On 13.05.21 06:33, Shin'ichiro Kawasaki wrote:
Commit f5ce815f34bc ("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 xas_next() calls with the RCU lock, adding
rcu_read_lock() and rcu_read_unlock().
[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
Fixes: f5ce815f34bc ("scsi: target: tcmu: Support DATA_BLOCK_SIZE = N * PAGE_SIZE")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx>
---
drivers/target/target_core_user.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index 198d25ae482a..8be0f40ffa2b 100644
--- a/drivers/target/target_core_user.c
+++ b/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 */
@@ -727,6 +729,7 @@ static inline void tcmu_copy_data(struct tcmu_dev *udev,
page_cnt = udev->data_pages_per_blk;
xas_set(&xas, dbi * udev->data_pages_per_blk);
+ rcu_read_lock();
for (page_inx = 0; page_inx < page_cnt && data_len; page_inx++) {
page = xas_next(&xas);
@@ -763,6 +766,7 @@ static inline void tcmu_copy_data(struct tcmu_dev *udev,
if (direction == TCMU_SG_TO_DATA_AREA)
flush_dcache_page(page);
}
+ rcu_read_unlock();
}
}
Thank you for catching and fixing this.
Regarding 2nd and 3rd hunk, I'm not sure using rcu_read_(un)lock is a
good choice. From a pure technical point of view, the missing RCU locks
are not a problem, since all all accesses to the xarray are protected by
the cmdr_lock mutex. If we now do rcu_read_lock() this might disable
preemtion for the duration of copying a complete data block, which might
be multiple MB worst case.
Therefore I think it would be better to add xas_(un)lock(&xas) before
and after the big while loop in tcmu_copy_data. Because we already hold
the cmdr_lock mutex, we know we will always immediately get the lock.
It will not disable preemption, but it will make the RCU warning
disappear.
-Bodo
