On 3/19/2026 8:46 AM, Koralahalli Channabasappa, Smita wrote:
Hi Jonathan and Alison,
Thanks for the report and suggestions. I took a look at Jonathan's
comments in Patch 6 and tying it together here.
On 3/18/2026 10:48 PM, Alison Schofield wrote:
On Thu, Mar 19, 2026 at 01:14:56AM +0000, Smita Koralahalli wrote:
From: Dan Williams <[email protected]>
Move hmem/ earlier in the dax Makefile so that hmem_init() runs before
dax_cxl.
In addition, defer registration of the dax_cxl driver to a workqueue
instead of using module_cxl_driver(). This ensures that dax_hmem has
an opportunity to initialize and register its deferred callback and make
ownership decisions before dax_cxl begins probing and claiming Soft
Reserved ranges.
Mark the dax_cxl driver as PROBE_PREFER_ASYNCHRONOUS so its probe runs
out of line from other synchronous probing avoiding ordering
dependencies while coordinating ownership decisions with dax_hmem.
Hi Smita,
Replying to this patch, as it's my best guess as to why I may be
seeing this WARN when I modprobe cxl-test.
We are able to pass all the CXL unit tests because it is only that
first load that causes the WARN. All subsequent reloads of cxl-test
do not unload dax_cxl and dax_hmem so they chug happily along.
I can reproduce by unloading each piece before reloading cxl-test
# modprobe -r cxl-test
# modprobe -r dax_cxl
# modprobe -r dax_hmem
# modprobe cxl-test
and the WARN repeats.
Guessing you may recognize what is going on. Let me know if I can
try anything else out.
# dmesg (trimmed to just the init calls)
[ 34.229033] calling fwctl_init+0x0/0xff0 [fwctl] @ 1057
[ 34.230616] initcall fwctl_init+0x0/0xff0 [fwctl] returned 0 after
186 usecs
[ 34.257096] calling cxl_core_init+0x0/0x100 [cxl_core] @ 1057
[ 34.258395] initcall cxl_core_init+0x0/0x100 [cxl_core] returned 0
after 538 usecs
[ 34.264170] calling cxl_port_init+0x0/0xff0 [cxl_port] @ 1057
[ 34.264982] initcall cxl_port_init+0x0/0xff0 [cxl_port] returned 0
after 110 usecs
[ 34.268058] calling cxl_mem_driver_init+0x0/0xff0 [cxl_mem] @ 1057
[ 34.268743] initcall cxl_mem_driver_init+0x0/0xff0 [cxl_mem]
returned 0 after 110 usecs
[ 34.274670] calling cxl_pmem_init+0x0/0xff0 [cxl_pmem] @ 1057
[ 34.277835] initcall cxl_pmem_init+0x0/0xff0 [cxl_pmem] returned 0
after 1671 usecs
[ 34.285807] calling cxl_acpi_init+0x0/0xff0 [cxl_acpi] @ 1057
[ 34.287105] initcall cxl_acpi_init+0x0/0xff0 [cxl_acpi] returned 0
after 262 usecs
[ 34.292967] calling cxl_test_init+0x0/0xff0 [cxl_test] @ 1057
[ 34.339841] initcall cxl_test_init+0x0/0xff0 [cxl_test] returned 0
after 45832 usecs
[ 34.342259] calling cxl_mock_mem_driver_init+0x0/0xff0
[cxl_mock_mem] @ 1063
[ 34.343459] initcall cxl_mock_mem_driver_init+0x0/0xff0
[cxl_mock_mem] returned 0 after 356 usecs
[ 34.658602] calling dax_hmem_init+0x0/0xff0 [dax_hmem] @ 1059
[ 34.670106] calling cxl_pci_driver_init+0x0/0xff0 [cxl_pci] @ 1100
[ 34.671023] initcall cxl_pci_driver_init+0x0/0xff0 [cxl_pci]
returned 0 after 197 usecs
[ 34.673051] initcall dax_hmem_init+0x0/0xff0 [dax_hmem] returned 0
after 2225 usecs
I agree with Jonathan's comments in Patch 6, using __WORK_INITIALIZER or
initializing work in dax_hmem_init() and gating flush on pdev will fix
the WARN — I will add both for v8. But I think the WARN is likely
indicating an ordering issue here..
On initial boot, the Makefile ordering ensures dax_hmem_init() runs
before cxl_dax_region_init(), so both work items land on system_long_wq
in the right order and dax_hmem's deferred work is queued before
dax_cxl's driver registration work.
