[src/trunk]: src/sys/kern Work around deadlock between fstchg and fstcnt

[manu <manu%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/d2c8523d1e77
branches:  trunk
changeset: 836109:d2c8523d1e77
user:      manu <manu%NetBSD.org@localhost>
date:      Thu Sep 27 01:03:40 2018 +0000

description:
Work around deadlock between fstchg and fstcnt

When suspending a filesystem in fstrans_setstate(), we wait on
fstcnt for threads to finish transactions. While we do this, any
thread trying to start a filesystem transaction will wait on fstchg
in fstrans_start(), a situation which can deadlock.

The wait for fstcnt in fstrans_setstate() can be interrupted by
a signal, but the wait for fstchg in fstrans_start() cannot. Once
most processes are stuck in fstchg, it is impossible to send a
signal to the thread that waits on fstcnt, because no process
respond anymore to user input.

We fix that by adding a timeout to the wait on fstcnt in
fstrans_setstate(). This means suspending a filesystem may fail,
but it was already the case when the sleep was interupted by
a signal, hence calling function must already handle a possible
failure.

Fixes kern/53624

diffstat:

 sys/kern/vfs_trans.c |  11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diffs (42 lines):

diff -r b1b3656dd757 -r d2c8523d1e77 sys/kern/vfs_trans.c
--- a/sys/kern/vfs_trans.c      Thu Sep 27 00:45:34 2018 +0000
+++ b/sys/kern/vfs_trans.c      Thu Sep 27 01:03:40 2018 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: vfs_trans.c,v 1.48 2017/06/18 14:00:17 hannken Exp $   */
+/*     $NetBSD: vfs_trans.c,v 1.49 2018/09/27 01:03:40 manu Exp $      */
 
 /*-
  * Copyright (c) 2007 The NetBSD Foundation, Inc.
@@ -30,7 +30,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_trans.c,v 1.48 2017/06/18 14:00:17 hannken Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_trans.c,v 1.49 2018/09/27 01:03:40 manu Exp $");
 
 /*
  * File system transaction operations.
@@ -42,6 +42,7 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/kernel.h>
 #include <sys/atomic.h>
 #include <sys/buf.h>
 #include <sys/kmem.h>
@@ -532,10 +533,14 @@
        /*
         * All threads see the new state now.
         * Wait for transactions invalid at this state to leave.
+        * We cannot wait forever because many processes would
+        * get stuck waiting for fstcnt in fstrans_start(). This
+        * is acute when suspending the root filesystem.
         */
        error = 0;
        while (! state_change_done(mp)) {
-               error = cv_wait_sig(&fstrans_count_cv, &fstrans_lock);
+               error = cv_timedwait_sig(&fstrans_count_cv,
+                                         &fstrans_lock, hz / 4);
                if (error) {
                        new_state = fmi->fmi_state = FSTRANS_NORMAL;
                        break;