[PATCH] use mutex instead of semaphore in tty_io.c

[PATCH] use mutex instead of semaphore in tty_io.c

[Matthias Kaehlcke]

drivers/char/tty_io.c uses a semaphore as mutex. use the mutex API
instead of the (binary) semaphore

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>

--

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 7a32df5..4496fd2 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -137,7 +137,7 @@ EXPORT_SYMBOL(tty_mutex);
 extern struct tty_driver *ptm_driver;	/* Unix98 pty masters; for /dev/ptmx */
 extern int pty_limit;		/* Config limit on Unix98 ptys */
 static DEFINE_IDR(allocated_ptys);
-static DECLARE_MUTEX(allocated_ptys_lock);
+static DEFINE_MUTEX(allocated_ptys_lock);
 static int ptmx_open(struct inode *, struct file *);
 #endif
 
@@ -2471,9 +2471,9 @@ static void release_dev(struct file * filp)
 #ifdef CONFIG_UNIX98_PTYS
 	/* Make this pty number available for reallocation */
 	if (devpts) {
-		down(&allocated_ptys_lock);
+		mutex_lock(&allocated_ptys_lock);
 		idr_remove(&allocated_ptys, idx);
-		up(&allocated_ptys_lock);
+		mutex_unlock(&allocated_ptys_lock);
 	}
 #endif
 
@@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
 	nonseekable_open(inode, filp);
 
 	/* find a device that is not in use. */
-	down(&allocated_ptys_lock);
+	mutex_lock(&allocated_ptys_lock);
 	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
-		up(&allocated_ptys_lock);
+		mutex_unlock(&allocated_ptys_lock);
 		return -ENOMEM;
 	}
 	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
 	if (idr_ret < 0) {
-		up(&allocated_ptys_lock);
+		mutex_unlock(&allocated_ptys_lock);
 		if (idr_ret == -EAGAIN)
 			return -ENOMEM;
 		return -EIO;
 	}
 	if (index >= pty_limit) {
 		idr_remove(&allocated_ptys, index);
-		up(&allocated_ptys_lock);
+		mutex_unlock(&allocated_ptys_lock);
 		return -EIO;
 	}
-	up(&allocated_ptys_lock);
+	mutex_unlock(&allocated_ptys_lock);
 
 	mutex_lock(&tty_mutex);
 	retval = init_dev(ptm_driver, index, &tty);
@@ -2681,9 +2681,9 @@ out1:
 	release_dev(filp);
 	return retval;
 out:
-	down(&allocated_ptys_lock);
+	mutex_lock(&allocated_ptys_lock);
 	idr_remove(&allocated_ptys, index);
-	up(&allocated_ptys_lock);
+	mutex_unlock(&allocated_ptys_lock);
 	return retval;
 }
 #endif

-- 
Matthias Kaehlcke
Linux Application Developer
Barcelona

            You can chain me, you can torture me, you can even
          destroy this body, but you will never imprison my mind
                            (Mahatma Gandhi)
                                                                 .''`.
    using free software / Debian GNU/Linux | http://debian.org  : :'  :
                                                                `. `'`
gpg --keyserver pgp.mit.edu --recv-keys 47D8E5D4                  `-

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Christoph Hellwig]

On Wed, Apr 25, 2007 at 05:49:34PM +0200, Matthias Kaehlcke wrote:
> drivers/char/tty_io.c uses a semaphore as mutex. use the mutex API
> instead of the (binary) semaphore

This looks like it should be a spinlock:

> -		down(&allocated_ptys_lock);
> +		mutex_lock(&allocated_ptys_lock);
>  		idr_remove(&allocated_ptys, idx);
> -		up(&allocated_ptys_lock);
> +		mutex_unlock(&allocated_ptys_lock);

idr_remove is a quick operation that doesn't sleep.

> @@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
>  	nonseekable_open(inode, filp);
>  
>  	/* find a device that is not in use. */
> -	down(&allocated_ptys_lock);
> +	mutex_lock(&allocated_ptys_lock);
>  	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> -		up(&allocated_ptys_lock);

The idr_pre_get should be moved out of the lock, that's the whole
point for it's existance..

