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Commit d79a5809 authored by Jeff Dike's avatar Jeff Dike Committed by Linus Torvalds
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[PATCH] uml: console locking fixes 



Clean up the console driver locking.  There are various problems here,
including sleeping under a spinlock and spinlock recursion, some of which are
fixed here.  This patch deals with the locking involved with opens and closes.
 The problem is that an mconsole request to change a console's configuration
can race with an open.  Changing a configuration should only be done when a
console isn't opened.  Also, an open must be looking at a stable
configuration.  In addition, a get configuration request must observe the same
locking since it must also see a stable configuration.  With the old locking,
it was possible for this to hang indefinitely in some cases because open would
block for a long time waiting for a connection from the host while holding the
lock needed by the mconsole request.

As explained in the long comment, this is fixed by adding a spinlock for the
use count and configuration and a mutex for the actual open and close.

Signed-off-by: default avatarJeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5cf885d0

arch/um/drivers/line.c

+122 −64
Original line number Diff line number Diff line
@@ -191,7 +191,6 @@ void line_flush_buffer(struct tty_struct *tty) 
	/*XXX: copied from line_write, verify if it is correct!*/

	if(tty->stopped)

		return;

		//return 0;



	spin_lock_irqsave(&line->lock, flags);

	err = flush_buffer(line);
@@ -421,27 +420,68 @@ int line_setup_irq(int fd, int input, int output, struct line *line, void *data) 
	return err;

}



/* Normally, a driver like this can rely mostly on the tty layer

 * locking, particularly when it comes to the driver structure.

 * 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.

 *

 * 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.

 */



int line_open(struct line *lines, struct tty_struct *tty)

{

	struct line *line;

	struct line *line = &lines[tty->index];

	int err = -ENODEV;



	line = &lines[tty->index];

	tty->driver_data = line;

	spin_lock(&line->count_lock);

	if(!line->valid)

		goto out_unlock;



	/* The IRQ which takes this lock is not yet enabled and won't be run

	 * before the end, so we don't need to use spin_lock_irq.*/

	spin_lock(&line->lock);

	err = 0;

	if(tty->count > 1)

		goto out_unlock;



	mutex_lock(&line->open_mutex);

	spin_unlock(&line->count_lock);



	tty->driver_data = line;

	line->tty = tty;

	if(!line->valid)

		goto out;



	if(tty->count == 1){

		/* Here the device is opened, if necessary, and interrupt

		 * is registered.

		 */

	enable_chan(line);

	INIT_DELAYED_WORK(&line->task, line_timer_cb);
@@ -452,11 +492,12 @@ int line_open(struct line *lines, struct tty_struct *tty) 


	chan_window_size(&line->chan_list, &tty->winsize.ws_row,

			 &tty->winsize.ws_col);

	}



	err = 0;

out:

	spin_unlock(&line->lock);

	mutex_unlock(&line->open_mutex);

	return err;



out_unlock:

	spin_unlock(&line->count_lock);

	return err;

}
@@ -466,15 +507,25 @@ void line_close(struct tty_struct *tty, struct file * filp) 
{

	struct line *line = tty->driver_data;



	/* XXX: I assume this should be called in process context, not with

         *  interrupts disabled!

	/* If line_open fails (and tty->driver_data is never set),

	 * tty_open will call line_close.  So just return in this case.

	 */

	spin_lock_irq(&line->lock);

	if(line == NULL)

		return;



	/* We ignore the error anyway! */

	flush_buffer(line);



	if(tty->count == 1){

	spin_lock(&line->count_lock);

	if(!line->valid)

		goto out_unlock;



	if(tty->count > 1)

		goto out_unlock;



	mutex_lock(&line->open_mutex);

	spin_unlock(&line->count_lock);



	line->tty = NULL;

	tty->driver_data = NULL;
@@ -482,9 +533,12 @@ void line_close(struct tty_struct *tty, struct file * filp) 
		unregister_winch(tty);

		line->sigio = 0;

        }

        }



	spin_unlock_irq(&line->lock);

	mutex_unlock(&line->open_mutex);

	return;



out_unlock:

	spin_unlock(&line->count_lock);

}



void close_lines(struct line *lines, int nlines)
@@ -495,6 +549,30 @@ void close_lines(struct line *lines, int nlines) 
		close_chan(&lines[i].chan_list, 0);

}



static void setup_one_line(struct line *lines, int n, char *init, int init_prio)

{

	struct line *line = &lines[n];



	spin_lock(&line->count_lock);



	if(line->tty != NULL){

		printk("line_setup - device %d is open\n", n);

		goto out;

	}



	if (line->init_pri <= init_prio){

		line->init_pri = init_prio;

		if (!strcmp(init, "none"))

			line->valid = 0;

		else {

			line->init_str = init;

			line->valid = 1;

		}

	}

out:

	spin_unlock(&line->count_lock);

}



/* Common setup code for both startup command line and mconsole initialization.

