Re: [PATCH v3 3/3] dmaengine: wait for RCU readers before releasing dma_device

[Garg, Shivank]

On Mon, 2026-08-17 at 09:29 -0500, Frank Li wrote:
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> 
> On Sun, Aug 16, 2026 at 03:49:27PM +0000, Shivank Garg wrote:
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> > 
> > dma_issue_pending_all() walks the dma_device_list with
> > list_for_each_entry_rcu() under rcu_read_lock(). dma_device_release()
> > unlinks the device with list_del_rcu() and then calls
> > device->device_release() (which in many drivers, such as plx_dma.c,
> > directly calls kfree()).
> > 
> > Because there is no grace period between unlinking the device and
> > freeing it, concurrent RCU readers in dma_issue_pending_all() can
> > access the device after it has been freed.
> > 
> > The lockless walk originally relied on clients holding a dmaengine
> > reference to pin the provider module, and therefore the device, for as
> > long as they might traverse the list. Commit 8ad342a86359 ("dmaengine:
> > Add reference counting to dma_device struct") decoupled the dma_device
> > lifetime from the module reference, so the device can now be released
> > while a reader is still walking the list.
> > 
> > Add synchronize_rcu() before the device is freed, so RCU readers are
> > guaranteed to have finished. Keep it unconditional: providers that do
> > not implement device_release() free the device themselves once
> > dma_async_device_unregister() returns, so they need the same grace
> > period.
> > 
> > Fixes: 2ba05622b8b1 ("dmaengine: provide a common 'issue_pending_all' implementation")
> > Suggested-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> > Link: https://sashiko.dev/#/patchset/20260526-dmaengine-kref-fix-v2-0-3df60afac01d@xxxxxxx
> > Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
> > ---
> 
> Reviewed-by: Frank Li <Frank.Li@xxxxxxx>
> 
> > 

Thank you for the review.

Best regards,
Shivank