> +		mutex_unlock(&allocated_ptys_lock);
>  		return -ENOMEM;
>  	}
>  	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
>  	if (idr_ret < 0) {
> -		up(&allocated_ptys_lock);
> +		mutex_unlock(&allocated_ptys_lock);
>  		if (idr_ret == -EAGAIN)
>  			return -ENOMEM;
>  		return -EIO;
>  	}
>  	if (index >= pty_limit) {
>  		idr_remove(&allocated_ptys, index);
> -		up(&allocated_ptys_lock);
> +		mutex_unlock(&allocated_ptys_lock);
>  		return -EIO;
>  	}
> -	up(&allocated_ptys_lock);
> +	mutex_unlock(&allocated_ptys_lock);

And idr_get_new is another quick, non-blocking operation.

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Christoph Hellwig]

On Wed, Apr 25, 2007 at 09:46:33PM +0200, Matthias Kaehlcke wrote:
> thanks for your remarks. is the following patch what you are
> proposing?

Yes, exactly.  Looks good to me this way.:w

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Matthias Kaehlcke]

El Wed, Apr 25, 2007 at 08:13:59PM +0100 Christoph Hellwig ha dit:

> On Wed, Apr 25, 2007 at 05:49:34PM +0200, Matthias Kaehlcke wrote:
> > drivers/char/tty_io.c uses a semaphore as mutex. use the mutex API
> > instead of the (binary) semaphore
> 
> This looks like it should be a spinlock:
> 
> > -		down(&allocated_ptys_lock);
> > +		mutex_lock(&allocated_ptys_lock);
> >  		idr_remove(&allocated_ptys, idx);
> > -		up(&allocated_ptys_lock);
> > +		mutex_unlock(&allocated_ptys_lock);
> 
> idr_remove is a quick operation that doesn't sleep.
> 
> > @@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
> >  	nonseekable_open(inode, filp);
> >  
> >  	/* find a device that is not in use. */
> > -	down(&allocated_ptys_lock);
> > +	mutex_lock(&allocated_ptys_lock);
> >  	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> > -		up(&allocated_ptys_lock);
> 
> The idr_pre_get should be moved out of the lock, that's the whole
> point for it's existance..
> 
> > +		mutex_unlock(&allocated_ptys_lock);
> >  		return -ENOMEM;
> >  	}
> >  	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
> >  	if (idr_ret < 0) {
> > -		up(&allocated_ptys_lock);
> > +		mutex_unlock(&allocated_ptys_lock);
> >  		if (idr_ret == -EAGAIN)
> >  			return -ENOMEM;
> >  		return -EIO;
> >  	}
> >  	if (index >= pty_limit) {
> >  		idr_remove(&allocated_ptys, index);
> > -		up(&allocated_ptys_lock);
> > +		mutex_unlock(&allocated_ptys_lock);
> >  		return -EIO;
> >  	}
> > -	up(&allocated_ptys_lock);
> > +	mutex_unlock(&allocated_ptys_lock);
> 
> And idr_get_new is another quick, non-blocking operation.

thanks for your remarks. is the following patch what you are
proposing?

--
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 7a32df5..ff27587 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -137,7 +137,7 @@ EXPORT_SYMBOL(tty_mutex);
 extern struct tty_driver *ptm_driver;	/* Unix98 pty masters; for /dev/ptmx */
 extern int pty_limit;		/* Config limit on Unix98 ptys */
 static DEFINE_IDR(allocated_ptys);
-static DECLARE_MUTEX(allocated_ptys_lock);
+static DEFINE_SPINLOCK(allocated_ptys_lock);
 static int ptmx_open(struct inode *, struct file *);
 #endif
 
@@ -2471,9 +2471,9 @@ static void release_dev(struct file * filp)
 #ifdef CONFIG_UNIX98_PTYS
 	/* Make this pty number available for reallocation */
 	if (devpts) {
-		down(&allocated_ptys_lock);
+		spin_lock(&allocated_ptys_lock);
 		idr_remove(&allocated_ptys, idx);
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 	}
 #endif
 
@@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
 	nonseekable_open(inode, filp);
 
 	/* find a device that is not in use. */
-	down(&allocated_ptys_lock);
-	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
-		up(&allocated_ptys_lock);
+	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL))
 		return -ENOMEM;
-	}
+
+	spin_lock(&allocated_ptys_lock);
+
 	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
 	if (idr_ret < 0) {
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 		if (idr_ret == -EAGAIN)
 			return -ENOMEM;
 		return -EIO;
 	}
 	if (index >= pty_limit) {
 		idr_remove(&allocated_ptys, index);
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 		return -EIO;
 	}
-	up(&allocated_ptys_lock);
+	spin_unlock(&allocated_ptys_lock);
 
 	mutex_lock(&tty_mutex);
 	retval = init_dev(ptm_driver, index, &tty);
@@ -2681,9 +2681,9 @@ out1:
 	release_dev(filp);
 	return retval;
 out:
-	down(&allocated_ptys_lock);
+	spin_lock(&allocated_ptys_lock);
 	idr_remove(&allocated_ptys, index);
-	up(&allocated_ptys_lock);
+	spin_unlock(&allocated_ptys_lock);
 	return retval;
 }
 #endif

