Re: Unnecessary BKL contention in video1394

Re: Unnecessary BKL contention in video1394

[Stefan Richter]

(added Cc: lkml to pull in some clues)

Daniel Drake wrote at linux1394-devel:
> Hi,
> 
> I noticed that video1394 calls lock_kernel in it's file_operations. I
> thought about converting these into a per-instance mutex or something,
> but in the end I couldn't find a reason why this locking is needed. (The
> more important I/O system is protected by separate spinlocks)
> 
> The BKL is only contended when operations such as ioctl() or release()
> are invoked. My knowledge lacks at this point, but I'm reasonably sure
> that some upper layer must ensure that these invokations are serialized,
> as opposed to leaving it up to the driver to handle nasty potential
> situations e.g. release() being called halfway through a read(). Can
> anyone clarify?
> 
> Thanks,
> Daniel

I think you are right. Same with dv1394. Although we need to
double-check whether something needs replacement protection then.

> ------------------------------------------------------------------------
> 
> Index: linux/drivers/ieee1394/video1394.c
> ===================================================================
> --- linux.orig/drivers/ieee1394/video1394.c
> +++ linux/drivers/ieee1394/video1394.c
> @@ -1161,9 +1161,7 @@ static int __video1394_ioctl(struct file
>  static long video1394_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>  {
>  	int err;
> -	lock_kernel();
>  	err = __video1394_ioctl(file, cmd, arg);
> -	unlock_kernel();
>  	return err;
>  }
>  
> @@ -1181,13 +1179,11 @@ static int video1394_mmap(struct file *f
>  	struct file_ctx *ctx = (struct file_ctx *)file->private_data;
>  	int res = -EINVAL;
>  
> -	lock_kernel();
>  	if (ctx->current_ctx == NULL) {
>  		PRINT(KERN_ERR, ctx->ohci->host->id,
>  				"Current iso context not set");
>  	} else
>  		res = dma_region_mmap(&ctx->current_ctx->dma, file, vma);
> -	unlock_kernel();
>  
>  	return res;
>  }
> @@ -1200,7 +1196,6 @@ static unsigned int video1394_poll(struc
>  	struct dma_iso_ctx *d;
>  	int i;
>  
> -	lock_kernel();
>  	ctx = file->private_data;
>  	d = ctx->current_ctx;
>  	if (d == NULL) {
> @@ -1221,7 +1216,6 @@ static unsigned int video1394_poll(struc
>  	}
>  	spin_unlock_irqrestore(&d->lock, flags);
>  done:
> -	unlock_kernel();
>  
>  	return mask;
>  }
> @@ -1257,7 +1251,6 @@ static int video1394_release(struct inod
>  	struct list_head *lh, *next;
>  	u64 mask;
>  
> -	lock_kernel();
>  	list_for_each_safe(lh, next, &ctx->context_list) {
>  		struct dma_iso_ctx *d;
>  		d = list_entry(lh, struct dma_iso_ctx, link);
> @@ -1278,7 +1271,6 @@ static int video1394_release(struct inod
>  	kfree(ctx);
>  	file->private_data = NULL;
>  
> -	unlock_kernel();
>  	return 0;
>  }
>  
> 

-- 
Stefan Richter
-=====-=-==- =-=- =--=-
http://arcgraph.de/sr/

Re: Unnecessary BKL contention in video1394

[Daniel Drake]

On Wed, 2006-10-18 at 23:36 +0200, Stefan Richter wrote:
> (added Cc: lkml to pull in some clues)
> 
> Daniel Drake wrote at linux1394-devel:
> > Hi,
> > 
> > I noticed that video1394 calls lock_kernel in it's file_operations. I
> > thought about converting these into a per-instance mutex or something,
> > but in the end I couldn't find a reason why this locking is needed. (The
> > more important I/O system is protected by separate spinlocks)
> > 
> > The BKL is only contended when operations such as ioctl() or release()
> > are invoked. My knowledge lacks at this point, but I'm reasonably sure
> > that some upper layer must ensure that these invokations are serialized,
> > as opposed to leaving it up to the driver to handle nasty potential
> > situations e.g. release() being called halfway through a read(). Can
> > anyone clarify?

> I think you are right. Same with dv1394. Although we need to
> double-check whether something needs replacement protection then.

Adding Andi Kleen to CC, who added the BKL around __video1394_ioctl a
long while back (when converting video1394 to compat_ioctl).

I don't feel that any replacement protection is needed, since the
critical sections (where structures are used both in interrupts and in
file_operations) are already protected by spinlocks.

