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

Matthias Kaehlcke
2007-04-25T19:43:47Z

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

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