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[PATCH] uml: fix previous console locking

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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@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jeff Dike 2007-02-10 01:44:08 -08:00 committed by Linus Torvalds
parent d5c9ffc6c6
commit c6256c6824
2 changed files with 8 additions and 41 deletions
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48
arch/um/drivers/line.c
View File

@ -425,42 +425,15 @@ int line_setup_irq(int fd, int input, int output, struct line *line, void *data)
* 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 tty_struct *tty)
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 tty_struct *tty)
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, struct file * filp)
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, struct file * filp)
line->sigio = 0;
}
mutex_unlock(&line->open_mutex);
return;
out_unlock:
@ -755,7 +724,6 @@ void lines_init(struct line *lines, int nlines, struct chan_opts *opts)
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;

1
arch/um/include/line.h
View File

@ -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;