Re: [PATCH] cxl/pmem: Fix module reload vs workqueue state

["Verma, Vishal L" <vishal.l.verma@intel.com>]

On Thu, 2021-11-11 at 10:19 -0800, Dan Williams wrote:
> A test of the form:
> 
>     while true; do modprobe -r cxl_pmem; modprobe cxl_pmem; done
> 
> May lead to a crash signature of the form:
> 
>     BUG: unable to handle page fault for address: ffffffffc0660030
>     #PF: supervisor instruction fetch in kernel mode
>     #PF: error_code(0x0010) - not-present page
>     [..]
>     Workqueue: cxl_pmem 0xffffffffc0660030
>     RIP: 0010:0xffffffffc0660030
>     Code: Unable to access opcode bytes at RIP 0xffffffffc0660006.
>     [..]
>     Call Trace:
>      ? process_one_work+0x4ec/0x9c0
>      ? pwq_dec_nr_in_flight+0x100/0x100
>      ? rwlock_bug.part.0+0x60/0x60
>      ? worker_thread+0x2eb/0x700
> 
> In that report the 0xffffffffc0660030 address corresponds to the former
> function address of cxl_nvb_update_state() from a previous load of the
> module, not the current address. Fix that by arranging for ->state_work
> in the 'struct cxl_nvdimm_bridge' object to be reinitialized on cxl_pmem
> module reload.
> 
> Details:
> 
> Recall that CXL subsystem wants to link a CXL memory expander device to
> an NVDIMM sub-hierarchy when both a persistent memory range has been
> registered by the CXL platform driver (cxl_acpi) *and* when that CXL
> memory expander has published persistent memory capacity (Get Partition
> Info). To this end the cxl_nvdimm_bridge driver arranges to rescan the
> CXL bus when either of those conditions change. The helper
> bus_rescan_devices() can not be called underneath the device_lock() for
> any device on that bus, so the cxl_nvdimm_bridge driver uses a workqueue
> for the rescan.
> 
> Typically a driver allocates driver data to hold a 'struct work_struct'
> for a driven device, but for a workqueue that may run after ->remove()
> returns, driver data will have been freed. The 'struct
> cxl_nvdimm_bridge' object holds the state and work_struct directly.
> Unfortunately it was only arranging for that infrastructure to be
> initialized once per device creation rather than the necessary once per
> workqueue (cxl_pmem_wq) creation.
> 
> Introduce is_cxl_nvdimm_bridge() and cxl_nvdimm_bridge_reset() in
> support of invalidating stale references to a recently destroyed
> cxl_pmem_wq.
> 
> Cc: <stable@vger.kernel.org>
> Fixes: 8fdcb1704f61 ("cxl/pmem: Add initial infrastructure for pmem support")
> Reported-by: Vishal Verma <vishal.l.verma@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Tried this out and works fine now, thanks for the quick fix!

