scsi: target: tcmu: Fix xarray RCU warning

Commit f5ce815f ("scsi: target: tcmu: Support DATA_BLOCK_SIZE = N *
PAGE_SIZE") introduced xas_next() calls to iterate xarray elements.  These
calls triggered the WARNING "suspicious RCU usage" at tcmu device set up
[1]. In the call stack of xas_next(), xas_load() was called.  According to
its comment, this function requires "the xa_lock or the RCU lock".

To avoid the warning:

 - Guard the small loop calling xas_next() in tcmu_get_empty_block with RCU
   lock.

 - In the large loop in tcmu_copy_data using RCU lock would possibly
   disable preemtion for a long time (copy multi MBs). Therefore replace
   XA_STATE, xas_set and xas_next with a single xa_load.

[1]

[ 1899.867091] =============================
[ 1899.871199] WARNING: suspicious RCU usage
[ 1899.875310] 5.13.0-rc1+ #41 Not tainted
[ 1899.879222] -----------------------------
[ 1899.883299] include/linux/xarray.h:1182 suspicious rcu_dereference_check() usage!
[ 1899.890940] other info that might help us debug this:
[ 1899.899082] rcu_scheduler_active = 2, debug_locks = 1
[ 1899.905719] 3 locks held by kworker/0:1/1368:
[ 1899.910161]  #0: ffffa1f8c8b98738 ((wq_completion)target_submission){+.+.}-{0:0}, at: process_one_work+0x1ee/0x580
[ 1899.920732]  #1: ffffbd7040cd7e78 ((work_completion)(&q->sq.work)){+.+.}-{0:0}, at: process_one_work+0x1ee/0x580
[ 1899.931146]  #2: ffffa1f8d1c99768 (&udev->cmdr_lock){+.+.}-{3:3}, at: tcmu_queue_cmd+0xea/0x160 [target_core_user]
[ 1899.941678] stack backtrace:
[ 1899.946093] CPU: 0 PID: 1368 Comm: kworker/0:1 Not tainted 5.13.0-rc1+ #41
[ 1899.953070] Hardware name: System manufacturer System Product Name/PRIME Z270-A, BIOS 1302 03/15/2018
[ 1899.962459] Workqueue: target_submission target_queued_submit_work [target_core_mod]
[ 1899.970337] Call Trace:
[ 1899.972839]  dump_stack+0x6d/0x89
[ 1899.976222]  xas_descend+0x10e/0x120
[ 1899.979875]  xas_load+0x39/0x50
[ 1899.983077]  tcmu_get_empty_blocks+0x115/0x1c0 [target_core_user]
[ 1899.989318]  queue_cmd_ring+0x1da/0x630 [target_core_user]
[ 1899.994897]  ? rcu_read_lock_sched_held+0x3f/0x70
[ 1899.999695]  ? trace_kmalloc+0xa6/0xd0
[ 1900.003501]  ? __kmalloc+0x205/0x380
[ 1900.007167]  tcmu_queue_cmd+0x12f/0x160 [target_core_user]
[ 1900.012746]  __target_execute_cmd+0x23/0xa0 [target_core_mod]
[ 1900.018589]  transport_generic_new_cmd+0x1f3/0x370 [target_core_mod]
[ 1900.025046]  transport_handle_cdb_direct+0x34/0x50 [target_core_mod]
[ 1900.031517]  target_queued_submit_work+0x43/0xe0 [target_core_mod]
[ 1900.037837]  process_one_work+0x268/0x580
[ 1900.041952]  ? process_one_work+0x580/0x580
[ 1900.046195]  worker_thread+0x55/0x3b0
[ 1900.049921]  ? process_one_work+0x580/0x580
[ 1900.054192]  kthread+0x143/0x160
[ 1900.057499]  ? kthread_create_worker_on_cpu+0x40/0x40
[ 1900.062661]  ret_from_fork+0x1f/0x30

Link: https://lore.kernel.org/r/20210519135440.26773-1-bostroesser@gmail.com
Fixes: f5ce815f ("scsi: target: tcmu: Support DATA_BLOCK_SIZE = N * PAGE_SIZE")
Reported-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_user.c

@@ -516,8 +516,10 @@ static inline int tcmu_get_empty_block(struct tcmu_dev *udev,
 	dpi = dbi * udev->data_pages_per_blk;
 	/* Count the number of already allocated pages */
 	xas_set(&xas, dpi);
+	rcu_read_lock();
 	for (cnt = 0; xas_next(&xas) && cnt < page_cnt;)
 		cnt++;
+	rcu_read_unlock();
 
 	for (i = cnt; i < page_cnt; i++) {
 		/* try to get new page from the mm */
@@ -699,11 +701,10 @@ static inline void tcmu_copy_data(struct tcmu_dev *udev,
 				  struct scatterlist *sg, unsigned int sg_nents,
 				  struct iovec **iov, size_t data_len)
 {
-	XA_STATE(xas, &udev->data_pages, 0);
 	/* start value of dbi + 1 must not be a valid dbi */
 	int dbi = -2;
 	size_t page_remaining, cp_len;
-	int page_cnt, page_inx;
+	int page_cnt, page_inx, dpi;
 	struct sg_mapping_iter sg_iter;
 	unsigned int sg_flags;
 	struct page *page;
@@ -726,9 +727,10 @@ static inline void tcmu_copy_data(struct tcmu_dev *udev,
 		if (page_cnt > udev->data_pages_per_blk)
 			page_cnt = udev->data_pages_per_blk;
 
-		xas_set(&xas, dbi * udev->data_pages_per_blk);
-		for (page_inx = 0; page_inx < page_cnt && data_len; page_inx++) {
-			page = xas_next(&xas);
+		dpi = dbi * udev->data_pages_per_blk;
+		for (page_inx = 0; page_inx < page_cnt && data_len;
+		     page_inx++, dpi++) {
+			page = xa_load(&udev->data_pages, dpi);
 
 			if (direction == TCMU_DATA_AREA_TO_SG)
 				flush_dcache_page(page);