Re: [PATCH v7 3/7] dax/cxl, hmem: Initialize hmem early and defer dax_cxl binding

Koralahalli Channabasappa, Smita
2026-03-19T15:46:50Z

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 <dan.j.williams@intel.com>
>>
>> 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

> [ 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 <dan.j.williams@intel.com>
>> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
>> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
>> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
>> ---
>> 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
>>
>>