Re: [PATCH 1/2] scsi: mpi3mr: Poll a reply queue that an interrupt found busy

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] scsi: mpi3mr: unhandled missed reply race under PREEMPT_RT
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commit c0b0a317901b4ce93a734871bf41b604287947d5
Author: Usama Arif <usama.arif@linux.dev>

scsi: mpi3mr: Poll a reply queue that an interrupt found busy

This patch sets a flag to trigger a thread wakeup when an interrupt finds a
reply queue busy. It aims to prevent missed replies when another context
owns the queue.

> diff --git a/drivers/scsi/mpi3mr/mpi3mr_fw.c b/drivers/scsi/mpi3mr/mpi3mr_fw.c
> index f0d3cd398dd00..102f84667a5cf 100644
> --- a/drivers/scsi/mpi3mr/mpi3mr_fw.c
> +++ b/drivers/scsi/mpi3mr/mpi3mr_fw.c
[ ... ]
> @@ -588,8 +588,11 @@ int mpi3mr_process_op_reply_q(struct mpi3mr_ioc *mrioc,
>  
>  	reply_qidx = op_reply_q->qid - 1;
>  
> -	if (!atomic_add_unless(&op_reply_q->in_use, 1, 1))
> +	if (!atomic_add_unless(&op_reply_q->in_use, 1, 1)) {
> +		/* The owner may have missed a reply, let the thread poll */
> +		WRITE_ONCE(op_reply_q->enable_irq_poll, true);

[Severity: High]
When this code executes in mpi3mr_process_op_reply_q() under PREEMPT_RT,
does it leave missed IO replies stranded?

Under CONFIG_PREEMPT_RT, the interrupt handler is mpi3mr_isr_primary(),
which exits without checking the enable_irq_poll flag or returning
IRQ_WAKE_THREAD.

Additionally, looking at how the interrupt is requested:

drivers/scsi/mpi3mr/mpi3mr_fw.c:mpi3mr_request_irq() {
    ...
#else
	retval = request_threaded_irq(pci_irq_vector(pdev, index), mpi3mr_isr_primary,
	    NULL, IRQF_SHARED, intr_info->name, intr_info);
#endif
    ...
}

Because the polling thread handler is explicitly NULL under PREEMPT_RT,
there is no polling thread to wake up to process the missed replies.
Could this cause SCSI command timeouts and host resets during concurrent
IO under PREEMPT_RT?

>  		return 0;
> +	}
>  
>  	exp_phase = op_reply_q->ephase;
>  	reply_ci = op_reply_q->ci;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260930145606.2632749-1-usama.arif@linux.dev?part=1