Re: [PATCHv3 1/1] SCSI: update hosts module to use idr index management

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2015-10-06 at 12:08 -0700, Lee Duncan wrote:
> Update the SCSI hosts module to use idr to manage
> its host_no index instead of an ATOMIC integer. This
> also allows using idr_find() to look up the SCSI
> host structure given the host number.
> 
> This means that the SCSI host number will now
> be reclaimable.
> 
> Signed-off-by: Lee Duncan <lduncan@suse.com>
> Reviewed-by: Hannes Reinecke <hare@suse.com>
> ---
>  drivers/scsi/hosts.c | 61 ++++++++++++++++++++++++++--------------------------
>  1 file changed, 31 insertions(+), 30 deletions(-
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index 8bb173e01084..afe7bd962ddb 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
[...]
> +	spin_lock(&host_index_lock);
> +	shost = idr_find(&host_index_idr, hostnum);
> +	spin_unlock(&host_index_lock);
> +
> +	return shost ? scsi_host_get(shost) : NULL;

So the thing I don't like here is that there's a race between
scsi_host_get() and the final put.  What could happen is that idr_find()
returns the host just before but scsi_host_dev_release() is executed
before the return.  In that instance, we'll reference freed memory in
scsi_host_get() ... probably completely harmlessly, but it will show up
occasionally on some of the traces ... particularly the ones doing a
fuzz/stress test around host create/destroy.

James