[PATCH] pch_uart: Add eg20t_port lock field, avoid recursive spinlocks

From: Darren Hart
Date: Wed Sep 05 2012 - 20:04:47 EST

The following patch has been included in linux-next
(fe89def79c48e2149abdd1e816523e69a9067191) but has not yet landed in mainline
nor been queued for stable so far as I can determine. This patch addresses a
deadlock in mainline and is a prerequisite for an additional fix required by the
PREEMPT_RT kernel. Can we get this pulled into 3.4.11 please? Perhaps I am
jumping the gun, but this patch was originally pulled on June 19, 2012.

pch_uart_interrupt() takes priv->port.lock which leads to two recursive
spinlock calls if low_latency==1 or CONFIG_PREEMPT_RT_FULL=y (one
otherwise):

pch_uart_interrupt
spin_lock_irqsave(priv->port.lock, flags)
case PCH_UART_IID_RDR_TO (data ready)
handle_rx_to
push_rx
tty_port_tty_get
spin_lock_irqsave(&port->lock, flags) <--- already hold this lock
...
tty_flip_buffer_push
...
flush_to_ldisc
spin_lock_irqsave(&tty->buf.lock)
spin_lock_irqsave(&tty->buf.lock)
disc->ops->receive_buf(tty, char_buf)
n_tty_receive_buf
tty->ops->flush_chars()
uart_flush_chars
uart_start
spin_lock_irqsave(&port->lock) <--- already hold this lock

Avoid this by using a dedicated lock to protect the eg20t_port structure
and IO access to its membase. This is more consistent with the 8250
driver. Ensure priv->lock is always take prior to priv->port.lock when
taken at the same time.

V2: Remove inadvertent whitespace change.
V3: Account for oops_in_progress for the private lock in
pch_console_write().

Note: Like the 8250 driver, if a printk is introduced anywhere inside
the pch_console_write() critical section, the kernel will hang
on a recursive spinlock on the private lock. The oops case is
handled by using a trylock in the oops_in_progress case.

Signed-off-by: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
CC: Tomoya MORINAGA <tomoya.rohm@xxxxxxxxx>
CC: Feng Tang <feng.tang@xxxxxxxxx>
CC: Alexander Stein <alexander.stein@xxxxxxxxxxxxxxxxxxxxx>
Acked-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/tty/serial/pch_uart.c | 38 ++++++++++++++++++++++++++------------
1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/drivers/tty/serial/pch_uart.c b/drivers/tty/serial/pch_uart.c
index c2816f4..c4d8731 100644
--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -252,6 +252,9 @@ struct eg20t_port {
dma_addr_t rx_buf_dma;

struct dentry *debugfs;
+
+ /* protect the eg20t_port private structure and io access to membase */
+ spinlock_t lock;
};

/**
@@ -1056,7 +1059,7 @@ static irqreturn_t pch_uart_interrupt(int irq, void *dev_id)
unsigned int iid;
unsigned long flags;

- spin_lock_irqsave(&priv->port.lock, flags);
+ spin_lock_irqsave(&priv->lock, flags);
handled = 0;
while ((iid = pch_uart_hal_get_iid(priv)) > 1) {
switch (iid) {
@@ -1107,7 +1110,7 @@ static irqreturn_t pch_uart_interrupt(int irq, void *dev_id)
priv->int_dis_flag = 0;
}

- spin_unlock_irqrestore(&priv->port.lock, flags);
+ spin_unlock_irqrestore(&priv->lock, flags);
return IRQ_RETVAL(handled);
}

@@ -1218,9 +1221,9 @@ static void pch_uart_break_ctl(struct uart_port *port, int ctl)
unsigned long flags;

priv = container_of(port, struct eg20t_port, port);
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&priv->lock, flags);
pch_uart_hal_set_break(priv, ctl);
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&priv->lock, flags);
}

/* Grab any interrupt resources and initialise any low level driver state. */
@@ -1368,7 +1371,8 @@ static void pch_uart_set_termios(struct uart_port *port,

baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);

- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&priv->lock, flags);
+ spin_lock(&port->lock);

uart_update_timeout(port, termios->c_cflag, baud);
rtn = pch_uart_hal_set_line(priv, baud, parity, bits, stb);
@@ -1381,7 +1385,8 @@ static void pch_uart_set_termios(struct uart_port *port,
tty_termios_encode_baud_rate(termios, baud, baud);

out:
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock(&port->lock);
+ spin_unlock_irqrestore(&priv->lock, flags);
}

static const char *pch_uart_type(struct uart_port *port)
@@ -1531,8 +1536,9 @@ pch_console_write(struct console *co, const char *s, unsigned int count)
{
struct eg20t_port *priv;
unsigned long flags;
+ int priv_locked = 1;
+ int port_locked = 1;
u8 ier;
- int locked = 1;

priv = pch_uart_ports[co->index];

@@ -1540,12 +1546,16 @@ pch_console_write(struct console *co, const char *s, unsigned int count)

local_irq_save(flags);
if (priv->port.sysrq) {
- /* serial8250_handle_port() already took the lock */
- locked = 0;
+ spin_lock(&priv->lock);
+ /* serial8250_handle_port() already took the port lock */
+ port_locked = 0;
} else if (oops_in_progress) {
- locked = spin_trylock(&priv->port.lock);
- } else
+ priv_locked = spin_trylock(&priv->lock);
+ port_locked = spin_trylock(&priv->port.lock);
+ } else {
+ spin_lock(&priv->lock);
spin_lock(&priv->port.lock);
+ }

/*
* First save the IER then disable the interrupts
@@ -1563,8 +1573,10 @@ pch_console_write(struct console *co, const char *s, unsigned int count)
wait_for_xmitr(priv, BOTH_EMPTY);
iowrite8(ier, priv->membase + UART_IER);

- if (locked)
+ if (port_locked)
spin_unlock(&priv->port.lock);
+ if (priv_locked)
+ spin_unlock(&priv->lock);
local_irq_restore(flags);
}

@@ -1662,6 +1674,8 @@ static struct eg20t_port *pch_uart_init_port(struct pci_dev *pdev,
pci_enable_msi(pdev);
pci_set_master(pdev);

+ spin_lock_init(&priv->lock);
+
iobase = pci_resource_start(pdev, 0);
mapbase = pci_resource_start(pdev, 1);
priv->mapbase = mapbase;
--
1.7.11.4

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