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Commit fe89def7 authored by Darren Hart's avatar Darren Hart Committed by Greg Kroah-Hartman
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pch_uart: Add eg20t_port lock field, avoid recursive spinlocks 



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: default avatarDarren 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: default avatarAlan Cox <alan@linux.intel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent e643f87f

drivers/tty/serial/pch_uart.c

+26 −12
Original line number Diff line number Diff line
@@ -253,6 +253,9 @@ 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;

};



/**
@@ -1058,7 +1061,7 @@ static irqreturn_t pch_uart_interrupt(int irq, void *dev_id) 
	int next = 1;

	u8 msr;



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

	spin_lock_irqsave(&priv->lock, flags);

	handled = 0;

	while (next) {

		iid = pch_uart_hal_get_iid(priv);
@@ -1116,7 +1119,7 @@ static irqreturn_t pch_uart_interrupt(int irq, void *dev_id) 
		handled |= (unsigned int)ret;

	}



	spin_unlock_irqrestore(&priv->port.lock, flags);

	spin_unlock_irqrestore(&priv->lock, flags);

	return IRQ_RETVAL(handled);

}
@@ -1226,9 +1229,9 @@ 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. */
@@ -1376,7 +1379,8 @@ 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);
@@ -1389,7 +1393,8 @@ 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)
@@ -1538,8 +1543,9 @@ 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];
@@ -1547,12 +1553,16 @@ 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
@@ -1570,8 +1580,10 @@ 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);

}
@@ -1669,6 +1681,8 @@ 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;