linux-kernel - Re: [PATCH 1/2] zram: free meta out of init

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Date:	Wed, 28 Jan 2015 13:55:01 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nitin Gupta <ngupta@...are.org>
Subject: Re: [PATCH 1/2] zram: free meta out of init_lock

On Wed, Jan 28, 2015 at 12:53:54PM +0900, Sergey Senozhatsky wrote:
> On (01/28/15 11:57), Minchan Kim wrote:
> [..]
> > > second,
> > > after kick_all_cpus_sync() new RW operations will see false init_done().
> > > bdev->bd_holders protects from resetting device which has read/write
> > > operation ongoing on the onther CPU.
> > > 
> > > I need to refresh on how ->bd_holders actually incremented/decremented.
> > > can the following race condition take a place?
> > > 
> > > 	CPU0					CPU1
> > > reset_store()
> > > bdev->bd_holders == false
> > > 					zram_make_request
> > > 						-rm- down_read(&zram->init_lock);
> > > 					init_done(zram) == true
> > > zram_reset_device()			valid_io_request()
> > > 					__zram_make_request
> > > down_write(&zram->init_lock);		zram_bvec_rw
> > > [..]
> > > set_capacity(zram->disk, 0);
> > > zram->init_done = false;
> > > kick_all_cpus_sync();			zram_bvec_write or zram_bvec_read()
> > > zram_meta_free(zram->meta);		
> > > zcomp_destroy(zram->comp);		zcomp_compress() or zcomp_decompress()
> > 
> > You're absolutely right. I forgot rw path is blockable so
> > kick_all_cpus_sync doesn't work for our case, unfortunately.
> > So, I want to go with srcu. Do you agree? or another suggestion?
> 
> yes, I think we need to take a second look on srcu approach.
> 
> 	-ss

Another idea is to introduce atomic refcount on zram for meta's lifetime management
so that rw path should get a ref for right before using the meta and put it on done.
If the refcount is negative, anyone shouldn't go with it.

However, I guess we can do it simple and more scalable with srcu rather than
introducing new atomic count. ;-)

-- 
Kind regards,
Minchan Kim
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