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Old 02-09-2009, 12:51 PM
Andy Whitcroft
Default serial: 8250: fix shared interrupts issues with SMP and RT kernels

From: Anton Vorontsov <avorontsov@ru.mvista.com>

Bug: #280821
commit 768aec0b5bccbd460bcf6e9131f19b5a26f3862d upstream

With SMP kernels _irqsave spinlock disables only local interrupts, while
the shared serial interrupt could be assigned to the CPU that is not
currently starting up the serial port.

This might cause issues because serial8250_startup() routine issues
IRQ-triggering operations before registering the port in the IRQ chain
(though, this is fine to do and done explicitly because we don't want to
process any interrupts on the port startup).

With RT kernels and preemptable hardirqs, _irqsave spinlock does not
disable local hardirqs, and the bug could be reproduced much easily:

$ cat /dev/ttyS0 &
$ cat /dev/ttyS1
irq 42: nobody cared (try booting with the "irqpoll" option)
Call Trace:
[C0475EB0] [C0008A98] show_stack+0x4c/0x1ac (unreliable)
[C0475EF0] [C004BBD4] __report_bad_irq+0x34/0xb8
[C0475F10] [C004BD38] note_interrupt+0xe0/0x308
[C0475F50] [C004B09C] thread_simple_irq+0xdc/0x104
[C0475F70] [C004B3FC] do_irqd+0x338/0x3c8
[C0475FC0] [C00398E0] kthread+0xf8/0x100
[C0475FF0] [C0011FE0] original_kernel_thread+0x44/0x60
[<c02112c4>] (serial8250_interrupt+0x0/0x138)
Disabling IRQ #42

After this, all serial ports on the given IRQ are non-functional.

To fix the issue we should explicitly disable shared IRQ before
issuing any IRQ-triggering operations.

I also changed spin_lock_irqsave to the ordinary spin_lock, since it
seems to be safe: chain does not contain new port (yet), thus nobody
will interfere us from the ISRs.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
drivers/serial/8250.c | 8 ++++++--
1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index f94109c..9b434a6 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1821,7 +1821,9 @@ static int serial8250_startup(struct uart_port *port)
* the interrupt is enabled. Delays are necessary to
* allow register changes to become visible.
- spin_lock_irqsave(&up->port.lock, flags);
+ spin_lock(&up->port.lock);
+ if (up->port.flags & UPF_SHARE_IRQ)
+ disable_irq_nosync(up->port.irq);

wait_for_xmitr(up, UART_LSR_THRE);
serial_out_sync(up, UART_IER, UART_IER_THRI);
@@ -1833,7 +1835,9 @@ static int serial8250_startup(struct uart_port *port)
iir = serial_in(up, UART_IIR);
serial_out(up, UART_IER, 0);

- spin_unlock_irqrestore(&up->port.lock, flags);
+ if (up->port.flags & UPF_SHARE_IRQ)
+ enable_irq(up->port.irq);
+ spin_unlock(&up->port.lock);

* If the interrupt is not reasserted, setup a timer to

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