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

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

[Darren Hart]

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@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>
---
 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

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

[Greg Kroah-Hartman]

On Wed, Sep 05, 2012 at 05:04:07PM -0700, Darren Hart wrote:
> 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?

3.4.11?  It has to hit Linus's tree first.

> Perhaps I am
> jumping the gun, but this patch was originally pulled on June 19, 2012.

Remember, we missed a pull cycle for tty due to other problems, I
thought I picked all of the different pieces needed for 3.6, but I must
of missed this one.  Should it go there now?  Also, if you want a patch
in the stable tree, please tell me when you submit it by marking it with
a
	cc: stable <stable@vger.kernel.org>
in the signed-off-by area of the patch.

thanks,

greg k-h

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

[Greg Kroah-Hartman]

On Wed, Sep 05, 2012 at 05:14:48PM -0700, Greg Kroah-Hartman wrote:
> On Wed, Sep 05, 2012 at 05:04:07PM -0700, Darren Hart wrote:
> > 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?
> 
> 3.4.11?  It has to hit Linus's tree first.
> 
> > Perhaps I am
> > jumping the gun, but this patch was originally pulled on June 19, 2012.
> 
> Remember, we missed a pull cycle for tty due to other problems, I
> thought I picked all of the different pieces needed for 3.6, but I must
> of missed this one.

Nope, it made it, it is commit 2588aba002d14e938c2f56d299ecf3e7ce1302a5.

Now, do you want that patch in the -stable releases?  If so, how far
back? :)

Sorry for the mess,

greg k-h

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

[Darren Hart]

On 09/05/2012 05:18 PM, Greg Kroah-Hartman wrote:
> On Wed, Sep 05, 2012 at 05:14:48PM -0700, Greg Kroah-Hartman wrote:
>> On Wed, Sep 05, 2012 at 05:04:07PM -0700, Darren Hart wrote:
>>> 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?
>>
>> 3.4.11?  It has to hit Linus's tree first.
>>
>>> Perhaps I am
>>> jumping the gun, but this patch was originally pulled on June 19, 2012.
>>
>> Remember, we missed a pull cycle for tty due to other problems, I
>> thought I picked all of the different pieces needed for 3.6, but I must
>> of missed this one.
> 
> Nope, it made it, it is commit 2588aba002d14e938c2f56d299ecf3e7ce1302a5.

Doh, I pulled master and stable, but only checked stable. Sigh. My
apologies Greg.

> 
> Now, do you want that patch in the -stable releases?  If so, how far
> back? :)

Yes, back to 3.0 would be ideal. It needs mangling for 3.2 and back
though. I will send patches for 3.4, 3.2 and possibly 3.0 following the
stable_kernel_rules.txt procedure.

> 
> Sorry for the mess,

Looks like it was my mess today :-)

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel

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

[Darren Hart]

On 09/05/2012 05:25 PM, Darren Hart wrote:
> 
> 
> On 09/05/2012 05:18 PM, Greg Kroah-Hartman wrote:
>> On Wed, Sep 05, 2012 at 05:14:48PM -0700, Greg Kroah-Hartman wrote:
>>> On Wed, Sep 05, 2012 at 05:04:07PM -0700, Darren Hart wrote:
>>>> 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?
>>>
>>> 3.4.11?  It has to hit Linus's tree first.
>>>
>>>> Perhaps I am
>>>> jumping the gun, but this patch was originally pulled on June 19, 2012.
>>>
>>> Remember, we missed a pull cycle for tty due to other problems, I
>>> thought I picked all of the different pieces needed for 3.6, but I must
>>> of missed this one.
>>
>> Nope, it made it, it is commit 2588aba002d14e938c2f56d299ecf3e7ce1302a5.
> 
> Doh, I pulled master and stable, but only checked stable. Sigh. My
> apologies Greg.
> 
>>
>> Now, do you want that patch in the -stable releases?  If so, how far
>> back? :)
> 
> Yes, back to 3.0 would be ideal. It needs mangling for 3.2 and back
> though. I will send patches for 3.4, 3.2 and possibly 3.0 following the
> stable_kernel_rules.txt procedure.

On second thought, there are way too many changes to pch_uart that are
required before this patch can really be applied prior to 3.4. I suspect
these are not all appropriate for -stable. I'd be happy just getting
this into 3.4.11. 2588aba002d14e938c2f56d299ecf3e7ce1302a5 cherry-picks
cleanly to 3.4.

Thanks,

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel

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

[Greg Kroah-Hartman]

On Wed, Sep 05, 2012 at 05:37:25PM -0700, Darren Hart wrote:
> On 09/05/2012 05:25 PM, Darren Hart wrote:
> > 
> > 
> > On 09/05/2012 05:18 PM, Greg Kroah-Hartman wrote:
> >> On Wed, Sep 05, 2012 at 05:14:48PM -0700, Greg Kroah-Hartman wrote:
> >>> On Wed, Sep 05, 2012 at 05:04:07PM -0700, Darren Hart wrote:
> >>>> 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?
> >>>
> >>> 3.4.11?  It has to hit Linus's tree first.
> >>>
> >>>> Perhaps I am
> >>>> jumping the gun, but this patch was originally pulled on June 19, 2012.
> >>>
> >>> Remember, we missed a pull cycle for tty due to other problems, I
> >>> thought I picked all of the different pieces needed for 3.6, but I must
> >>> of missed this one.
> >>
> >> Nope, it made it, it is commit 2588aba002d14e938c2f56d299ecf3e7ce1302a5.
> > 
> > Doh, I pulled master and stable, but only checked stable. Sigh. My
> > apologies Greg.
> > 
> >>
> >> Now, do you want that patch in the -stable releases?  If so, how far
> >> back? :)
> > 
> > Yes, back to 3.0 would be ideal. It needs mangling for 3.2 and back
> > though. I will send patches for 3.4, 3.2 and possibly 3.0 following the
> > stable_kernel_rules.txt procedure.
> 
> On second thought, there are way too many changes to pch_uart that are
> required before this patch can really be applied prior to 3.4. I suspect
> these are not all appropriate for -stable. I'd be happy just getting
> this into 3.4.11. 2588aba002d14e938c2f56d299ecf3e7ce1302a5 cherry-picks
> cleanly to 3.4.

Now queued up for 3.4.y and 3.5.y, thanks.

greg k-h