Re: [PATCH v5 05/23] sg: bitops in sg_device

[Hannes Reinecke <hare@suse.de>]

On 10/8/19 9:50 AM, Douglas Gilbert wrote:
> Introduce bitops in sg_device to replace an atomic, a bool and a
> char. That char (sgdebug) had been reduced to only two states.
> Add some associated macros to make the code a little clearer.
> 
> Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
> ---
>  drivers/scsi/sg.c | 104 +++++++++++++++++++++++-----------------------
>  1 file changed, 53 insertions(+), 51 deletions(-)
> 
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index 9aa1b1030033..97ce84f0c51b 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -74,6 +74,11 @@ static char *sg_version_date = "20190606";
>  
>  #define SG_DEFAULT_TIMEOUT mult_frac(SG_DEFAULT_TIMEOUT_USER, HZ, USER_HZ)
>  
> +/* Bit positions (flags) for sg_device::fdev_bm bitmask follow */
> +#define SG_FDEV_EXCLUDE		0	/* have fd open with O_EXCL */
> +#define SG_FDEV_DETACHING	1	/* may be unexpected device removal */
> +#define SG_FDEV_LOG_SENSE	2	/* set by ioctl(SG_SET_DEBUG) */
> +
>  int sg_big_buff = SG_DEF_RESERVED_SIZE;
>  /* N.B. This variable is readable and writeable via
>     /proc/scsi/sg/def_reserved_size . Each time sg_open() is called a buffer
> @@ -155,14 +160,12 @@ struct sg_device { /* holds the state of each scsi generic device */
>  	struct scsi_device *device;
>  	wait_queue_head_t open_wait;    /* queue open() when O_EXCL present */
>  	struct mutex open_rel_lock;     /* held when in open() or release() */
> -	int sg_tablesize;	/* adapter's max scatter-gather table size */
> -	u32 index;		/* device index number */
>  	struct list_head sfds;
>  	rwlock_t sfd_lock;      /* protect access to sfd list */
> -	atomic_t detaching;     /* 0->device usable, 1->device detaching */
> -	bool exclude;		/* 1->open(O_EXCL) succeeded and is active */
> +	int sg_tablesize;	/* adapter's max scatter-gather table size */
> +	u32 index;		/* device index number */
>  	int open_cnt;		/* count of opens (perhaps < num(sfds) ) */
> -	char sgdebug;		/* 0->off, 1->sense, 9->dump dev, 10-> all devs */
> +	unsigned long fdev_bm[1];	/* see SG_FDEV_* defines above */
>  	struct gendisk *disk;
>  	struct cdev * cdev;	/* char_dev [sysfs: /sys/cdev/major/sg<n>] */
>  	struct kref d_ref;
> @@ -200,6 +203,9 @@ static void sg_device_destroy(struct kref *kref);
>  #define SZ_SG_IO_HDR ((int)sizeof(struct sg_io_hdr))	/* v3 header */
>  #define SZ_SG_REQ_INFO ((int)sizeof(struct sg_req_info))
>  
> +#define SG_IS_DETACHING(sdp) test_bit(SG_FDEV_DETACHING, (sdp)->fdev_bm)
> +#define SG_HAVE_EXCLUDE(sdp) test_bit(SG_FDEV_EXCLUDE, (sdp)->fdev_bm)
> +
>  /*
>   * Kernel needs to be built with CONFIG_SCSI_LOGGING to see log messages.