-- 
Matthias Kaehlcke
Linux Application Developer
Barcelona

             You must have a plan. If you don't have a plan,
               you'll become part of somebody else's plan
                                                                 .''`.
    using free software / Debian GNU/Linux | http://debian.org  : :'  :
                                                                `. `'`
gpg --keyserver pgp.mit.edu --recv-keys 47D8E5D4                  `-

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Andrew Morton]

On Wed, 25 Apr 2007 20:13:59 +0100 Christoph Hellwig <hch@infradead.org> wrote:

> On Wed, Apr 25, 2007 at 05:49:34PM +0200, Matthias Kaehlcke wrote:
> > drivers/char/tty_io.c uses a semaphore as mutex. use the mutex API
> > instead of the (binary) semaphore
> 
> This looks like it should be a spinlock:
> 
> > -		down(&allocated_ptys_lock);
> > +		mutex_lock(&allocated_ptys_lock);
> >  		idr_remove(&allocated_ptys, idx);
> > -		up(&allocated_ptys_lock);
> > +		mutex_unlock(&allocated_ptys_lock);
> 
> idr_remove is a quick operation that doesn't sleep.
> 
> > @@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
> >  	nonseekable_open(inode, filp);
> >  
> >  	/* find a device that is not in use. */
> > -	down(&allocated_ptys_lock);
> > +	mutex_lock(&allocated_ptys_lock);
> >  	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> > -		up(&allocated_ptys_lock);
> 
> The idr_pre_get should be moved out of the lock, that's the whole
> point for it's existance..
> 

I think having it inside the lock makes sense:

	mutex_lock()
	idr_pre_get()
	idr_get_new()
	mutex_unlock()

here, if idr_pre_get() succeeded, we know that idr_get_new() will succeed.

otoh:

try_again:
	idr_pre_get()
	mutex_lock()
	if (idr_get_new() == failed) {
		mutex_unlock()
		goto try_again;
	}
	mutex_unlock()

is not nice.


the IDR api is awful.  A little project is to rip out all its internal
locking and to implement caller-provided locking.

Unfortunately the fact that the library allocates memory means that we
might need to do awkward things like radix_tree_preload() to make it
reliable for callers who use spinlocking.

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Matthias Kaehlcke]

drivers/char/tty_io.c: Use spinlock instead of a (binary) semaphore

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>

--

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 7a32df5..ff27587 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -137,7 +137,7 @@ EXPORT_SYMBOL(tty_mutex);
 extern struct tty_driver *ptm_driver;	/* Unix98 pty masters; for /dev/ptmx */
 extern int pty_limit;		/* Config limit on Unix98 ptys */
 static DEFINE_IDR(allocated_ptys);
-static DECLARE_MUTEX(allocated_ptys_lock);
+static DEFINE_SPINLOCK(allocated_ptys_lock);
 static int ptmx_open(struct inode *, struct file *);
 #endif
 
@@ -2471,9 +2471,9 @@ static void release_dev(struct file * filp)
 #ifdef CONFIG_UNIX98_PTYS
 	/* Make this pty number available for reallocation */
 	if (devpts) {
-		down(&allocated_ptys_lock);
+		spin_lock(&allocated_ptys_lock);
 		idr_remove(&allocated_ptys, idx);
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 	}
 #endif
 
@@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
 	nonseekable_open(inode, filp);
 
 	/* find a device that is not in use. */
-	down(&allocated_ptys_lock);
-	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
-		up(&allocated_ptys_lock);
+	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL))
 		return -ENOMEM;
-	}
+
+	spin_lock(&allocated_ptys_lock);
+
 	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
 	if (idr_ret < 0) {
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 		if (idr_ret == -EAGAIN)
 			return -ENOMEM;
 		return -EIO;
 	}
 	if (index >= pty_limit) {
 		idr_remove(&allocated_ptys, index);
-		up(&allocated_ptys_lock);
+		spin_unlock(&allocated_ptys_lock);
 		return -EIO;
 	}
-	up(&allocated_ptys_lock);
+	spin_unlock(&allocated_ptys_lock);
 
 	mutex_lock(&tty_mutex);
 	retval = init_dev(ptm_driver, index, &tty);
@@ -2681,9 +2681,9 @@ out1:
 	release_dev(filp);
 	return retval;
 out:
-	down(&allocated_ptys_lock);
+	spin_lock(&allocated_ptys_lock);
 	idr_remove(&allocated_ptys, index);
-	up(&allocated_ptys_lock);
+	spin_unlock(&allocated_ptys_lock);
 	return retval;
 }
 #endif

