Re: [PATCH v3 1/2] Ensure that the SCSI error handler gets woken up

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

On 12/04/2017 09:06 PM, Bart Van Assche wrote:
> If scsi_eh_scmd_add() is called concurrently with
> scsi_host_queue_ready() while shost->host_blocked > 0 then it can
> happen that neither function wakes up the SCSI error handler. Fix
> this by making every function that decreases the host_busy counter
> wake up the error handler if necessary and by protecting the
> host_failed checks with the SCSI host lock.
> 
> Reported-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> References: https://marc.info/?l=linux-kernel&m=150461610630736
> Fixes: commit 746650160866 ("scsi: convert host_busy to atomic_t")
> Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com> > Cc: Konstantin Khorenko <khorenko@virtuozzo.com>
> Cc: Stuart Hayes <stuart.w.hayes@gmail.com>
> Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Johannes Thumshirn <jthumshirn@suse.de>
> Cc: <stable@vger.kernel.org>
> ---
>   drivers/scsi/hosts.c      |  6 ++++++
>   drivers/scsi/scsi_error.c | 18 ++++++++++++++++--
>   drivers/scsi/scsi_lib.c   | 39 ++++++++++++++++++++++++++++-----------
>   include/scsi/scsi_host.h  |  2 ++
>   4 files changed, 52 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index a306af6a5ea7..a0a7e4ff255c 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -324,6 +324,9 @@ static void scsi_host_dev_release(struct device *dev)
>   
>   	scsi_proc_hostdir_rm(shost->hostt);
>   
> +	/* Wait for functions invoked through call_rcu(&shost->rcu, ...) */
> +	rcu_barrier();
> +
>   	if (shost->tmf_work_q)
>   		destroy_workqueue(shost->tmf_work_q);
>   	if (shost->ehandler)
> @@ -331,6 +334,8 @@ static void scsi_host_dev_release(struct device *dev)
>   	if (shost->work_q)
>   		destroy_workqueue(shost->work_q);
>   
> +	destroy_rcu_head(&shost->rcu);
> +
>   	if (shost->shost_state == SHOST_CREATED) {
>   		/*
>   		 * Free the shost_dev device name here if scsi_host_alloc()
> @@ -399,6 +404,7 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
>   	INIT_LIST_HEAD(&shost->starved_list);
>   	init_waitqueue_head(&shost->host_wait);
>   	mutex_init(&shost->scan_mutex);
> +	init_rcu_head(&shost->rcu);
>   
>   	index = ida_simple_get(&host_index_ida, 0, 0, GFP_KERNEL);
>   	if (index < 0)
> diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
> index 5e89049e9b4e..258b8a741992 100644
> --- a/drivers/scsi/scsi_error.c
> +++ b/drivers/scsi/scsi_error.c
> @@ -226,6 +226,17 @@ static void scsi_eh_reset(struct scsi_cmnd *scmd)
>   	}
>   }
>   
> +static void scsi_eh_inc_host_failed(struct rcu_head *head)
> +{
> +	struct Scsi_Host *shost = container_of(head, typeof(*shost), rcu);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(shost->host_lock, flags);
> +	shost->host_failed++;

May be we need to increment host_failed before call_rcu(), so that all 
rcu protected readers already see a change at these point?

> +	scsi_eh_wakeup(shost);
> +	spin_unlock_irqrestore(shost->host_lock, flags);
> +}
> +
>   /**
>    * scsi_eh_scmd_add - add scsi cmd to error handling.
>    * @scmd:	scmd to run eh on.
> @@ -248,9 +259,12 @@ void scsi_eh_scmd_add(struct scsi_cmnd *scmd)
>   
>   	scsi_eh_reset(scmd);
>   	list_add_tail(&scmd->eh_entry, &shost->eh_cmd_q);
> -	shost->host_failed++;
> -	scsi_eh_wakeup(shost);
>   	spin_unlock_irqrestore(shost->host_lock, flags);
> +	/*
> +	 * Ensure that all tasks observe the host state change before the
> +	 * host_failed change.
> +	 */
> +	call_rcu(&shost->rcu, scsi_eh_inc_host_failed);
>   }
>   
>   /**
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index b6d3842b6809..5cbc69b2b1ae 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -318,22 +318,39 @@ static void scsi_init_cmd_errh(struct scsi_cmnd *cmd)
>   		cmd->cmd_len = scsi_command_size(cmd->cmnd);
>   }
>   
> -void scsi_device_unbusy(struct scsi_device *sdev)
> +/*
> + * Decrement the host_busy counter and wake up the error handler if necessary.
> + * Avoid as follows that the error handler is not woken up if shost->host_busy
> + * == shost->host_failed: use call_rcu() in scsi_eh_scmd_add() in combination
> + * with an RCU read lock in this function to ensure that this function in its
> + * entirety either finishes before scsi_eh_scmd_add() increases the
> + * host_failed counter or that it notices the shost state change made by
> + * scsi_eh_scmd_add().
> + */
> +static void scsi_dec_host_busy(struct Scsi_Host *shost)
>   {
> -	struct Scsi_Host *shost = sdev->host;
> -	struct scsi_target *starget = scsi_target(sdev);
>   	unsigned long flags;
>   
> +	rcu_read_lock();
>   	atomic_dec(&shost->host_busy);
> -	if (starget->can_queue > 0)
> -		atomic_dec(&starget->target_busy);
> -
> -	if (unlikely(scsi_host_in_recovery(shost) &&
> -		     (shost->host_failed || shost->host_eh_scheduled))) {
> +	if (unlikely(scsi_host_in_recovery(shost))) {
>   		spin_lock_irqsave(shost->host_lock, flags);
> -		scsi_eh_wakeup(shost);
> +		if (shost->host_failed || shost->host_eh_scheduled)
> +			scsi_eh_wakeup(shost);
>   		spin_unlock_irqrestore(shost->host_lock, flags);
>   	}
> +	rcu_read_unlock();
> +}
> +
> +void scsi_device_unbusy(struct scsi_device *sdev)
> +{
> +	struct Scsi_Host *shost = sdev->host;
> +	struct scsi_target *starget = scsi_target(sdev);
> +
> +	scsi_dec_host_busy(shost);
> +
> +	if (starget->can_queue > 0)
> +		atomic_dec(&starget->target_busy);
>   
>   	atomic_dec(&sdev->device_busy);
>   }
> @@ -1531,7 +1548,7 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
>   		list_add_tail(&sdev->starved_entry, &shost->starved_list);
>   	spin_unlock_irq(shost->host_lock);
>   out_dec:
> -	atomic_dec(&shost->host_busy);
> +	scsi_dec_host_busy(shost);
>   	return 0;
>   }
>   
> @@ -2017,7 +2034,7 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
>   	return BLK_STS_OK;
>   
>   out_dec_host_busy:
> -       atomic_dec(&shost->host_busy);
> +	scsi_dec_host_busy(shost);
>   out_dec_target_busy:
>   	if (scsi_target(sdev)->can_queue > 0)
>   		atomic_dec(&scsi_target(sdev)->target_busy);
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index a8b7bf879ced..1a1df0d21ee3 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -571,6 +571,8 @@ struct Scsi_Host {
>   		struct blk_mq_tag_set	tag_set;
>   	};
>   
> +	struct rcu_head rcu;
> +
>   	atomic_t host_busy;		   /* commands actually active on low-level */
>   	atomic_t host_blocked;
>   
> 

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.