Re: [PATCH] block: Fix a race in the runtime power management code

From: Bart Van Assche <bvanassche@acm.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Stanley Chu <stanley.chu@mediatek.com>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Ming Lei <ming.lei@redhat.com>, stable <stable@vger.kernel.org>,
	Can Guo <cang@codeaurora.org>,
	"Martin K. Petersen" <martin.petersen@oracle.com>
Subject: Re: [PATCH] block: Fix a race in the runtime power management code
Date: Thu, 27 Aug 2020 20:27:49 -0700	[thread overview]
Message-ID: <5da883fe-b5ec-b98d-ae0c-bc053b6e22cb@acm.org> (raw)
In-Reply-To: <20200827203321.GB449067@rowland.harvard.edu>

On 2020-08-27 13:33, Alan Stern wrote:
> It may not need to be that complicated.  what about something like this?
> 
> Index: usb-devel/block/blk-core.c
> ===================================================================
> --- usb-devel.orig/block/blk-core.c
> +++ usb-devel/block/blk-core.c
> @@ -420,11 +420,11 @@ EXPORT_SYMBOL(blk_cleanup_queue);
>  /**
>   * blk_queue_enter() - try to increase q->q_usage_counter
>   * @q: request queue pointer
> - * @flags: BLK_MQ_REQ_NOWAIT and/or BLK_MQ_REQ_PREEMPT
> + * @flags: BLK_MQ_REQ_NOWAIT and/or BLK_MQ_REQ_PM
>   */
>  int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
>  {
> -	const bool pm = flags & BLK_MQ_REQ_PREEMPT;
> +	const bool pm = flags & BLK_MQ_REQ_PM;
>  
>  	while (true) {
>  		bool success = false;
> @@ -626,7 +626,8 @@ struct request *blk_get_request(struct r
>  	struct request *req;
>  
>  	WARN_ON_ONCE(op & REQ_NOWAIT);
> -	WARN_ON_ONCE(flags & ~(BLK_MQ_REQ_NOWAIT | BLK_MQ_REQ_PREEMPT));
> +	WARN_ON_ONCE(flags & ~(BLK_MQ_REQ_NOWAIT | BLK_MQ_REQ_PREEMPT |
> +			BLK_MQ_REQ_PM));
>  
>  	req = blk_mq_alloc_request(q, op, flags);
>  	if (!IS_ERR(req) && q->mq_ops->initialize_rq_fn)
> Index: usb-devel/drivers/scsi/scsi_lib.c
> ===================================================================
> --- usb-devel.orig/drivers/scsi/scsi_lib.c
> +++ usb-devel/drivers/scsi/scsi_lib.c
> @@ -245,11 +245,15 @@ int __scsi_execute(struct scsi_device *s
>  {
>  	struct request *req;
>  	struct scsi_request *rq;
> +	blk_mq_req_flags_t mq_req_flags;
>  	int ret = DRIVER_ERROR << 24;
>  
> +	mq_req_flags = BLK_MQ_REQ_PREEMPT;
> +	if (rq_flags & RQF_PM)
> +		mq_req_flags |= BLK_MQ_REQ_PM;
>  	req = blk_get_request(sdev->request_queue,
>  			data_direction == DMA_TO_DEVICE ?
> -			REQ_OP_SCSI_OUT : REQ_OP_SCSI_IN, BLK_MQ_REQ_PREEMPT);
> +			REQ_OP_SCSI_OUT : REQ_OP_SCSI_IN, mq_req_flags);
>  	if (IS_ERR(req))
>  		return ret;
>  	rq = scsi_req(req);
> Index: usb-devel/include/linux/blk-mq.h
> ===================================================================
> --- usb-devel.orig/include/linux/blk-mq.h
> +++ usb-devel/include/linux/blk-mq.h
> @@ -435,6 +435,8 @@ enum {
>  	BLK_MQ_REQ_RESERVED	= (__force blk_mq_req_flags_t)(1 << 1),
>  	/* set RQF_PREEMPT */
>  	BLK_MQ_REQ_PREEMPT	= (__force blk_mq_req_flags_t)(1 << 3),
> +	/* used for power management */
> +	BLK_MQ_REQ_PM		= (__force blk_mq_req_flags_t)(1 << 4),
>  };
>  
>  struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,

I think this patch will break SCSI domain validation. The SCSI domain
validation code calls scsi_device_quiesce() and that function in turn calls
blk_set_pm_only(). The SCSI domain validation code submits SCSI commands with
the BLK_MQ_REQ_PREEMPT flag. Since the above code postpones such requests
while blk_set_pm_only() is in effect, I think the above patch will cause the
SCSI domain validation code to deadlock.

Bart.