linux-kernel - Re: [PATCH v5 6/7] module: Improve support for asynchronous module exit code

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Date:   Wed, 28 Sep 2022 12:27:07 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>,
        Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
        John Garry <john.garry@...wei.com>,
        Mike Christie <michael.christie@...cle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v5 6/7] module: Improve support for asynchronous module
 exit code

On 9/27/22 18:09, Ming Lei wrote:
> On Wed, Sep 14, 2022 at 03:56:20PM -0700, Bart Van Assche wrote:
>> Some kernel modules call device_del() from their module exit code and
>> schedule asynchronous work from inside the .release callback without waiting
>> until that callback has finished. As an example, many SCSI LLD drivers call
> 
> It isn't only related with device, any kobject has such issue, or any
> reference counter usage has similar potential risk, see previous discussion:
> 
> https://lore.kernel.org/lkml/YsZm7lSXYAHT14ui@T590/
> 
> IMO, it is one fundamental problem wrt. module vs. reference counting or
> kobject uses at least, since the callback depends on module code
> segment.
> 
>> scsi_remove_host() from their module exit code. scsi_remove_host() may
>> invoke scsi_device_dev_release_usercontext() asynchronously.
>> scsi_device_dev_release_usercontext() uses the host template pointer and
>> that pointer usually exists in static storage in the SCSI LLD. Support
>> using the module reference count to keep the module around until
>> asynchronous module exiting has completed by waiting in the delete_module()
>> system call until the module reference count drops to zero.
> 
> The issue can't be addressed by the normal mod->refcnt, since user need
> to unload module when the device isn't used.

Hi Ming,

How about removing support for calling scsi_device_put() from atomic context
as is done in the untested patch below?

Thanks,

Bart.

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index c59eac7a32f2..661753a10b47 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -561,6 +561,8 @@ EXPORT_SYMBOL(scsi_report_opcode);
   */
  int scsi_device_get(struct scsi_device *sdev)
  {
+	might_sleep();
+
  	if (sdev->sdev_state == SDEV_DEL || sdev->sdev_state == SDEV_CANCEL)
  		goto fail;
  	if (!get_device(&sdev->sdev_gendev))
@@ -588,6 +590,7 @@ void scsi_device_put(struct scsi_device *sdev)
  {
  	struct module *mod = sdev->host->hostt->module;

+	might_sleep();
  	put_device(&sdev->sdev_gendev);
  	module_put(mod);
  }
diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index a3aaafdeac1d..4cfc9317b4ad 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -441,7 +441,7 @@ static void scsi_device_cls_release(struct device *class_dev)
  	put_device(&sdev->sdev_gendev);
  }

-static void scsi_device_dev_release_usercontext(struct work_struct *work)
+static void scsi_device_dev_release(struct device *dev)
  {
  	struct scsi_device *sdev;
  	struct device *parent;
@@ -450,11 +450,8 @@ static void scsi_device_dev_release_usercontext(struct work_struct *work)
  	struct scsi_vpd *vpd_pg0 = NULL, *vpd_pg89 = NULL;
  	struct scsi_vpd *vpd_pgb0 = NULL, *vpd_pgb1 = NULL, *vpd_pgb2 = NULL;
  	unsigned long flags;
-	struct module *mod;
-
-	sdev = container_of(work, struct scsi_device, ew.work);

-	mod = sdev->host->hostt->module;
+	sdev = to_scsi_device(dev);

  	parent = sdev->sdev_gendev.parent;

@@ -516,19 +513,6 @@ static void scsi_device_dev_release_usercontext(struct work_struct *work)

  	if (parent)
  		put_device(parent);
-	module_put(mod);
-}
-
-static void scsi_device_dev_release(struct device *dev)
-{
-	struct scsi_device *sdp = to_scsi_device(dev);
-
-	/* Set module pointer as NULL in case of module unloading */
-	if (!try_module_get(sdp->host->hostt->module))
-		sdp->host->hostt->module = NULL;
-
-	execute_in_process_context(scsi_device_dev_release_usercontext,
-				   &sdp->ew);
  }

  static struct class sdev_class = {