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mailbox: qcom-cpucp: fix PREEMPT_RT self-deadlock in IRQ handler

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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: 0e2a9a0310 ("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>
This commit is contained in:
Jia Yang 2026-08-06 15:03:56 +08:00 committed by Jassi Brar
parent 9e9afcb436
commit 3690aaa6d1
1 changed files with 14 additions and 3 deletions
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17
drivers/mailbox/qcom-cpucp-mbox.c
View File

@ -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;