4 files changed, 57 insertions, 1 deletions

diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt
index 9e592c718afb..afa2946892da 100644
--- a/Documentation/sysrq.txt
+++ b/Documentation/sysrq.txt
@@ -81,6 +81,8 @@ On all -  write a character to /proc/sysrq-trigger.  e.g.:
 
 'i'     - Send a SIGKILL to all processes, except for init.
 
+'j'     - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
+
 'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
           console. NOTE: See important comments below in SAK section.
 
@@ -160,6 +162,9 @@ t'E'rm and k'I'll are useful if you have some sort of runaway process you
 are unable to kill any other way, especially if it's spawning other
 processes.
 
+"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
+(probably root) filesystem via the FIFREEZE ioctl.
+
 *  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 That happens to me, also. I've found that tapping shift, alt, and control
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 */
diff --git a/fs/buffer.c b/fs/buffer.c
index c77b848c3d43..f5f8b15a6e40 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -547,6 +547,39 @@ repeat:
         return err;
 }
 
+void do_thaw_all(unsigned long unused)
+{
+        struct super_block *sb;
+        char b[BDEVNAME_SIZE];
+
+        spin_lock(&sb_lock);
+restart:
+        list_for_each_entry(sb, &super_blocks, s_list) {
+                sb->s_count++;
+                spin_unlock(&sb_lock);
+                down_read(&sb->s_umount);
+                while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
+                        printk(KERN_WARNING "Emergency Thaw on %s\n",
+                               bdevname(sb->s_bdev, b));
+                up_read(&sb->s_umount);
+                spin_lock(&sb_lock);
+                if (__put_super_and_need_restart(sb))
+                        goto restart;
+        }
+        spin_unlock(&sb_lock);
+        printk(KERN_WARNING "Emergency Thaw complete\n");
+}
+
+/**
+ * emergency_thaw_all -- forcibly thaw every frozen filesystem
+ *
+ * Used for emergency unfreeze of all filesystems via SysRq
+ */
+void emergency_thaw_all(void)
+{
+        pdflush_operation(do_thaw_all, 0);
+}
+
 /**
  * sync_mapping_buffers - write out & wait upon a mapping's "associated" buffers
  * @mapping: the mapping which wants those buffers written
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 87e7bfc5ebd7..61211ad823fe 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1878,6 +1878,7 @@ extern struct block_device *open_by_devnum(dev_t, fmode_t);
 extern void invalidate_bdev(struct block_device *);
 extern int sync_blockdev(struct block_device *bdev);
 extern struct super_block *freeze_bdev(struct block_device *);
+extern void emergency_thaw_all(void);
 extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
 extern int fsync_bdev(struct block_device *);
 extern int fsync_super(struct super_block *);