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Subject[PATCH] serial: pl011: Fix lockdep splat when handling magic-sysrq interrupt
Date
From: Peter Zijlstra <peterz@infradead.org>

Issuing a magic-sysrq via the PL011 causes the following lockdep splat,
which is easily reproducible under QEMU:

| sysrq: Changing Loglevel
| sysrq: Loglevel set to 9
|
| ======================================================
| WARNING: possible circular locking dependency detected
| 5.9.0-rc7 #1 Not tainted
| ------------------------------------------------------
| systemd-journal/138 is trying to acquire lock:
| ffffab133ad950c0 (console_owner){-.-.}-{0:0}, at: console_lock_spinning_enable+0x34/0x70
|
| but task is already holding lock:
| ffff0001fd47b098 (&port_lock_key){-.-.}-{2:2}, at: pl011_int+0x40/0x488
|
| which lock already depends on the new lock.

[...]

| Possible unsafe locking scenario:
|
| CPU0 CPU1
| ---- ----
| lock(&port_lock_key);
| lock(console_owner);
| lock(&port_lock_key);
| lock(console_owner);
|
| *** DEADLOCK ***

The issue being that CPU0 takes 'port_lock' on the irq path in pl011_int()
before taking 'console_owner' on the printk() path, whereas CPU1 takes
the two locks in the opposite order on the printk() path due to setting
the "console_owner" prior to calling into into the actual console driver.

Fix this in the same way as the msm-serial driver by dropping 'port_lock'
before handling the sysrq.

Cc: <stable@vger.kernel.org> # 4.19+
Cc: Russell King <linux@armlinux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20200811101313.GA6970@willie-the-truck
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Tested-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
drivers/tty/serial/amba-pl011.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 67498594d7d7..87dc3fc15694 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -308,8 +308,9 @@ static void pl011_write(unsigned int val, const struct uart_amba_port *uap,
*/
static int pl011_fifo_to_tty(struct uart_amba_port *uap)
{
- u16 status;
unsigned int ch, flag, fifotaken;
+ int sysrq;
+ u16 status;

for (fifotaken = 0; fifotaken != 256; fifotaken++) {
status = pl011_read(uap, REG_FR);
@@ -344,10 +345,12 @@ static int pl011_fifo_to_tty(struct uart_amba_port *uap)
flag = TTY_FRAME;
}

- if (uart_handle_sysrq_char(&uap->port, ch & 255))
- continue;
+ spin_unlock(&uap->port.lock);
+ sysrq = uart_handle_sysrq_char(&uap->port, ch & 255);
+ spin_lock(&uap->port.lock);

- uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
+ if (!sysrq)
+ uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
}

return fifotaken;
--
2.28.0.709.gb0816b6eb0-goog
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