filesystem freeze: allow SysRq emergency thaw to thaw frozen filesystems

Now that the filesystem freeze operation has been elevated to the VFS, and
is just an ioctl away, some sort of safety net for unintentionally frozen
root filesystems may be in order.

The timeout thaw originally proposed did not get merged, but perhaps
something like this would be useful in emergencies.

For example, freeze /path/to/mountpoint may freeze your root filesystem if
you forgot that you had that unmounted.

I chose 'j' as the last remaining character other than 'h' which is sort
of reserved for help (because help is generated on any unknown character).

I've tested this on a non-root fs with multiple (nested) freezers, as well
as on a system rendered unresponsive due to a frozen root fs.

[randy.dunlap@oracle.com: emergency thaw only if CONFIG_BLOCK enabled]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: Takashi Sato <t-sato@yk.jp.nec.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 1 deletions

diff --git a/drivers/char/sysrq.c b/drivers/char/sysrq.c
index 33a9351c896d..5afe7316c72e 100644
--- a/drivers/char/sysrq.c
+++ b/drivers/char/sysrq.c
@@ -346,6 +346,19 @@ static struct sysrq_key_op sysrq_moom_op = {
         .enable_mask    = SYSRQ_ENABLE_SIGNAL,
 };
 
+#ifdef CONFIG_BLOCK
+static void sysrq_handle_thaw(int key, struct tty_struct *tty)
+{
+        emergency_thaw_all();
+}
+static struct sysrq_key_op sysrq_thaw_op = {
+        .handler        = sysrq_handle_thaw,
+        .help_msg       = "thaw-filesystems(J)",
+        .action_msg     = "Emergency Thaw of all frozen filesystems",
+        .enable_mask    = SYSRQ_ENABLE_SIGNAL,
+};
+#endif
+
 static void sysrq_handle_kill(int key, struct tty_struct *tty)
 {
         send_sig_all(SIGKILL);
@@ -396,9 +409,13 @@ static struct sysrq_key_op *sysrq_key_table[36] = {
         &sysrq_moom_op,                 /* f */
         /* g: May be registered by ppc for kgdb */
         NULL,                           /* g */
-        NULL,                           /* h */
+        NULL,                           /* h - reserved for help */
         &sysrq_kill_op,                 /* i */
+#ifdef CONFIG_BLOCK
+        &sysrq_thaw_op,                 /* j */
+#else
         NULL,                           /* j */
+#endif
         &sysrq_SAK_op,                  /* k */
 #ifdef CONFIG_SMP
         &sysrq_showallcpus_op,          /* l */