[PATCH] vt: refactor console SAK processing

This does several things.
- It moves looking up of the current foreground console into process
  context where we can safely take the semaphore that protects this
  operation.
- It uses the new flavor of work queue processing.
- This generates a factor of do_SAK, __do_SAK that runs immediately.
- This calls __do_SAK with the console semaphore held ensuring nothing
  else happens to the console while we process the SAK operation.
- With the console SAK processing moved into process context this
  patch removes the xchg operations that I used to attempt to attomically
  update struct pid, because of the strange locking used in the SAK processing.
  With SAK using the normal console semaphore nothing special is needed.

Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 41 insertions, 18 deletions

diff --git a/drivers/char/keyboard.c b/drivers/char/keyboard.c
index 7a6c1c0b7a95..c654a3e0c697 100644
--- a/drivers/char/keyboard.c
+++ b/drivers/char/keyboard.c
@@ -595,15 +595,9 @@ static void fn_spawn_con(struct vc_data *vc)
 
 static void fn_SAK(struct vc_data *vc)
 {
-        struct tty_struct *tty = vc->vc_tty;
-
-        /*
-         * SAK should also work in all raw modes and reset
-         * them properly.
-         */
-        if (tty)
-                do_SAK(tty);
-        reset_vc(vc);
+        struct work_struct *SAK_work = &vc_cons[fg_console].SAK_work;
+        PREPARE_WORK(SAK_work, vc_SAK);
+        schedule_work(SAK_work);
 }
 
 static void fn_null(struct vc_data *vc)
diff --git a/drivers/char/sysrq.c b/drivers/char/sysrq.c
index 7fd3cd5ddf21..3757610b7835 100644
--- a/drivers/char/sysrq.c
+++ b/drivers/char/sysrq.c
@@ -88,9 +88,9 @@ static struct sysrq_key_op sysrq_loglevel_op = {
 #ifdef CONFIG_VT
 static void sysrq_handle_SAK(int key, struct tty_struct *tty)
 {
-        if (tty)
-                do_SAK(tty);
-        reset_vc(vc_cons[fg_console].d);
+        struct work_struct *SAK_work = &vc_cons[fg_console].SAK_work;
+        PREPARE_WORK(SAK_work, vc_SAK);
+        schedule_work(SAK_work);
 }
 static struct sysrq_key_op sysrq_SAK_op = {
         .handler        = sysrq_handle_SAK,
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 47a6eacb10bc..c57b1f434652 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -3324,10 +3324,8 @@ int tty_ioctl(struct inode * inode, struct file * file,
  * Nasty bug: do_SAK is being called in interrupt context.  This can
  * deadlock.  We punt it up to process context.  AKPM - 16Mar2001
  */
-static void __do_SAK(struct work_struct *work)
+void __do_SAK(struct tty_struct *tty)
 {
-        struct tty_struct *tty =
-                container_of(work, struct tty_struct, SAK_work);
 #ifdef TTY_SOFT_SAK
         tty_hangup(tty);
 #else
@@ -3394,6 +3392,13 @@ static void __do_SAK(struct work_struct *work)
 #endif
 }
 
+static void do_SAK_work(struct work_struct *work)
+{
+        struct tty_struct *tty =
+                container_of(work, struct tty_struct, SAK_work);
+        __do_SAK(tty);
+}
+
 /*
  * The tq handling here is a little racy - tty->SAK_work may already be queued.
  * Fortunately we don't need to worry, because if ->SAK_work is already queued,
@@ -3404,7 +3409,7 @@ void do_SAK(struct tty_struct *tty)
 {
         if (!tty)
                 return;
-        PREPARE_WORK(&tty->SAK_work, __do_SAK);
+        PREPARE_WORK(&tty->SAK_work, do_SAK_work);
         schedule_work(&tty->SAK_work);
 }
 
diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c
index dc8368ebb1ac..3a5d301e783b 100644
--- a/drivers/char/vt_ioctl.c
+++ b/drivers/char/vt_ioctl.c
@@ -672,7 +672,8 @@ int vt_ioctl(struct tty_struct *tty, struct file * file,
                 vc->vt_mode = tmp;
                 /* the frsig is ignored, so we set it to 0 */
                 vc->vt_mode.frsig = 0;
-                put_pid(xchg(&vc->vt_pid, get_pid(task_pid(current))));
+                put_pid(vc->vt_pid);
+                vc->vt_pid = get_pid(task_pid(current));
                 /* no switch is required -- saw@shade.msu.ru */
                 vc->vt_newvt = -1;
                 release_console_sem();
@@ -1063,12 +1064,35 @@ void reset_vc(struct vc_data *vc)
         vc->vt_mode.relsig = 0;
         vc->vt_mode.acqsig = 0;
         vc->vt_mode.frsig = 0;
-        put_pid(xchg(&vc->vt_pid, NULL));
+        put_pid(vc->vt_pid);
+        vc->vt_pid = NULL;
         vc->vt_newvt = -1;
         if (!in_interrupt())    /* Via keyboard.c:SAK() - akpm */
                 reset_palette(vc);
 }
 
+void vc_SAK(struct work_struct *work)
+{
+        struct vc *vc_con =
+                container_of(work, struct vc, SAK_work);
+        struct vc_data *vc;
+        struct tty_struct *tty;
+
+        acquire_console_sem();
+        vc = vc_con->d;
+        if (vc) {
+                tty = vc->vc_tty;
+                /*
+                 * SAK should also work in all raw modes and reset
+                 * them properly.
+                 */
+                if (tty)
+                        __do_SAK(tty);
+                reset_vc(vc);
+        }
+        release_console_sem();
+}
+
 /*
  * Performs the back end of a vt switch
  */