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

["Koralahalli Channabasappa, Smita" <skoralah@amd.com>]

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
>>
>>