format_corename(char *corename, int nr_threads, long signr)
 {
-	const char *pat_ptr = pattern;
+	const char *pat_ptr = core_pattern;
+	int ispipe = (*pat_ptr == '|');
 	char *out_ptr = corename;
 	char *const out_end = corename + CORENAME_MAX_SIZE;
 	int rc;
 	int pid_in_pattern = 0;
-	int ispipe = 0;
-
-	if (*pattern == '|')
-		ispipe = 1;
 
 	/* Repeat as long as we have more pattern to process and more output
 	   space */
@@ -1490,7 +1487,7 @@ static int format_corename(char *corename, const char *pattern, long signr)
 	 * and core_uses_pid is set, then .%pid will be appended to
 	 * the filename. Do not do this for piped commands. */
 	if (!ispipe && !pid_in_pattern
-            && (core_uses_pid || atomic_read(&current->mm->mm_users) != 1)) {
+	    && (core_uses_pid || nr_threads)) {
 		rc = snprintf(out_ptr, out_end - out_ptr,
 			      ".%d", task_tgid_vnr(current));
 		if (rc > out_end - out_ptr)
@@ -1753,7 +1750,7 @@ int do_coredump(long signr, int exit_code, struct pt_regs * regs)
 	 * uses lock_kernel()
 	 */
  	lock_kernel();
-	ispipe = format_corename(corename, core_pattern, signr);
+	ispipe = format_corename(corename, retval, signr);
 	unlock_kernel();
 	/*
 	 * Don't bother to check the RLIMIT_CORE value if core_pattern points