From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Jia Yang <jia.yang@oss.qualcomm.com>,
	Jassi Brar <jassisinghbrar@gmail.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 1143/1376] mailbox: qcom-cpucp: fix PREEMPT_RT self-deadlock in IRQ handler
Date: Sat, 12 Sep 2026 08:59:29 +0200
Message-ID: <20260912065633.031881281@linuxfoundation.org>
In-Reply-To: <20260912065607.535295758@linuxfoundation.org>
References: <20260912065607.535295758@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jia Yang <jia.yang@oss.qualcomm.com>

[ Upstream commit 3690aaa6d18f6775c3e7932fb8af8c5bf6a6b69c ]

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 <jia.yang@oss.qualcomm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 44f4ed15f8184..f68298b2a1912 100644
--- a/drivers/mailbox/qcom-cpucp-mbox.c
+++ b/drivers/mailbox/qcom-cpucp-mbox.c
@@ -56,14 +56,25 @@ static irqreturn_t qcom_cpucp_mbox_irq_fn(int irq, void *data)
 	for_each_set_bit(i, (unsigned long *)&status, APSS_CPUCP_IPC_CHAN_SUPPORTED) {
 		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;
-- 
2.53.0