Re: [PATCH v2] vfio/ccw: Flush notoper_work before returning from reset

[sashiko-bot@kernel.org]

> vfio_ccw_dma_unmap() is the DMA invalidation callback, which must unpin
> the pages covering the range before it returns.  It calls
> vfio_ccw_mdev_reset(), which raises a CLOSE event.
> 
> On the normal path fsm_close() calls cp_free() itself, so the pages are
> unpinned synchronously.  But if the subchannel cannot be disabled or
> quiesced, fsm_close() falls through to a NOT_OPER event.  Since commit
> 0c11f61a876e ("s390/vfio_ccw: Move cp cleanup out of not operational"),
> fsm_notoper() only queues notoper_work, which is where cp_free() then
> runs.  That commit flushes the work in vfio_ccw_mdev_close_device() and
> vfio_ccw_mdev_release_dev(), but nothing waits for it on the reset
> path, so the callback returns with the pages still pinned.
> 
> Because notoper_work runs asynchronously, vfio_dma_do_unmap() can exhaust
> its 10 retries before cp_free() completes, hitting BUG_ON(++retries > 10)
> and taking the host down.  A further CLOSE cannot help either, as both
> CLOSE and OPEN are fsm_nop in the NOT_OPER state.
> 
> Flush notoper_work after the CLOSE event, as vfio_ccw_mdev_close_device()
> already does.  The flush returns immediately when the work was never
> queued, so the normal path is unaffected.
> 
> Fixes: 0c11f61a876e ("s390/vfio_ccw: Move cp cleanup out of not operational")
> Signed-off-by: Yuho Choi <oss.patchbox@gmail.com>

Sashiko has reviewed this patch and found no issues.

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