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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Tyrel Datwyler <tyreld@linux.ibm.com>,
        james.bottomley@hansenpartnership.com
Cc:     Tyrel Datwyler <tyreld@linux.ibm.com>, martin.petersen@oracle.com,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        brking@linux.ibm.com, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] ibmvscsi: fix race potential race after loss of transport
In-Reply-To: <20201025001355.4527-1-tyreld@linux.ibm.com>
References: <20201025001355.4527-1-tyreld@linux.ibm.com>
Date:   Wed, 28 Oct 2020 16:21:54 +1100
Message-ID: <87o8knvsb1.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Tyrel Datwyler <tyreld@linux.ibm.com> writes:
> After a loss of tranport due to an adatper migration or crash/disconnect from
> the host partner there is a tiny window where we can race adjusting the
> request_limit of the adapter. The request limit is atomically inc/dec to track
> the number of inflight requests against the allowed limit of our VIOS partner.
> After a transport loss we set the request_limit to zero to reflect this state.
> However, there is a window where the adapter may attempt to queue a command
> because the transport loss event hasn't been fully processed yet and
> request_limit is still greater than zero. The hypercall to send the event will
> fail and the error path will increment the request_limit as a result. If the
> adapter processes the transport event prior to this increment the request_limit
> becomes out of sync with the adapter state and can result in scsi commands being
> submitted on the now reset connection prior to an SRP Login resulting in a
> protocol violation.
>
> Fix this race by protecting request_limit with the host lock when changing the
> value via atomic_set() to indicate no transport.
>
> Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
> ---
>  drivers/scsi/ibmvscsi/ibmvscsi.c | 36 +++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/scsi/ibmvscsi/ibmvscsi.c b/drivers/scsi/ibmvscsi/ibmvscsi.c
> index b1f3017b6547..188ed75417a5 100644
> --- a/drivers/scsi/ibmvscsi/ibmvscsi.c
> +++ b/drivers/scsi/ibmvscsi/ibmvscsi.c
> @@ -806,6 +806,22 @@ static void purge_requests(struct ibmvscsi_host_data *hostdata, int error_code)
>  	spin_unlock_irqrestore(hostdata->host->host_lock, flags);
>  }
>  
> +/**
> + * ibmvscsi_set_request_limit - Set the adapter request_limit in response to
> + * an adapter failure, reset, or SRP Login. Done under host lock to prevent
> + * race with scsi command submission.
> + * @hostdata:	adapter to adjust
> + * @limit:	new request limit
> + */
> +static void ibmvscsi_set_request_limit(struct ibmvscsi_host_data *hostdata, int limit)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(hostdata->host->host_lock, flags);
> +	atomic_set(&hostdata->request_limit, limit);
> +	spin_unlock_irqrestore(hostdata->host->host_lock, flags);
> +}
> +
>  /**
>   * ibmvscsi_reset_host - Reset the connection to the server
>   * @hostdata:	struct ibmvscsi_host_data to reset
...
> @@ -2137,12 +2153,12 @@ static void ibmvscsi_do_work(struct ibmvscsi_host_data *hostdata)
>  	}
>  
>  	hostdata->action = IBMVSCSI_HOST_ACTION_NONE;
> +	spin_unlock_irqrestore(hostdata->host->host_lock, flags);

You drop the lock ...

>  	if (rc) {
> -		atomic_set(&hostdata->request_limit, -1);
> +		ibmvscsi_set_request_limit(hostdata, -1);

.. then retake it, then drop it again in ibmvscsi_set_request_limit().

Which introduces the possibility that something else gets the lock
before you can set the limit to -1.

I'm not sure that's a bug, but it's not obviously correct either?

cheers

>  		dev_err(hostdata->dev, "error after %s\n", action);
>  	}
> -	spin_unlock_irqrestore(hostdata->host->host_lock, flags);
>  
>  	scsi_unblock_requests(hostdata->host);
>  }