[ 061/108] pch_uart: Add eg20t_port lock field, avoid recursive spinlocks

Ben Hutchings
2012-10-07T23:33:23Z

From: Darren Hart <dvhart@linux.intel.com>

commit 2588aba002d14e938c2f56d299ecf3e7ce1302a5 upstream.

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@linux.intel.com>
CC: Tomoya MORINAGA <tomoya.rohm@gmail.com>
CC: Feng Tang <feng.tang@intel.com>
CC: Alexander Stein <alexander.stein@systec-electronic.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2:
- Adjust context
- Drop changes to pch_console_write()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/tty/serial/pch_uart.c | 38 ++++++++++++++++++++++++++------------
1 file changed, 26 insertions(+), 12 deletions(-)

--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -236,6 +236,9 @@ struct eg20t_port {
int tx_dma_use;
void *rx_buf_virt;
dma_addr_t rx_buf_dma;
+
+ /* protect the eg20t_port private structure and io access to membase */
+ spinlock_t lock;
};

/**
@@ -964,7 +967,7 @@ static irqreturn_t pch_uart_interrupt(in
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) {
@@ -1017,7 +1020,7 @@ static irqreturn_t pch_uart_interrupt(in
priv->int_dis_flag = 0;
}

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

@@ -1131,9 +1134,9 @@ static void pch_uart_break_ctl(struct ua
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. */
@@ -1284,7 +1287,8 @@ static void pch_uart_set_termios(struct

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);
@@ -1297,7 +1301,8 @@ static void pch_uart_set_termios(struct
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)
@@ -1449,6 +1454,8 @@ static struct eg20t_port *pch_uart_init_
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;