On module reload which Alison is trying here I dont think, modules are
loaded by Makefile order. I think dax_cxl's workqueue is calling
dax_hmem_flush_work() before dax_hmem probe has had a chance to queue
its work, so flush_work() flushes nothing and dax_cxl registers its
driver without waiting.
__WORK_INITIALIZER fixes the WARN, but doesn't fix the race I guess if
we are hitting that here..
[ 34.673051] initcall dax_hmem_init+0x0/0xff0 [dax_hmem] returned 0
after 2225 usecs
[ 34.676011] calling cxl_dax_region_init+0x0/0xff0 [dax_cxl] @ 1059
These two lines indicate cxl_dax started after dax_hmem_init() returns
but I dont think that guarantees dax_hmem_platform_probe() has actually
run..
I dont know if wait_for_device_probe() in cxl_dax_region_driver_register
might help..
Thanks
Smita
Actually, thinking about this more..
dax_hmem_initial_probe lives in device.c (built-in) so it survives
module reload. On reload it's still true from the first boot. This means
hmem_register_device() skips the deferral path entirely..
The problem is this bypasses the cxl_region_contains_resource() check
that the deferred work normally does. On first boot,
process_defer_work() walks each range and decides per-range: if CXL
covers it, skip. If not, register with HMEM. On reload, that check never
happens — whoever registers first via alloc_dax_region() wins,
regardless of whether CXL actually covers the range.
So if dax_cxl registers first on reload, it could claim a range that CXL
doesn't actually cover, and dax_hmem would lose a range it should own..
I dont know if Im thinking through this right..
Thanks
Smita
[ 34.676011] calling cxl_dax_region_init+0x0/0xff0 [dax_cxl] @ 1059
[ 34.676856] ------------[ cut here ]------------
[ 34.677533] WARNING: kernel/workqueue.c:4289 at
__flush_work+0x4f9/0x550, CPU#3: kworker/3:2/136
[ 34.678596] Modules linked in: dax_cxl(+) cxl_pci dax_hmem
cxl_mock_mem(O) cxl_test(O) cxl_acpi(O) cxl_pmem(O) cxl_mem(O)
cxl_port(O) cxl_mock(O) cxl_core(O) fwctl nd_pmem nd_btt dax_pmem nfit
nd_e820 libnvdimm
[ 34.680632] initcall cxl_dax_region_init+0x0/0xff0 [dax_cxl]
returned 0 after 3842 usecs
[ 34.680918] CPU: 3 UID: 0 PID: 136 Comm: kworker/3:2 Tainted:
G O 7.0.0-rc4+ #156 PREEMPT(full)
[ 34.684368] Tainted: [O]=OOT_MODULE
[ 34.684993] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
BIOS 0.0.0 02/06/2015
[ 34.686098] Workqueue: events_long cxl_dax_region_driver_register
[dax_cxl]
[ 34.687108] RIP: 0010:__flush_work+0x4f9/0x550
That addr is this line in flush_work()
if (WARN_ON(!work->func))
return false;
[ 34.687811] Code: ff 49 8b 45 00 49 8b 55 08 89 c7 48 c1 e8 04 83
e7 08 83 e0 0f 83 cf 02 49 0f ba 6d 00 03 e9 a1 fc ff ff 0f 0b e9 e6
fe ff ff <0f> 0b e9 df fe ff ff e8 9b 48 15 01 85 c0 0f 84 26 ff ff ff
80 3d
[ 34.690107] RSP: 0018:ffffc900020b7cf8 EFLAGS: 00010246
[ 34.690673] RAX: 0000000000000000 RBX: ffffffffa0ea2088 RCX:
ffff8880088b2b78
[ 34.691388] RDX: 00000000834fb194 RSI: 0000000000000000 RDI:
ffffffffa0ea2088
[ 34.692135] RBP: ffffc900020b7de0 R08: 0000000031ab93b0 R09:
00000000effb42e8
[ 34.692876] R10: 000000008effb42e R11: 0000000000000000 R12:
ffff88807d9bb340
[ 34.693588] R13: ffffffffa0ea2088 R14: ffffffffa0ed2020 R15:
0000000000000001
[ 34.694358] FS: 0000000000000000(0000) GS:ffff8880fa45f000(0000)
knlGS:0000000000000000
