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    Subject[PATCH 4.19 51/71] scsi: core: Dont start concurrent async scan on same host
    Date
    From: Ming Lei <ming.lei@redhat.com>

    [ Upstream commit 831e3405c2a344018a18fcc2665acc5a38c3a707 ]

    The current scanning mechanism is supposed to fall back to a synchronous
    host scan if an asynchronous scan is in progress. However, this rule isn't
    strictly respected, scsi_prep_async_scan() doesn't hold scan_mutex when
    checking shost->async_scan. When scsi_scan_host() is called concurrently,
    two async scans on same host can be started and a hang in do_scan_async()
    is observed.

    Fixes this issue by checking & setting shost->async_scan atomically with
    shost->scan_mutex.

    Link: https://lore.kernel.org/r/20201010032539.426615-1-ming.lei@redhat.com
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: Ewan D. Milne <emilne@redhat.com>
    Cc: Hannes Reinecke <hare@suse.de>
    Cc: Bart Van Assche <bvanassche@acm.org>
    Reviewed-by: Lee Duncan <lduncan@suse.com>
    Reviewed-by: Bart Van Assche <bvanassche@acm.org>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/scsi/scsi_scan.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
    index 9a7e3a3bd5ce8..009a5b2aa3d02 100644
    --- a/drivers/scsi/scsi_scan.c
    +++ b/drivers/scsi/scsi_scan.c
    @@ -1722,15 +1722,16 @@ static void scsi_sysfs_add_devices(struct Scsi_Host *shost)
    */
    static struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
    {
    - struct async_scan_data *data;
    + struct async_scan_data *data = NULL;
    unsigned long flags;

    if (strncmp(scsi_scan_type, "sync", 4) == 0)
    return NULL;

    + mutex_lock(&shost->scan_mutex);
    if (shost->async_scan) {
    shost_printk(KERN_DEBUG, shost, "%s called twice\n", __func__);
    - return NULL;
    + goto err;
    }

    data = kmalloc(sizeof(*data), GFP_KERNEL);
    @@ -1741,7 +1742,6 @@ static struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
    goto err;
    init_completion(&data->prev_finished);

    - mutex_lock(&shost->scan_mutex);
    spin_lock_irqsave(shost->host_lock, flags);
    shost->async_scan = 1;
    spin_unlock_irqrestore(shost->host_lock, flags);
    @@ -1756,6 +1756,7 @@ static struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
    return data;

    err:
    + mutex_unlock(&shost->scan_mutex);
    kfree(data);
    return NULL;
    }
    --
    2.27.0


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