>   * 'depth' is a number between 1 (most severe) and 7 (most noisy, most
> @@ -273,26 +279,26 @@ sg_wait_open_event(struct sg_device *sdp, bool o_excl)
>  		while (sdp->open_cnt > 0) {
>  			mutex_unlock(&sdp->open_rel_lock);
>  			retval = wait_event_interruptible(sdp->open_wait,
> -					(atomic_read(&sdp->detaching) ||
> +					(SG_IS_DETACHING(sdp) ||
>  					 !sdp->open_cnt));
>  			mutex_lock(&sdp->open_rel_lock);
>  
>  			if (retval) /* -ERESTARTSYS */
>  				return retval;
> -			if (atomic_read(&sdp->detaching))
> +			if (SG_IS_DETACHING(sdp))
>  				return -ENODEV;
>  		}
>  	} else {
> -		while (sdp->exclude) {
> +		while (SG_HAVE_EXCLUDE(sdp)) {
>  			mutex_unlock(&sdp->open_rel_lock);
>  			retval = wait_event_interruptible(sdp->open_wait,
> -					(atomic_read(&sdp->detaching) ||
> -					 !sdp->exclude));
> +					(SG_IS_DETACHING(sdp) ||
> +					 !SG_HAVE_EXCLUDE(sdp)));
>  			mutex_lock(&sdp->open_rel_lock);
>  
>  			if (retval) /* -ERESTARTSYS */
>  				return retval;
> -			if (atomic_read(&sdp->detaching))
> +			if (SG_IS_DETACHING(sdp))
>  				return -ENODEV;
>  		}
>  	}
> @@ -354,7 +360,7 @@ sg_open(struct inode *inode, struct file *filp)
>  				goto error_mutex_locked;
>  			}
>  		} else {
> -			if (sdp->exclude) {
> +			if (SG_HAVE_EXCLUDE(sdp)) {
>  				retval = -EBUSY;
>  				goto error_mutex_locked;
>  			}
> @@ -367,10 +373,10 @@ sg_open(struct inode *inode, struct file *filp)
>  
>  	/* N.B. at this point we are holding the open_rel_lock */
>  	if (o_excl)
> -		sdp->exclude = true;
> +		set_bit(SG_FDEV_EXCLUDE, sdp->fdev_bm);
>  
>  	if (sdp->open_cnt < 1) {  /* no existing opens */
> -		sdp->sgdebug = 0;
> +		clear_bit(SG_FDEV_LOG_SENSE, sdp->fdev_bm);
>  		q = sdp->device->request_queue;
>  		sdp->sg_tablesize = queue_max_segments(q);
>  	}
> @@ -393,8 +399,8 @@ sg_open(struct inode *inode, struct file *filp)
>  	return retval;
>  
>  out_undo:
> -	if (o_excl) {
> -		sdp->exclude = false;   /* undo if error */
> +	if (o_excl) {		/* undo if error */
> +		clear_bit(SG_FDEV_EXCLUDE, sdp->fdev_bm);
>  		wake_up_interruptible(&sdp->open_wait);
>  	}
>  error_mutex_locked:
> @@ -428,12 +434,10 @@ sg_release(struct inode *inode, struct file *filp)
>  
>  	/* possibly many open()s waiting on exlude clearing, start many;
>  	 * only open(O_EXCL)s wait on 0==open_cnt so only start one */
> -	if (sdp->exclude) {
> -		sdp->exclude = false;
> +	if (test_and_clear_bit(SG_FDEV_EXCLUDE, sdp->fdev_bm))
>  		wake_up_interruptible_all(&sdp->open_wait);
> -	} else if (0 == sdp->open_cnt) {
> +	else if (sdp->open_cnt == 0)
>  		wake_up_interruptible(&sdp->open_wait);
> -	}
>  	mutex_unlock(&sdp->open_rel_lock);
>  	return 0;
>  }
> @@ -467,7 +471,7 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
>  	SG_LOG(3, sfp, "%s: write(3rd arg) count=%d\n", __func__, (int)count);
>  	if (!sdp)
>  		return -ENXIO;
> -	if (atomic_read(&sdp->detaching))
> +	if (SG_IS_DETACHING(sdp))
>  		return -ENODEV;
>  	if (!((filp->f_flags & O_NONBLOCK) ||
>  	      scsi_block_when_processing_errors(sdp->device)))
> @@ -666,7 +670,7 @@ sg_common_write(struct sg_fd *sfp, struct sg_request *srp,
>  		sg_remove_request(sfp, srp);
>  		return k;	/* probably out of space --> ENOMEM */
>  	}
> -	if (atomic_read(&sdp->detaching)) {
> +	if (SG_IS_DETACHING(sdp)) {
>  		if (srp->bio) {
>  			scsi_req_free_cmd(scsi_req(srp->rq));
>  			blk_put_request(srp->rq);
> @@ -831,7 +835,7 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
>  	}
>  	srp = sg_get_rq_mark(sfp, req_pack_id);
>  	if (!srp) {		/* now wait on packet to arrive */
> -		if (atomic_read(&sdp->detaching)) {
> +		if (SG_IS_DETACHING(sdp)) {
>  			retval = -ENODEV;
>  			goto free_old_hdr;
>  		}
> @@ -840,9 +844,9 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
