linux-kernel - Re: [PATCH 2/2] dmaengine: idxd: Fix refcount underflow on module unload

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Message-ID: <78dd14f8-8344-49a4-95eb-14ff005c5ba5@linux.alibaba.com>
Date: Fri, 30 May 2025 13:39:44 +0800
From: Shuai Xue <xueshuai@...ux.alibaba.com>
To: Yi Sun <yi.sun@...el.com>, Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc: dave.jiang@...el.com, dmaengine@...r.kernel.org,
 linux-kernel@...r.kernel.org, gordon.jin@...el.com
Subject: Re: [PATCH 2/2] dmaengine: idxd: Fix refcount underflow on module
 unload



在 2025/5/30 11:06, Yi Sun 写道:
> On 29.05.2025 10:04, Vinicius Costa Gomes wrote:
>> Yi Sun <yi.sun@...el.com> writes:
>>
>>> A recent refactor introduced a misplaced put_device() call, resulting in
>>> reference count underflow when the module is unloaded.
>>>
>>> Expand the idxd_cleanup() function to handle proper cleanup, and remove
>>> idxd_cleanup_internals() as it was not part of the driver unload path.
>>>
>>
>> 'idxd_cleanup_internals()' frees a bunch of stuff. I would expect an
>> explanation of when those things are being free'd now that removed that
>> call.
>>
> 
> I believe the call to idxd_unregister_devices(), which is invoked at the
> very beginning of idxd_remove(), already takes care of the necessary
> put_device() through the following call path:
> idxd_unregister_devices() -> device_unregister() -> put_device()
> 
> Therefore, there's no need to add additional put_device() calls for idxd
> groups, engines, or workqueues. While the commit message for a409e919ca3
> states: "Note, this also fixes the missing put_device() for idxd groups,
> engines, and wqs."
> it appears that no such omission actually existed, this part of the flow
> was already correctly handled.
> 
> Moreover, this refcount underflow issue appears to be a a clear
> regression. Prior to this commit, idxd_cleanup_internals() was not part of
> the driver unload path. The commit did not provide a strong justification
> for calling idxd_cleanup_internals() within idxd_cleanup().
> 
> For reference, the both two related bugs produce nearly identical call
> traces, and I think both are blocking issues.
> 
> Thanks
>     --Sun, Yi

Hi, Sun, Yi

idxd_pci_probe_alloc() {
	rc = idxd_probe(idxd);
	rc = idxd_register_devices(idxd);
	if (rc) {
		dev_err(dev, "IDXD sysfs setup failed\n");
		goto err_dev_register;
	}
// ...
  err_dev_register:			<-
	idxd_cleanup(idxd);
}

We use idxd_cleanup() when register devices failed to undo the
idxd_probe(). idxd_probe() sets up idxd groups, engine and wqs and
get reference counts by device_initialize().

what's the difference between idxd_cleanup_internals() and
idxd_unregister_devices()?

Thanks.
Shuai