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SubjectRe: [PATCH 09/14] scsi: fix the {host,target,device}_blocked counter mess
On 06/25/2014 06:51 PM, Christoph Hellwig wrote:
> Seems like these counters are missing any sort of synchronization for
> updates, as a over 10 year old comment from me noted. Fix this by
> using atomic counters, and while we're at it also make sure they are
> in the same cacheline as the _busy counters and not needlessly stored
> to in every I/O completion.
>
> With the new model the _busy counters can temporarily go negative,
> so all the readers are updated to check for > 0 values. Longer
> term every successful I/O completion will reset the counters to zero,
> so the temporarily negative values will not cause any harm.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> drivers/scsi/scsi.c | 21 ++++++------
> drivers/scsi/scsi_lib.c | 82 +++++++++++++++++++++-----------------------
> drivers/scsi/scsi_sysfs.c | 10 +++++-
> include/scsi/scsi_device.h | 7 ++--
> include/scsi/scsi_host.h | 7 ++--
> 5 files changed, 64 insertions(+), 63 deletions(-)
>
> diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
> index 35a23e2..b362058 100644
> --- a/drivers/scsi/scsi.c
> +++ b/drivers/scsi/scsi.c
> @@ -729,17 +729,16 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
>
> scsi_device_unbusy(sdev);
>
> - /*
> - * Clear the flags which say that the device/host is no longer
> - * capable of accepting new commands. These are set in scsi_queue.c
> - * for both the queue full condition on a device, and for a
> - * host full condition on the host.
> - *
> - * XXX(hch): What about locking?
> - */
> - shost->host_blocked = 0;
> - starget->target_blocked = 0;
> - sdev->device_blocked = 0;
> + /*
> + * Clear the flags which say that the device/target/host is no longer
> + * capable of accepting new commands.
> + */
> + if (atomic_read(&shost->host_blocked))
> + atomic_set(&shost->host_blocked, 0);
> + if (atomic_read(&starget->target_blocked))
> + atomic_set(&starget->target_blocked, 0);
> + if (atomic_read(&sdev->device_blocked))
> + atomic_set(&sdev->device_blocked, 0);
>
> /*
> * If we have valid sense information, then some kind of recovery
Hmm. I guess there is a race window between
atomic_read() and atomic_set().
Doesn't this cause issues when someone calls atomic_set() just
before the call to atomic_read?

> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index e23fef5..a39d5ba 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -99,14 +99,16 @@ scsi_set_blocked(struct scsi_cmnd *cmd, int reason)
> */
> switch (reason) {
> case SCSI_MLQUEUE_HOST_BUSY:
> - host->host_blocked = host->max_host_blocked;
> + atomic_set(&host->host_blocked, host->max_host_blocked);
> break;
> case SCSI_MLQUEUE_DEVICE_BUSY:
> case SCSI_MLQUEUE_EH_RETRY:
> - device->device_blocked = device->max_device_blocked;
> + atomic_set(&device->device_blocked,
> + device->max_device_blocked);
> break;
> case SCSI_MLQUEUE_TARGET_BUSY:
> - starget->target_blocked = starget->max_target_blocked;
> + atomic_set(&starget->target_blocked,
> + starget->max_target_blocked);
> break;
> }
> }
> @@ -351,30 +353,39 @@ static void scsi_single_lun_run(struct scsi_device *current_sdev)
> spin_unlock_irqrestore(shost->host_lock, flags);
> }
>
> -static inline int scsi_device_is_busy(struct scsi_device *sdev)
> +static inline bool scsi_device_is_busy(struct scsi_device *sdev)
> {
> if (atomic_read(&sdev->device_busy) >= sdev->queue_depth)
> - return 1;
> - if (sdev->device_blocked)
> - return 1;
> + return true;
> + if (atomic_read(&sdev->device_blocked) > 0)
> + return true;
> return 0;
> }
>
> -static inline int scsi_target_is_busy(struct scsi_target *starget)
> +static inline bool scsi_target_is_busy(struct scsi_target *starget)
> {
> - return ((starget->can_queue > 0 &&
> - atomic_read(&starget->target_busy) >= starget->can_queue) ||
> - starget->target_blocked);
> + if (starget->can_queue > 0) {
> + if (atomic_read(&starget->target_busy) >= starget->can_queue)
> + return true;
> + if (atomic_read(&starget->target_blocked) > 0)
> + return true;
> + }
> +
> + return false;
> }
>
> -static inline int scsi_host_is_busy(struct Scsi_Host *shost)
> +static inline bool scsi_host_is_busy(struct Scsi_Host *shost)
> {
> - if ((shost->can_queue > 0 &&
> - atomic_read(&shost->host_busy) >= shost->can_queue) ||
> - shost->host_blocked || shost->host_self_blocked)
> - return 1;
> + if (shost->can_queue > 0) {
> + if (atomic_read(&shost->host_busy) >= shost->can_queue)
> + return true;
> + if (atomic_read(&shost->host_blocked) > 0)
> + return true;
> + if (shost->host_self_blocked)
> + return true;
> + }
>
> - return 0;
> + return false;
> }
>
> static void scsi_starved_list_run(struct Scsi_Host *shost)
> @@ -1283,11 +1294,8 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
> unsigned int busy;
>
> busy = atomic_inc_return(&sdev->device_busy) - 1;
> - if (busy == 0 && sdev->device_blocked) {
> - /*
> - * unblock after device_blocked iterates to zero
> - */
> - if (--sdev->device_blocked != 0) {
> + if (busy == 0 && atomic_read(&sdev->device_blocked) > 0) {
> + if (atomic_dec_return(&sdev->device_blocked) > 0) {
> blk_delay_queue(q, SCSI_QUEUE_DELAY);
> goto out_dec;
> }
> @@ -1297,7 +1305,7 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
>
> if (busy >= sdev->queue_depth)
> goto out_dec;
> - if (sdev->device_blocked)
> + if (atomic_read(&sdev->device_blocked) > 0)
> goto out_dec;
>
> return 1;
> @@ -1328,16 +1336,9 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
> }
>
> busy = atomic_inc_return(&starget->target_busy) - 1;
> - if (busy == 0 && starget->target_blocked) {
> - /*
> - * unblock after target_blocked iterates to zero
> - */
> - spin_lock_irq(shost->host_lock);
> - if (--starget->target_blocked != 0) {
> - spin_unlock_irq(shost->host_lock);
> + if (busy == 0 && atomic_read(&starget->target_blocked) > 0) {
> + if (atomic_dec_return(&starget->target_blocked) > 0)
> goto out_dec;
> - }
> - spin_unlock_irq(shost->host_lock);
>
> SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
> "unblocking target at zero depth\n"));
> @@ -1345,7 +1346,7 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
>
> if (starget->can_queue > 0 && busy >= starget->can_queue)
> goto starved;
> - if (starget->target_blocked)
> + if (atomic_read(&starget->target_blocked) > 0)
> goto starved;
>
> return 1;
> @@ -1374,16 +1375,9 @@ static inline int scsi_host_queue_ready(struct request_queue *q,
> return 0;
>
> busy = atomic_inc_return(&shost->host_busy) - 1;
> - if (busy == 0 && shost->host_blocked) {
> - /*
> - * unblock after host_blocked iterates to zero
> - */
> - spin_lock_irq(shost->host_lock);
> - if (--shost->host_blocked != 0) {
> - spin_unlock_irq(shost->host_lock);
> + if (busy == 0 && atomic_read(&shost->host_blocked) > 0) {
> + if (atomic_dec_return(&shost->host_blocked) > 0)
> goto out_dec;
> - }
> - spin_unlock_irq(shost->host_lock);
>
> SCSI_LOG_MLQUEUE(3,
> shost_printk(KERN_INFO, shost,
Same with this one.

Cheers,

Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare@suse.de +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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