>  			goto free_old_hdr;
>  		}
>  		retval = wait_event_interruptible(sfp->read_wait,
> -			(atomic_read(&sdp->detaching) ||
> +			(SG_IS_DETACHING(sdp) ||
>  			(srp = sg_get_rq_mark(sfp, req_pack_id))));
> -		if (atomic_read(&sdp->detaching)) {
> +		if (SG_IS_DETACHING(sdp)) {
>  			retval = -ENODEV;
>  			goto free_old_hdr;
>  		}
> @@ -997,7 +1001,7 @@ sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
>  
>  	switch (cmd_in) {
>  	case SG_IO:
> -		if (atomic_read(&sdp->detaching))
> +		if (SG_IS_DETACHING(sdp))
>  			return -ENODEV;
>  		if (!scsi_block_when_processing_errors(sdp->device))
>  			return -ENXIO;
> @@ -1008,8 +1012,8 @@ sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
>  		if (result < 0)
>  			return result;
>  		result = wait_event_interruptible(sfp->read_wait,
> -			(srp_done(sfp, srp) || atomic_read(&sdp->detaching)));
> -		if (atomic_read(&sdp->detaching))
> +			(srp_done(sfp, srp) || SG_IS_DETACHING(sdp)));
> +		if (SG_IS_DETACHING(sdp))
>  			return -ENODEV;
>  		write_lock_irq(&sfp->rq_list_lock);
>  		if (srp->done) {
> @@ -1052,7 +1056,7 @@ sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
>  		else {
>  			sg_scsi_id_t __user *sg_idp = p;
>  
> -			if (atomic_read(&sdp->detaching))
> +			if (SG_IS_DETACHING(sdp))
>  				return -ENODEV;
>  			__put_user((int) sdp->device->host->host_no,
>  				   &sg_idp->host_no);
> @@ -1176,18 +1180,18 @@ sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
>  			return result;
>  		}
>  	case SG_EMULATED_HOST:
> -		if (atomic_read(&sdp->detaching))
> +		if (SG_IS_DETACHING(sdp))
>  			return -ENODEV;
>  		return put_user(sdp->device->host->hostt->emulated, ip);
>  	case SCSI_IOCTL_SEND_COMMAND:
> -		if (atomic_read(&sdp->detaching))
> +		if (SG_IS_DETACHING(sdp))
>  			return -ENODEV;
>  		return sg_scsi_ioctl(sdp->device->request_queue, NULL, filp->f_mode, p);
>  	case SG_SET_DEBUG:
>  		result = get_user(val, ip);
>  		if (result)
>  			return result;
> -		sdp->sgdebug = (char) val;
> +		assign_bit(SG_FDEV_LOG_SENSE, sdp->fdev_bm, val);
>  		return 0;
>  	case BLKSECTGET:
>  		return put_user(max_sectors_bytes(sdp->device->request_queue),
> @@ -1208,7 +1212,7 @@ sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
>  	case SCSI_IOCTL_PROBE_HOST:
>  	case SG_GET_TRANSFORM:
>  	case SG_SCSI_RESET:
> -		if (atomic_read(&sdp->detaching))
> +		if (SG_IS_DETACHING(sdp))
>  			return -ENODEV;
>  		break;
>  	default:
> @@ -1271,7 +1275,7 @@ sg_poll(struct file *filp, poll_table * wait)
>  	}
>  	read_unlock_irqrestore(&sfp->rq_list_lock, iflags);
>  
> -	if (sfp->parentdp && atomic_read(&sfp->parentdp->detaching)) {
> +	if (sfp->parentdp && SG_IS_DETACHING(sfp->parentdp)) {
>  		p_res |= EPOLLHUP;
>  	} else if (!sfp->cmd_q) {
>  		if (count == 0)
> @@ -1419,7 +1423,7 @@ sg_rq_end_io(struct request *rq, blk_status_t status)
>  		return;
>  
>  	sdp = sfp->parentdp;
> -	if (unlikely(atomic_read(&sdp->detaching)))
> +	if (unlikely(SG_IS_DETACHING(sdp)))
>  		pr_info("%s: device detaching\n", __func__);
>  
>  	sense = req->sense;
> @@ -1440,9 +1444,9 @@ sg_rq_end_io(struct request *rq, blk_status_t status)
>  		srp->header.msg_status = msg_byte(result);
>  		srp->header.host_status = host_byte(result);
>  		srp->header.driver_status = driver_byte(result);
> -		if ((sdp->sgdebug > 0) &&
> -		    ((CHECK_CONDITION == srp->header.masked_status) ||
> -		     (COMMAND_TERMINATED == srp->header.masked_status)))
> +		if (test_bit(SG_FDEV_LOG_SENSE, sdp->fdev_bm) &&
> +		    (srp->header.masked_status == CHECK_CONDITION ||
> +		     srp->header.masked_status == COMMAND_TERMINATED))
>  			__scsi_print_sense(sdp->device, __func__, sense,
>  					   SCSI_SENSE_BUFFERSIZE);
>  
