scsi: core: Fix a regression triggered by scsi_host_busy()

JIRA: https://issues.redhat.com/browse/RHEL-123046

commit a0b7780
Author: Bart Van Assche <bvanassche@acm.org>
Date:   Tue Oct 7 14:48:00 2025 -0700

    scsi: core: Fix a regression triggered by scsi_host_busy()

    Commit 995412e ("blk-mq: Replace tags->lock with SRCU for tag
    iterators") introduced the following regression:

    Call trace:
     __srcu_read_lock+0x30/0x80 (P)
     blk_mq_tagset_busy_iter+0x44/0x300
     scsi_host_busy+0x38/0x70
     ufshcd_print_host_state+0x34/0x1bc
     ufshcd_link_startup.constprop.0+0xe4/0x2e0
     ufshcd_init+0x944/0xf80
     ufshcd_pltfrm_init+0x504/0x820
     ufs_rockchip_probe+0x2c/0x88
     platform_probe+0x5c/0xa4
     really_probe+0xc0/0x38c
     __driver_probe_device+0x7c/0x150
     driver_probe_device+0x40/0x120
     __driver_attach+0xc8/0x1e0
     bus_for_each_dev+0x7c/0xdc
     driver_attach+0x24/0x30
     bus_add_driver+0x110/0x230
     driver_register+0x68/0x130
     __platform_driver_register+0x20/0x2c
     ufs_rockchip_pltform_init+0x1c/0x28
     do_one_initcall+0x60/0x1e0
     kernel_init_freeable+0x248/0x2c4
     kernel_init+0x20/0x140
     ret_from_fork+0x10/0x20

    Fix this regression by making scsi_host_busy() check whether the SCSI
    host tag set has already been initialized. tag_set->ops is set by
    scsi_mq_setup_tags() just before blk_mq_alloc_tag_set() is called. This
    fix is based on the assumption that scsi_host_busy() and
    scsi_mq_setup_tags() calls are serialized. This is the case in the UFS
    driver.

    Reported-by: Sebastian Reichel <sebastian.reichel@collabora.com>
    Closes: https://lore.kernel.org/linux-block/pnezafputodmqlpumwfbn644ohjybouveehcjhz2hmhtcf2rka@sdhoiivync4y/
    Cc: Ming Lei <ming.lei@redhat.com>
    Cc: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Bart Van Assche <bvanassche@acm.org>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
    Link: https://patch.msgid.link/20251007214800.1678255-1-bvanassche@acm.org
    Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

drivers/scsi/hosts.c

@@ -607,8 +607,9 @@ int scsi_host_busy(struct Scsi_Host *shost)
 {
 	int cnt = 0;
 
-	blk_mq_tagset_busy_iter(&shost->tag_set,
-				scsi_host_check_in_flight, &cnt);
+	if (shost->tag_set.ops)
+		blk_mq_tagset_busy_iter(&shost->tag_set,
+					scsi_host_check_in_flight, &cnt);
 	return cnt;
 }
 EXPORT_SYMBOL(scsi_host_busy);