[PATCH 1/2] mailbox: qcom-cpucp: fix PREEMPT_RT self-deadlock in IRQ handler

qcom_cpucp_mbox_irq_fn() calls mbox_chan_received_data() while holding
chan->lock. Under PREEMPT_RT, spin_lock_irqsave() is converted to an
rt_spinlock (rtmutex-based), which tracks ownership and can sleep.

The callback chain triggered by mbox_chan_received_data() eventually
reaches mailbox_clear_channel() -> mbox_send_message() -> add_to_rbuf(),
which attempts to re-acquire the same chan->lock. Since rtmutex detects
the re-entrant lock attempt by the same owner, the thread blocks waiting
for a lock it already holds, causing a permanent deadlock.

This deadlock manifests as 'irq/N-apss_cpucp_mbox' stuck in D state
with the following call trace:
  rt_spin_lock -> mbox_send_message -> mailbox_clear_channel ->
  scmi_rx_callback -> mbox_chan_received_data [<- held chan->lock here]

Fix by saving chan->cl locally and clearing the HW interrupt register
inside the lock, then invoking mbox_chan_received_data() after releasing
the lock. This preserves the mutual exclusion for chan->cl access while
avoiding the lock re-entrancy that causes the PREEMPT_RT deadlock.

Fixes: 0e2a9a03106c ("mailbox: Add support for QTI CPUCP mailbox controller")
Signed-off-by: Jia Yang <[email protected]>
---
 drivers/mailbox/qcom-cpucp-mbox.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/mailbox/qcom-cpucp-mbox.c b/drivers/mailbox/qcom-cpucp-mbox.c
index 862e45e8fbd5..0f7fe189e8b1 100644
--- a/drivers/mailbox/qcom-cpucp-mbox.c
+++ b/drivers/mailbox/qcom-cpucp-mbox.c
@@ -63,14 +63,25 @@ static irqreturn_t qcom_cpucp_mbox_irq_fn(int irq, void *data)
 	for_each_set_bit(i, (unsigned long *)&status, cpucp->mbox.num_chans) {
 		u32 val = readl(cpucp->rx_base + APSS_CPUCP_RX_MBOX_CMD(i) + APSS_CPUCP_MBOX_CMD_OFF);
 		struct mbox_chan *chan = &cpucp->chans[i];
+		struct mbox_client *cl;
 		unsigned long flags;
 
-		/* Provide mutual exclusion with changes to chan->cl */
+		/*
+		 * Provide mutual exclusion with changes to chan->cl.
+		 * Save cl locally and clear the HW interrupt inside the lock,
+		 * then invoke mbox_chan_received_data() outside the lock to
+		 * avoid a PREEMPT_RT self-deadlock: mbox_chan_received_data()
+		 * can call back into mbox_send_message() via scmi_rx_callback()
+		 * -> mailbox_clear_channel(), which re-acquires chan->lock
+		 * (converted to an rt_spinlock under PREEMPT_RT).
+		 */
 		spin_lock_irqsave(&chan->lock, flags);
-		if (chan->cl)
-			mbox_chan_received_data(chan, &val);
+		cl = chan->cl;
 		writeq(BIT(i), cpucp->rx_base + APSS_CPUCP_RX_MBOX_CLEAR);
 		spin_unlock_irqrestore(&chan->lock, flags);
+
+		if (cl)
+			mbox_chan_received_data(chan, &val);
 	}
 
 	return IRQ_HANDLED;

-- 
2.43.0