Tested-by: Vishal Verma <vishal.l.verma@intel.com>

> ---
>  drivers/cxl/core/pmem.c |    8 +++++++-
>  drivers/cxl/cxl.h       |    8 ++++++++
>  drivers/cxl/pmem.c      |   29 +++++++++++++++++++++++++++--
>  3 files changed, 42 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/core/pmem.c b/drivers/cxl/core/pmem.c
> index 76a4fa39834c..cc402cb7a905 100644
> --- a/drivers/cxl/core/pmem.c
> +++ b/drivers/cxl/core/pmem.c
> @@ -51,10 +51,16 @@ struct cxl_nvdimm_bridge *to_cxl_nvdimm_bridge(struct device *dev)
>  }
>  EXPORT_SYMBOL_NS_GPL(to_cxl_nvdimm_bridge, CXL);
>  
> -__mock int match_nvdimm_bridge(struct device *dev, const void *data)
> +bool is_cxl_nvdimm_bridge(struct device *dev)
>  {
>  	return dev->type == &cxl_nvdimm_bridge_type;
>  }
> +EXPORT_SYMBOL_NS_GPL(is_cxl_nvdimm_bridge, CXL);
> +
> +__mock int match_nvdimm_bridge(struct device *dev, const void *data)
> +{
> +	return is_cxl_nvdimm_bridge(dev);
> +}
>  
>  struct cxl_nvdimm_bridge *cxl_find_nvdimm_bridge(struct cxl_nvdimm *cxl_nvd)
>  {
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 5e2e93451928..ca979ee11017 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -221,6 +221,13 @@ struct cxl_decoder {
>  };
>  
>  
> +/**
> + * enum cxl_nvdimm_brige_state - state machine for managing bus rescans
> + * @CXL_NVB_NEW: Set at bridge create and after cxl_pmem_wq is destroyed
> + * @CXL_NVB_DEAD: Set at brige unregistration to preclude async probing
> + * @CXL_NVB_ONLINE: Target state after successful ->probe()
> + * @CXL_NVB_OFFLINE: Target state after ->remove() or failed ->probe()
> + */
>  enum cxl_nvdimm_brige_state {
>  	CXL_NVB_NEW,
>  	CXL_NVB_DEAD,
> @@ -333,6 +340,7 @@ struct cxl_nvdimm_bridge *devm_cxl_add_nvdimm_bridge(struct device *host,
>  						     struct cxl_port *port);
>  struct cxl_nvdimm *to_cxl_nvdimm(struct device *dev);
>  bool is_cxl_nvdimm(struct device *dev);
> +bool is_cxl_nvdimm_bridge(struct device *dev);
>  int devm_cxl_add_nvdimm(struct device *host, struct cxl_memdev *cxlmd);
>  struct cxl_nvdimm_bridge *cxl_find_nvdimm_bridge(struct cxl_nvdimm *cxl_nvd);
>  
> diff --git a/drivers/cxl/pmem.c b/drivers/cxl/pmem.c
> index 17e82ae90456..b65a272a2d6d 100644
> --- a/drivers/cxl/pmem.c
> +++ b/drivers/cxl/pmem.c
> @@ -315,6 +315,31 @@ static struct cxl_driver cxl_nvdimm_bridge_driver = {
>  	.id = CXL_DEVICE_NVDIMM_BRIDGE,
>  };
>  
> +/*
> + * Return all bridges to the CXL_NVB_NEW state to invalidate any
> + * ->state_work referring to the now destroyed cxl_pmem_wq.
> + */
> +static int cxl_nvdimm_bridge_reset(struct device *dev, void *data)
> +{
> +	struct cxl_nvdimm_bridge *cxl_nvb;
> +
> +	if (!is_cxl_nvdimm_bridge(dev))
> +		return 0;
> +
> +	cxl_nvb = to_cxl_nvdimm_bridge(dev);
> +	device_lock(dev);
> +	cxl_nvb->state = CXL_NVB_NEW;
> +	device_unlock(dev);
> +
> +	return 0;
> +}
> +
> +static void destroy_cxl_pmem_wq(void)
> +{
> +	destroy_workqueue(cxl_pmem_wq);
> +	bus_for_each_dev(&cxl_bus_type, NULL, NULL, cxl_nvdimm_bridge_reset);
> +}
> +
>  static __init int cxl_pmem_init(void)
>  {
>  	int rc;
> @@ -340,7 +365,7 @@ static __init int cxl_pmem_init(void)
>  err_nvdimm:
>  	cxl_driver_unregister(&cxl_nvdimm_bridge_driver);
>  err_bridge:
> -	destroy_workqueue(cxl_pmem_wq);
> +	destroy_cxl_pmem_wq();
>  	return rc;
>  }
>  
> @@ -348,7 +373,7 @@ static __exit void cxl_pmem_exit(void)
>  {
>  	cxl_driver_unregister(&cxl_nvdimm_driver);
>  	cxl_driver_unregister(&cxl_nvdimm_bridge_driver);
> -	destroy_workqueue(cxl_pmem_wq);
> +	destroy_cxl_pmem_wq();
>  }
>  
>  MODULE_LICENSE("GPL v2");
>