[ 34.695179] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 34.695775] CR2: 00007fe888b4e34c CR3: 00000000090ed004 CR4:
0000000000370ef0
[ 34.696494] Call Trace:
[ 34.696889] <TASK>
[ 34.697238] ? __lock_acquire+0xb08/0x2930
[ 34.697730] ? __this_cpu_preempt_check+0x13/0x20
[ 34.698277] flush_work+0x17/0x30
[ 34.698705] dax_hmem_flush_work+0x10/0x20 [dax_hmem]
[ 34.699270] cxl_dax_region_driver_register+0x9/0x30 [dax_cxl]
[ 34.699943] process_one_work+0x203/0x6c0
[ 34.700452] worker_thread+0x197/0x350
[ 34.700942] ? __pfx_worker_thread+0x10/0x10
[ 34.701455] kthread+0x108/0x140
[ 34.701915] ? __pfx_kthread+0x10/0x10
[ 34.702396] ret_from_fork+0x28a/0x310
[ 34.702880] ? __pfx_kthread+0x10/0x10
[ 34.703363] ret_from_fork_asm+0x1a/0x30
[ 34.703872] </TASK>
[ 34.704227] irq event stamp: 11015
[ 34.704656] hardirqs last enabled at (11025): [<ffffffff813486de>]
__up_console_sem+0x5e/0x80
[ 34.705493] hardirqs last disabled at (11036): [<ffffffff813486c3>]
__up_console_sem+0x43/0x80
[ 34.706354] softirqs last enabled at (10500): [<ffffffff812ab9f3>]
__irq_exit_rcu+0xc3/0x120
[ 34.707197] softirqs last disabled at (10495): [<ffffffff812ab9f3>]
__irq_exit_rcu+0xc3/0x120
[ 34.708015] ---[ end trace 0000000000000000 ]---
[ 34.752127] calling dax_init+0x0/0xff0 [device_dax] @ 1089
[ 34.754006] initcall dax_init+0x0/0xff0 [device_dax] returned 0
after 422 usecs
[ 34.759609] calling dax_kmem_init+0x0/0xff0 [kmem] @ 1089
[ 37.338377] initcall dax_kmem_init+0x0/0xff0 [kmem] returned 0
after 2577658 usecs
Signed-off-by: Dan Williams <[email protected]>
Signed-off-by: Smita Koralahalli
<[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Reviewed-by: Jonathan Cameron <[email protected]>
---
drivers/dax/Makefile | 3 +--
drivers/dax/cxl.c | 27 ++++++++++++++++++++++++++-
2 files changed, 27 insertions(+), 3 deletions(-)
diff --git a/drivers/dax/Makefile b/drivers/dax/Makefile
index 5ed5c39857c8..70e996bf1526 100644
--- a/drivers/dax/Makefile
+++ b/drivers/dax/Makefile
@@ -1,4 +1,5 @@
# SPDX-License-Identifier: GPL-2.0
+obj-y += hmem/
obj-$(CONFIG_DAX) += dax.o
obj-$(CONFIG_DEV_DAX) += device_dax.o
obj-$(CONFIG_DEV_DAX_KMEM) += kmem.o
@@ -10,5 +11,3 @@ dax-y += bus.o
device_dax-y := device.o
dax_pmem-y := pmem.o
dax_cxl-y := cxl.o
-
-obj-y += hmem/
diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
index 13cd94d32ff7..a2136adfa186 100644
--- a/drivers/dax/cxl.c
+++ b/drivers/dax/cxl.c
@@ -38,10 +38,35 @@ static struct cxl_driver cxl_dax_region_driver = {
.id = CXL_DEVICE_DAX_REGION,
.drv = {
.suppress_bind_attrs = true,
+ .probe_type = PROBE_PREFER_ASYNCHRONOUS,
},
};
-module_cxl_driver(cxl_dax_region_driver);
+static void cxl_dax_region_driver_register(struct work_struct *work)
+{
+ cxl_driver_register(&cxl_dax_region_driver);
+}
+
+static DECLARE_WORK(cxl_dax_region_driver_work,
cxl_dax_region_driver_register);
+
+static int __init cxl_dax_region_init(void)
+{
+ /*
+ * Need to resolve a race with dax_hmem wanting to drive regions
+ * instead of CXL
+ */
+ queue_work(system_long_wq, &cxl_dax_region_driver_work);
+ return 0;
+}
+module_init(cxl_dax_region_init);
+
+static void __exit cxl_dax_region_exit(void)
+{
+ flush_work(&cxl_dax_region_driver_work);
+ cxl_driver_unregister(&cxl_dax_region_driver);
+}
+module_exit(cxl_dax_region_exit);
+
MODULE_ALIAS_CXL(CXL_DEVICE_DAX_REGION);
MODULE_DESCRIPTION("CXL DAX: direct access to CXL regions");
MODULE_LICENSE("GPL");
--
2.17.1