> > ------------------------------------------------------------------------
> > 
> > Index: linux/drivers/ieee1394/video1394.c
> > ===================================================================
> > --- linux.orig/drivers/ieee1394/video1394.c
> > +++ linux/drivers/ieee1394/video1394.c
> > @@ -1161,9 +1161,7 @@ static int __video1394_ioctl(struct file
> >  static long video1394_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> >  {
> >  	int err;
> > -	lock_kernel();
> >  	err = __video1394_ioctl(file, cmd, arg);
> > -	unlock_kernel();
> >  	return err;
> >  }
> >  
> > @@ -1181,13 +1179,11 @@ static int video1394_mmap(struct file *f
> >  	struct file_ctx *ctx = (struct file_ctx *)file->private_data;
> >  	int res = -EINVAL;
> >  
> > -	lock_kernel();
> >  	if (ctx->current_ctx == NULL) {
> >  		PRINT(KERN_ERR, ctx->ohci->host->id,
> >  				"Current iso context not set");
> >  	} else
> >  		res = dma_region_mmap(&ctx->current_ctx->dma, file, vma);
> > -	unlock_kernel();
> >  
> >  	return res;
> >  }
> > @@ -1200,7 +1196,6 @@ static unsigned int video1394_poll(struc
> >  	struct dma_iso_ctx *d;
> >  	int i;
> >  
> > -	lock_kernel();
> >  	ctx = file->private_data;
> >  	d = ctx->current_ctx;
> >  	if (d == NULL) {
> > @@ -1221,7 +1216,6 @@ static unsigned int video1394_poll(struc
> >  	}
> >  	spin_unlock_irqrestore(&d->lock, flags);
> >  done:
> > -	unlock_kernel();
> >  
> >  	return mask;
> >  }
> > @@ -1257,7 +1251,6 @@ static int video1394_release(struct inod
> >  	struct list_head *lh, *next;
> >  	u64 mask;
> >  
> > -	lock_kernel();
> >  	list_for_each_safe(lh, next, &ctx->context_list) {
> >  		struct dma_iso_ctx *d;
> >  		d = list_entry(lh, struct dma_iso_ctx, link);
> > @@ -1278,7 +1271,6 @@ static int video1394_release(struct inod
> >  	kfree(ctx);
> >  	file->private_data = NULL;
> >  
> > -	unlock_kernel();
> >  	return 0;
> >  }
> >  
> > 
> 

Re: Unnecessary BKL contention in video1394

[Andi Kleen]

> Adding Andi Kleen to CC, who added the BKL around __video1394_ioctl a
> long while back (when converting video1394 to compat_ioctl).
> 
> I don't feel that any replacement protection is needed, since the
> critical sections (where structures are used both in interrupts and in
> file_operations) are already protected by spinlocks.

Fine by me. I just did it to preserve old semantics because I didn't want
to audit the 1394 locking.  But if you think it's not needed feel free to remove
them.

-andi

Re: Unnecessary BKL contention in video1394

[Stefan Richter]

Andi Kleen wrote:
[Daniel Drake wrote:]
>> Adding Andi Kleen to CC, who added the BKL around __video1394_ioctl a
>> long while back (when converting video1394 to compat_ioctl).
>> 
>> I don't feel that any replacement protection is needed, since the
>> critical sections (where structures are used both in interrupts and in
>> file_operations) are already protected by spinlocks.
> 
> Fine by me. I just did it to preserve old semantics because I didn't want
> to audit the 1394 locking.  But if you think it's not needed feel free to remove
> them.

Thanks for the info. Daniel, do you want to resend a signed-off patch?
And __video1394_ioctl and its wrapper video1394_ioctl can certainly be
merged then.
-- 
Stefan Richter
-=====-=-==- =-=- =--==
http://arcgraph.de/sr/

Re: Unnecessary BKL contention in video1394

[Daniel Drake]

On Thu, 2006-10-19 at 16:36 +0200, Stefan Richter wrote:
> Thanks for the info. Daniel, do you want to resend a signed-off patch?
> And __video1394_ioctl and its wrapper video1394_ioctl can certainly be
> merged then.

Yep, I had already made that change locally. I will run it overnight on
several cameras just to be sure, and will send a patch tomorrow.

I also did some more investigation and straightened out my knowledge of
file_operations: release() can never be called while inside a read() or
ioctl(): This would imply that separate threads are in use, and the
driver's release() function is not called until *all* threads have
closed the fd.  In other words I'm now much more confident that this
patch is not removing any necessary locking.

Daniel