Re: zram: lockdep spew for zram->init_lock

[Sergey Senozhatsky]

On (02/28/14 16:32), Andrew Morton wrote:
> Date: Fri, 28 Feb 2014 16:32:06 -0800
> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>, ngupta@xxxxxxxxxx, LKML
>  <linux-kernel@xxxxxxxxxxxxxxx>, Sergey Senozhatsky
>  <sergey.senozhatsky@xxxxxxxxx>
> Subject: Re: zram: lockdep spew for zram->init_lock
> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu)
> 
> On Fri, 28 Feb 2014 08:56:29 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> > Sasha reported following below lockdep spew of zram.
> > 
> > It was introduced by [1] in recent linux-next but it's false positive
> > because zram_meta_alloc with down_write(init_lock) couldn't be called
> > during zram is working as swap device so we could annotate the lock.
> > 
> > But I don't think it's worthy because it would make greate lockdep
> > less effective. Instead, move zram_meta_alloc out of the lock as good
> > old day so we could do unnecessary allocation/free of zram_meta for
> > initialied device as Sergey claimed in [1] but it wouldn't be common
> > and be harmful if someone might do it. Rather than, I'd like to respect
> > lockdep which is great tool to prevent upcoming subtle bugs.
> > 
> > [1] zram: delete zram_init_device
> >
> > ...
> >
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -537,26 +537,27 @@ static ssize_t disksize_store(struct device *dev,
> >  		struct device_attribute *attr, const char *buf, size_t len)
> >  {
> >  	u64 disksize;
> > +	struct zram_meta *meta;
> >  	struct zram *zram = dev_to_zram(dev);
> >  
> >  	disksize = memparse(buf, NULL);
> >  	if (!disksize)
> >  		return -EINVAL;
> >  
> > +	disksize = PAGE_ALIGN(disksize);
> > +	meta = zram_meta_alloc(disksize);
> > +	if (!meta)
> > +		return -ENOMEM;
> > +
> >  	down_write(&zram->init_lock);
> >  	if (init_done(zram)) {
> > +		zram_meta_free(meta);
> >  		up_write(&zram->init_lock);
> >  		pr_info("Cannot change disksize for initialized device\n");
> >  		return -EBUSY;
> >  	}
> >  
> > -	disksize = PAGE_ALIGN(disksize);
> > -	zram->meta = zram_meta_alloc(disksize);
> > -	if (!zram->meta) {
> > -		up_write(&zram->init_lock);
> > -		return -ENOMEM;
> > -	}
> > -
> > +	zram->meta = meta;
> >  	zram->disksize = disksize;
> >  	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> >  	up_write(&zram->init_lock);
> 
> When applying zram-use-zcomp-compressing-backends.patch on top of this
> we get a bit of a mess, and simple conflict resolution results in a
> leak.
> 
> disksize_store() was one of those nasty functions which does multiple
> "return" statements after performing locking and resource allocation. 
> As usual, this led to a resource leak.  Remember folks, "return" is a
> goto in disguise.
> 
> 
> Here's what I ended up with.  Please review.
> 

looks good to me.
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>

> static ssize_t disksize_store(struct device *dev,
> 		struct device_attribute *attr, const char *buf, size_t len)
> {
> 	u64 disksize;
> 	struct zram_meta *meta;
> 	struct zram *zram = dev_to_zram(dev);
> 	int err;
> 
> 	disksize = memparse(buf, NULL);
> 	if (!disksize)
> 		return -EINVAL;
> 
> 	disksize = PAGE_ALIGN(disksize);
> 	meta = zram_meta_alloc(disksize);
> 	if (!meta)
> 		return -ENOMEM;
> 
> 	down_write(&zram->init_lock);
> 	if (init_done(zram)) {
> 		pr_info("Cannot change disksize for initialized device\n");
> 		err = -EBUSY;
> 		goto out_free_meta;
> 	}
> 
> 	zram->comp = zcomp_create(default_compressor);
> 	if (!zram->comp) {
> 		pr_info("Cannot initialise %s compressing backend\n",
> 				default_compressor);
> 		err = -EINVAL;
> 		goto out_free_meta;
> 	}
> 
> 	zram->meta = meta;
> 	zram->disksize = disksize;
> 	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> 	up_write(&zram->init_lock);
> 
> 	return len;
> 
> out_free_meta:
> 	up_write(&zram->init_lock);
> 	zram_meta_free(meta);
> 	return err;
> }
> 
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