-- 
Matthias Kaehlcke
Linux Application Developer
Barcelona

     The assumption that what currently exists must necessarily
      exist is the acid that corrodes all visionary thinking
                                                                 .''`.
    using free software / Debian GNU/Linux | http://debian.org  : :'  :
                                                                `. `'`
gpg --keyserver pgp.mit.edu --recv-keys 47D8E5D4                  `-

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Christoph Hellwig]

On Thu, May 31, 2007 at 03:42:26PM +0200, Matthias Kaehlcke wrote:
> drivers/char/tty_io.c: Use spinlock instead of a (binary) semaphore

Looks good.

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Andrew Morton]

On Thu, 31 May 2007 15:42:26 +0200
Matthias Kaehlcke <matthias.kaehlcke@gmail.com> wrote:

> drivers/char/tty_io.c: Use spinlock instead of a (binary) semaphore
> 

hm.

> 
> --
> 
> diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
> index 7a32df5..ff27587 100644
> --- a/drivers/char/tty_io.c
> +++ b/drivers/char/tty_io.c

We end up with this:

	/* find a device that is not in use. */
	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL))
		return -ENOMEM;

	spin_lock(&allocated_ptys_lock);

	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
	if (idr_ret < 0) {
		spin_unlock(&allocated_ptys_lock);
		if (idr_ret == -EAGAIN)
			return -ENOMEM;
		return -EIO;
	}
	if (index >= pty_limit) {
		idr_remove(&allocated_ptys, index);
		spin_unlock(&allocated_ptys_lock);
		return -EIO;
	}
	spin_unlock(&allocated_ptys_lock);

this leaves a small window in which another thread can come in and steal
away the idr tree's reserves, causing the idr_get_new() to fail.  It's
highly improbable, but it's real.

Hence I think a straight semaphore->mutex conversion would be better.

The IDR API absolutely blows chunks: it should require caller-provided
locking, like radix-tree.  But then it'd need gunk like radix_tree_preload
to be reliable.  Fact is, storage librares which need to allocate memory at
insert-time are always going to be problematic in-kernel.

Re: [PATCH] use mutex instead of semaphore in tty_io.c

[Matthias Kaehlcke]

El Thu, May 31, 2007 at 03:37:12PM -0700 Andrew Morton ha dit:

> On Thu, 31 May 2007 15:42:26 +0200
> Matthias Kaehlcke <matthias.kaehlcke@gmail.com> wrote:
> 
> > drivers/char/tty_io.c: Use spinlock instead of a (binary) semaphore
> > 
> 
> hm.
> 
> > 
> 
> We end up with this:
> 
> 	/* find a device that is not in use. */
> 	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL))
> 		return -ENOMEM;
> 
> 	spin_lock(&allocated_ptys_lock);
> 
> 	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
> 	if (idr_ret < 0) {
> 		spin_unlock(&allocated_ptys_lock);
> 		if (idr_ret == -EAGAIN)
> 			return -ENOMEM;
> 		return -EIO;
> 	}
> 	if (index >= pty_limit) {
> 		idr_remove(&allocated_ptys, index);
> 		spin_unlock(&allocated_ptys_lock);
> 		return -EIO;
> 	}
> 	spin_unlock(&allocated_ptys_lock);
> 
> this leaves a small window in which another thread can come in and steal
> away the idr tree's reserves, causing the idr_get_new() to fail.  It's
> highly improbable, but it's real.

i agree, thanks for pointing it out
 
> Hence I think a straight semaphore->mutex conversion would be better.

that leads us back to the initial patch. christoph: is that ok for
you or do you have another proposal?

-- 
Matthias Kaehlcke
Linux Application Developer
Barcelona

     The assumption that what currently exists must necessarily
      exist is the acid that corrodes all visionary thinking
                                                                 .''`.
    using free software / Debian GNU/Linux | http://debian.org  : :'  :
                                                                `. `'`
gpg --keyserver pgp.mit.edu --recv-keys 47D8E5D4                  `-