 * @lines contains the array (of size @num) to modify;

 * @init is the setup string;
@@ -526,32 +604,11 @@ int line_setup(struct line *lines, unsigned int num, char *init) 
		       n, num - 1);

		return 0;

	}

	else if (n >= 0){

		if (lines[n].tty != NULL) {

			printk("line_setup - device %d is open\n", n);

			return 0;

		}

		if (lines[n].init_pri <= INIT_ONE){

			lines[n].init_pri = INIT_ONE;

			if (!strcmp(init, "none"))

				lines[n].valid = 0;

			else {

				lines[n].init_str = init;

				lines[n].valid = 1;

			}

		}

	}

	else {

		for(i = 0; i < num; i++){

			if(lines[i].init_pri <= INIT_ALL){

				lines[i].init_pri = INIT_ALL;

				if(!strcmp(init, "none")) lines[i].valid = 0;

	else if (n >= 0)

		setup_one_line(lines, n, init, INIT_ONE);

	else {

					lines[i].init_str = init;

					lines[i].valid = 1;

				}

			}

		}

		for(i = 0; i < num; i++)

			setup_one_line(lines, i, init, INIT_ALL);

	}

	return n == -1 ? num : n;

}
@@ -602,13 +659,13 @@ int line_get_config(char *name, struct line *lines, unsigned int num, char *str, 


	line = &lines[dev];



	spin_lock(&line->lock);

	spin_lock(&line->count_lock);

	if(!line->valid)

		CONFIG_CHUNK(str, size, n, "none", 1);

	else if(line->tty == NULL)

		CONFIG_CHUNK(str, size, n, line->init_str, 1);

	else n = chan_config_string(&line->chan_list, str, size, error_out);

	spin_unlock(&line->lock);

	spin_unlock(&line->count_lock);



	return n;

}
@@ -688,6 +745,7 @@ 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;

arch/um/drivers/stdio_console.c

+3 −3
Original line number Diff line number Diff line
@@ -83,7 +83,7 @@ static struct lines console_lines = LINES_INIT(MAX_TTYS); 
/* The array is initialized by line_init, which is an initcall.  The 

 * individual elements are protected by individual semaphores.

 */

struct line vts[MAX_TTYS] = { LINE_INIT(CONFIG_CON_ZERO_CHAN, &driver),

static struct line vts[MAX_TTYS] = { LINE_INIT(CONFIG_CON_ZERO_CHAN, &driver),

				     [ 1 ... MAX_TTYS - 1 ] =

				     LINE_INIT(CONFIG_CON_CHAN, &driver) };

arch/um/include/line.h

+9 −5
Original line number Diff line number Diff line
@@ -11,6 +11,7 @@ 
#include "linux/tty.h"

#include "linux/interrupt.h"

#include "linux/spinlock.h"

#include "linux/mutex.h"

#include "chan_user.h"

#include "mconsole_kern.h"
@@ -32,15 +33,17 @@ struct line_driver { 


struct line {

	struct tty_struct *tty;

	spinlock_t count_lock;

	int valid;



	struct mutex open_mutex;

	char *init_str;

	int init_pri;

	struct list_head chan_list;

	int valid;

	int count;

	int throttled;



	/*This lock is actually, mostly, local to*/

	spinlock_t lock;



	int throttled;

	/* Yes, this is a real circular buffer.

	 * XXX: And this should become a struct kfifo!

	 *
@@ -57,7 +60,8 @@ struct line { 
};



#define LINE_INIT(str, d) \

	{ .init_str =	str, \

	{ .count_lock =	SPIN_LOCK_UNLOCKED, \

	  .init_str =	str,	\

	  .init_pri =	INIT_STATIC, \

	  .valid =	1, \

	  .lock =	SPIN_LOCK_UNLOCKED, \