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    Subject[PATCH 06/14] scsi: convert target_busy to an atomic_t
    Date
    Avoid taking the host-wide host_lock to check the per-target queue limit.
    Instead we do an atomic_inc_return early on to grab our slot in the queue,
    and if nessecary decrement it after finishing all checks.

    Signed-off-by: Christoph Hellwig <hch@lst.de>
    ---
    drivers/scsi/scsi_lib.c | 52 ++++++++++++++++++++++++++------------------
    include/scsi/scsi_device.h | 4 ++--
    2 files changed, 33 insertions(+), 23 deletions(-)

    diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
    index 18e6449..5e269d6 100644
    --- a/drivers/scsi/scsi_lib.c
    +++ b/drivers/scsi/scsi_lib.c
    @@ -294,7 +294,7 @@ void scsi_device_unbusy(struct scsi_device *sdev)

    spin_lock_irqsave(shost->host_lock, flags);
    shost->host_busy--;
    - starget->target_busy--;
    + atomic_dec(&starget->target_busy);
    if (unlikely(scsi_host_in_recovery(shost) &&
    (shost->host_failed || shost->host_eh_scheduled)))
    scsi_eh_wakeup(shost);
    @@ -361,7 +361,7 @@ static inline int scsi_device_is_busy(struct scsi_device *sdev)
    static inline int scsi_target_is_busy(struct scsi_target *starget)
    {
    return ((starget->can_queue > 0 &&
    - starget->target_busy >= starget->can_queue) ||
    + atomic_read(&starget->target_busy) >= starget->can_queue) ||
    starget->target_blocked);
    }

    @@ -1305,37 +1305,49 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
    struct scsi_device *sdev)
    {
    struct scsi_target *starget = scsi_target(sdev);
    - int ret = 0;
    + unsigned int busy;

    - spin_lock_irq(shost->host_lock);
    if (starget->single_lun) {
    + spin_lock_irq(shost->host_lock);
    if (starget->starget_sdev_user &&
    - starget->starget_sdev_user != sdev)
    - goto out;
    + starget->starget_sdev_user != sdev) {
    + spin_unlock_irq(shost->host_lock);
    + return 0;
    + }
    starget->starget_sdev_user = sdev;
    + spin_unlock_irq(shost->host_lock);
    }

    - if (starget->target_busy == 0 && starget->target_blocked) {
    + busy = atomic_inc_return(&starget->target_busy) - 1;
    + if (busy == 0 && starget->target_blocked) {
    /*
    * unblock after target_blocked iterates to zero
    */
    - if (--starget->target_blocked != 0)
    - goto out;
    + spin_lock_irq(shost->host_lock);
    + if (--starget->target_blocked != 0) {
    + spin_unlock_irq(shost->host_lock);
    + goto out_dec;
    + }
    + spin_unlock_irq(shost->host_lock);

    SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
    "unblocking target at zero depth\n"));
    }

    - if (scsi_target_is_busy(starget)) {
    - list_move_tail(&sdev->starved_entry, &shost->starved_list);
    - goto out;
    - }
    + if (starget->can_queue > 0 && busy >= starget->can_queue)
    + goto starved;
    + if (starget->target_blocked)
    + goto starved;

    - scsi_target(sdev)->target_busy++;
    - ret = 1;
    -out:
    + return 1;
    +
    +starved:
    + spin_lock_irq(shost->host_lock);
    + list_move_tail(&sdev->starved_entry, &shost->starved_list);
    spin_unlock_irq(shost->host_lock);
    - return ret;
    +out_dec:
    + atomic_dec(&starget->target_busy);
    + return 0;
    }

    /*
    @@ -1445,7 +1457,7 @@ static void scsi_kill_request(struct request *req, struct request_queue *q)
    spin_unlock(sdev->request_queue->queue_lock);
    spin_lock(shost->host_lock);
    shost->host_busy++;
    - starget->target_busy++;
    + atomic_inc(&starget->target_busy);
    spin_unlock(shost->host_lock);
    spin_lock(sdev->request_queue->queue_lock);

    @@ -1615,9 +1627,7 @@ static void scsi_request_fn(struct request_queue *q)
    return;

    host_not_ready:
    - spin_lock_irq(shost->host_lock);
    - scsi_target(sdev)->target_busy--;
    - spin_unlock_irq(shost->host_lock);
    + atomic_dec(&scsi_target(sdev)->target_busy);
    not_ready:
    /*
    * lock q, handle tag, requeue req, and decrement device_busy. We
    diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
    index 816e8a2..446f741 100644
    --- a/include/scsi/scsi_device.h
    +++ b/include/scsi/scsi_device.h
    @@ -290,8 +290,8 @@ struct scsi_target {
    unsigned int expecting_lun_change:1; /* A device has reported
    * a 3F/0E UA, other devices on
    * the same target will also. */
    - /* commands actually active on LLD. protected by host lock. */
    - unsigned int target_busy;
    + /* commands actually active on LLD. */
    + atomic_t target_busy;
    /*
    * LLDs should set this in the slave_alloc host template callout.
    * If set to zero then there is not limit.
    --
    1.7.10.4


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