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Date: Fri, 30 Jan 2015 00:12:27 +0900
From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, Nitin Gupta <ngupta@vflare.org>,
        Jerome Marchand <jmarchan@redhat.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        sergey.senozhatsky.work@gmail.com
Subject: Re: [PATCH v1 2/2] zram: remove init_lock in zram_make_request
Message-ID: <20150129151227.GA936@swordfish>
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On (01/29/15 21:48), Ganesh Mahendran wrote:
> > Admin could reset zram during I/O operation going on so we have
> > used zram->init_lock as read-side lock in I/O path to prevent
> > sudden zram meta freeing.
> 
> When I/O operation is running, that means the /dev/zram0 is
> mounted or swaped on. Then the device could not be reset by
> below code:
> 
>     /* Do not reset an active device! */
>     if (bdev->bd_holders) {
>         ret = -EBUSY;
>         goto out;
>     }
> 
> So the zram->init_lock in I/O path is to check whether the device
> has been initialized(echo xxx > /sys/block/zram/disk_size).
> 

for mounted device (w/fs), we see initial (well, it goes up and down
many times while we create device, but this is not interesting here)
->bd_holders increment in:
  vfs_kern_mount -> mount_bdev -> blkdev_get_by_path -> blkdev_get

and it goes to zero in:
  cleanup_mnt -> deactivate_super -> kill_block_super -> blkdev_put


after umount we still have init device. so, *theoretically*, we
can see something like

	CPU0				CPU1
umount
reset_store			
bdev->bd_holders == 0			mount
...					zram_make_request()
zram_reset_device()

w/o zram->init_lock in both zram_reset_device() and zram_make_request()
one of CPUs will be a bit sad.

	-ss