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Date: Fri, 05 Jan 2007 13:42:16 -0500
From: Jeff Dike <jdike@...toit.com>
To: akpm@...l.org
cc: linux-kernel@...r.kernel.org,
user-mode-linux-devel@...ts.sourceforge.net,
Blaisorblade <blaisorblade@...oo.it>
Subject: [PATCH 7/9] UML - Fix previous console locking
Eliminate the open_mutex after complaints from Blaisorblade. It turns
out that the tty count provides the information needed to tell whether
we are the first opener or last closer.
Signed-off-by: Jeff Dike <jdike@...toit.com>
--
arch/um/drivers/line.c | 48 ++++++++----------------------------------------
arch/um/include/line.h | 1 -
2 files changed, 8 insertions(+), 41 deletions(-)
Index: linux-2.6.18-mm/arch/um/drivers/line.c
===================================================================
--- linux-2.6.18-mm.orig/arch/um/drivers/line.c 2007-01-03 12:08:02.000000000 -0500
+++ linux-2.6.18-mm/arch/um/drivers/line.c 2007-01-04 12:02:29.000000000 -0500
@@ -425,42 +425,15 @@ int line_setup_irq(int fd, int input, in
* However, in this case, mconsole requests can come in "from the
* side", and race with opens and closes.
*
- * The problem comes from line_setup not wanting to sleep if
- * the device is open or being opened. This can happen because the
- * first opener of a device is responsible for setting it up on the
- * host, and that can sleep. The open of a port device will sleep
- * until someone telnets to it.
+ * mconsole config requests will want to be sure the device isn't in
+ * use, and get_config, open, and close will want a stable
+ * configuration. The checking and modification of the configuration
+ * is done under a spinlock. Checking whether the device is in use is
+ * line->tty->count > 1, also under the spinlock.
*
- * The obvious solution of putting everything under a mutex fails
- * because then trying (and failing) to change the configuration of an
- * open(ing) device will block until the open finishes. The right
- * thing to happen is for it to fail immediately.
- *
- * We can put the opening (and closing) of the host device under a
- * separate lock, but that has to be taken before the count lock is
- * released. Otherwise, you open a window in which another open can
- * come through and assume that the host side is opened and working.
- *
- * So, if the tty count is one, open will take the open mutex
- * inside the count lock. Otherwise, it just returns. This will sleep
- * if the last close is pending, and will block a setup or get_config,
- * but that should not last long.
- *
- * So, what we end up with is that open and close take the count lock.
- * If the first open or last close are happening, then the open mutex
- * is taken inside the count lock and the host opening or closing is done.
- *
- * setup and get_config only take the count lock. setup modifies the
- * device configuration only if the open count is zero. Arbitrarily
- * long blocking of setup doesn't happen because something would have to be
- * waiting for an open to happen. However, a second open with
- * tty->count == 1 can't happen, and a close can't happen until the open
- * had finished.
- *
- * We can't maintain our own count here because the tty layer doesn't
- * match opens and closes. It will call close if an open failed, and
- * a tty hangup will result in excess closes. So, we rely on
- * tty->count instead. It is one on both the first open and last close.
+ * tty->count serves to decide whether the device should be enabled or
+ * disabled on the host. If it's equal to 1, then we are doing the
+ * first open or last close. Otherwise, open and close just return.
*/
int line_open(struct line *lines, struct tty_struct *tty)
@@ -476,7 +449,6 @@ int line_open(struct line *lines, struct
if(tty->count > 1)
goto out_unlock;
- mutex_lock(&line->open_mutex);
spin_unlock(&line->count_lock);
tty->driver_data = line;
@@ -493,7 +465,6 @@ int line_open(struct line *lines, struct
chan_window_size(&line->chan_list, &tty->winsize.ws_row,
&tty->winsize.ws_col);
- mutex_unlock(&line->open_mutex);
return err;
out_unlock:
@@ -523,7 +494,6 @@ void line_close(struct tty_struct *tty,
if(tty->count > 1)
goto out_unlock;
- mutex_lock(&line->open_mutex);
spin_unlock(&line->count_lock);
line->tty = NULL;
@@ -534,7 +504,6 @@ void line_close(struct tty_struct *tty,
line->sigio = 0;
}
- mutex_unlock(&line->open_mutex);
return;
out_unlock:
@@ -755,7 +724,6 @@ void lines_init(struct line *lines, int
for(i = 0; i < nlines; i++){
line = &lines[i];
INIT_LIST_HEAD(&line->chan_list);
- mutex_init(&line->open_mutex);
if(line->init_str == NULL)
continue;
Index: linux-2.6.18-mm/arch/um/include/line.h
===================================================================
--- linux-2.6.18-mm.orig/arch/um/include/line.h 2007-01-02 13:29:54.000000000 -0500
+++ linux-2.6.18-mm/arch/um/include/line.h 2007-01-03 16:31:32.000000000 -0500
@@ -35,7 +35,6 @@ struct line {
spinlock_t count_lock;
int valid;
- struct mutex open_mutex;
char *init_str;
int init_pri;
struct list_head chan_list;
-
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