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

[Alison Schofield <alison.schofield@intel.com>]

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
[   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
> 
>