> @@ -1557,7 +1561,7 @@ sg_alloc(struct gendisk *disk, struct scsi_device *scsidp)
>  	mutex_init(&sdp->open_rel_lock);
>  	INIT_LIST_HEAD(&sdp->sfds);
>  	init_waitqueue_head(&sdp->open_wait);
> -	atomic_set(&sdp->detaching, 0);
> +	clear_bit(SG_FDEV_DETACHING, sdp->fdev_bm);
>  	rwlock_init(&sdp->sfd_lock);
>  	sdp->sg_tablesize = queue_max_segments(q);
>  	sdp->index = k;
> @@ -1682,13 +1686,11 @@ sg_remove_device(struct device *cl_dev, struct class_interface *cl_intf)
>  	struct sg_device *sdp = dev_get_drvdata(cl_dev);
>  	unsigned long iflags;
>  	struct sg_fd *sfp;
> -	int val;
>  
>  	if (!sdp)
>  		return;
> -	/* want sdp->detaching non-zero as soon as possible */
> -	val = atomic_inc_return(&sdp->detaching);
> -	if (val > 1)
> +	/* set this flag as soon as possible as it could be a surprise */
> +	if (test_and_set_bit(SG_FDEV_DETACHING, sdp->fdev_bm))
>  		return; /* only want to do following once per device */
>  
>  	SCSI_LOG_TIMEOUT(3, sdev_printk(KERN_INFO, sdp->device,
> @@ -2218,7 +2220,7 @@ sg_add_sfp(struct sg_device *sdp)
>  	sfp->keep_orphan = SG_DEF_KEEP_ORPHAN;
>  	sfp->parentdp = sdp;
>  	write_lock_irqsave(&sdp->sfd_lock, iflags);
> -	if (atomic_read(&sdp->detaching)) {
> +	if (SG_IS_DETACHING(sdp)) {
>  		write_unlock_irqrestore(&sdp->sfd_lock, iflags);
>  		kfree(sfp);
>  		return ERR_PTR(-ENODEV);
> @@ -2315,8 +2317,8 @@ sg_get_dev(int dev)
>  	sdp = sg_lookup_dev(dev);
>  	if (!sdp)
>  		sdp = ERR_PTR(-ENXIO);
> -	else if (atomic_read(&sdp->detaching)) {
> -		/* If sdp->detaching, then the refcount may already be 0, in
> +	else if (SG_IS_DETACHING(sdp)) {
> +		/* If detaching, then the refcount may already be 0, in
>  		 * which case it would be a bug to do kref_get().
>  		 */
>  		sdp = ERR_PTR(-ENODEV);
> @@ -2530,8 +2532,7 @@ sg_proc_seq_show_dev(struct seq_file *s, void *v)
>  
>  	read_lock_irqsave(&sg_index_lock, iflags);
>  	sdp = it ? sg_lookup_dev(it->index) : NULL;
> -	if ((NULL == sdp) || (NULL == sdp->device) ||
> -	    (atomic_read(&sdp->detaching)))
> +	if (!sdp || !sdp->device || SG_IS_DETACHING(sdp))
>  		seq_puts(s, "-1\t-1\t-1\t-1\t-1\t-1\t-1\t-1\t-1\n");
>  	else {
>  		scsidp = sdp->device;
> @@ -2558,7 +2559,7 @@ sg_proc_seq_show_devstrs(struct seq_file *s, void *v)
>  	read_lock_irqsave(&sg_index_lock, iflags);
>  	sdp = it ? sg_lookup_dev(it->index) : NULL;
>  	scsidp = sdp ? sdp->device : NULL;
> -	if (sdp && scsidp && (!atomic_read(&sdp->detaching)))
> +	if (sdp && scsidp && !SG_IS_DETACHING(sdp))
>  		seq_printf(s, "%8.8s\t%16.16s\t%4.4s\n",
>  			   scsidp->vendor, scsidp->model, scsidp->rev);
>  	else
> @@ -2650,7 +2651,7 @@ sg_proc_seq_show_debug(struct seq_file *s, void *v)
>  	read_lock(&sdp->sfd_lock);
>  	if (!list_empty(&sdp->sfds)) {
>  		seq_printf(s, " >>> device=%s ", sdp->disk->disk_name);
> -		if (atomic_read(&sdp->detaching))
> +		if (SG_IS_DETACHING(sdp))
>  			seq_puts(s, "detaching pending close ");
>  		else if (sdp->device) {
>  			struct scsi_device *scsidp = sdp->device;
> @@ -2662,7 +2663,8 @@ sg_proc_seq_show_debug(struct seq_file *s, void *v)
>  				   scsidp->host->hostt->emulated);
>  		}
>  		seq_printf(s, " sg_tablesize=%d excl=%d open_cnt=%d\n",
> -			   sdp->sg_tablesize, sdp->exclude, sdp->open_cnt);
> +			   sdp->sg_tablesize, SG_HAVE_EXCLUDE(sdp),
> +			   sdp->open_cnt);
>  		sg_proc_debug_helper(s, sdp);
>  	}
>  	read_unlock(&sdp->sfd_lock);
> 
One thing to keep in mind here is that 'set_bit()' is not atomic; it
needs to be followed by a memory barrier or being replaced by
test_and_set_bit() if possible.
Please audit the code if that poses a problem.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      Teamlead Storage & Networking
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 247165 (AG München), GF: Felix Imendörffer