From: Ian Jackson <[email protected]>
Do not read IIR in serial8250_start_tx when UART_BUG_TXEN
Reading the IIR clears some oustanding interrupts so it is not safe.
Instead, simply transmit immediately if the buffer is empty without
regard to IIR.
Signed-off-by: Ian Jackson <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
---
Ian Jackson recently debugged a problem reported by Anders Kaseorg, and
posted a fix (see http://lkml.org/lkml/2009/2/11/240). As far as I can
see, the patch has been dropped on the floor.
I also experienced the problem, and Ian's patch fixes it for me.
The bug bites Xen HVM guests. Output to the serial console stalls until
some input happens. As Ian's analysis quoted below shows, it is a race
condition that could theoretically bite elsewhere as well.
Ian Jackson explained:
> The bugs in detail (this discussion applies to 2.6.20 and also to
> 2.6.28.4):
>
> 1. The hunk of serial8250_startup I quote below attempts to discover
> whether writing the IER re-asserts the THRI (transmit ready)
> interrupt. However the spinlock that it has taken out,
> port->lock, is not the one that the IRQ service routine takes
> before reading the IIR (i->lock). As a result, on an SMP system
> the generated interrupt races with the straight-line code in
> serial8250_startup.
>
> If serial8250_startup loses the race (perhaps because the system
> is a VM and its VCPU got preempted), UART_BUG_TXEN is spuriously
> added to bugs. This is quite unlikely in a normal system but in
> certain Xen configurations, particularly ones where there is CPU
> pressure, we may lose the race every time.
>
> It is not exactly clear to me how this ought to be resolved. One
> possibility is that the UART_BUG_TXEN problem might be worked
> around perfectly well by the new and very similar workaround
> UART_BUG_THRE[1] in 2.6.21ish in which case it could just be
> removed.
> 2. UART_BUG_TXEN's workaround appears to be intended to be
> harmless.
> However what it actually does is to read the IIR, thus clearing
> any actual interrupt (including incidentally non-THRI), and then
> only perform the intended servicing if the interrupt was _not_
> asserted. That is, it breaks on any serial port with the bug.
>
> As far as I can see there is not much use in UART_BUG_TXEN reading
> IIR at all, so a suitable change if we want to keep UART_BUG_TXEN
> might be the first patch I enclose below (again, not compiled
> or tested).
>
> If UART_BUG_TXEN is retained something along these lines should be
> done at the very least.
>
> Ian.
>
> [1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=40b36daad0ac704e6d5c1b75789f371ef5b053c1
> in which case UART
--- ../linux-2.6.28.4/drivers/serial/8250.c~ 2009-02-06 21:47:45.000000000 +0000
+++ ../linux-2.6.28.4/drivers/serial/8250.c 2009-02-11 15:55:24.000000000 +0000
@@ -1257,14 +1257,12 @@
serial_out(up, UART_IER, up->ier);
if (up->bugs & UART_BUG_TXEN) {
- unsigned char lsr, iir;
+ unsigned char lsr;
lsr = serial_in(up, UART_LSR);
up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
- iir = serial_in(up, UART_IIR) & 0x0f;
if ((up->port.type == PORT_RM9000) ?
- (lsr & UART_LSR_THRE &&
- (iir == UART_IIR_NO_INT || iir == UART_IIR_THRI)) :
- (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT))
+ (lsr & UART_LSR_THRE) :
+ (lsr & UART_LSR_TEMT))
transmit_chars(up);
}
}
> Ian Jackson recently debugged a problem reported by Anders Kaseorg, and
> posted a fix (see http://lkml.org/lkml/2009/2/11/240). As far as I can
> see, the patch has been dropped on the floor.
It was proposed as a band aid fix not a real fix. The real fix involves
finishing sorting out any locking issues Ian identified. I've not seen a
patch for that yet.
Alan Cox writes ("Re: [PATCH] IRQ handling race and spurious IIR read in serial/8250.c"):
> > Ian Jackson recently debugged a problem reported by Anders Kaseorg, and
> > posted a fix (see http://lkml.org/lkml/2009/2/11/240). As far as I can
> > see, the patch has been dropped on the floor.
>
> It was proposed as a band aid fix not a real fix. The real fix involves
> finishing sorting out any locking issues Ian identified. I've not seen a
> patch for that yet.
The patch I have provided and which Markus has reposted has these
properties:
1. It is definitely correct and does not introduce any new bugs.
2. It makes the existing bug definitely go away. That is, after my
patch is applied the code drives the UART correctly - that is,
according to the specification and in a manner guaranteed to be
reliable - even though the code may still mistakenly set a bug
flag;
3. Without it the code is definitely wrong;
4. Any fix which does not involve completely removing UART_BUG_TXEN
will need my change.
Or to put it another way
UART_BUG_TXEN UART_BUG_TXEN
incorrectly set behaviour
Current code Sometimes in VMs Breaks correct systems
Rarely in baremetal? (observed in production)
With my patch Sometimes in VMs Always correct.
(behaviour unchanged) Minor possible
performance problem.
Perfect code Perhaps feature If retained, should
should be removed? be correct. Minor
performance impact
is acceptable.
My patch is therefore a strict improvement and also a likely component
of many of the possible ultimate fixes; the other ultimate fixes
involve removing entirely the code which I am now proposing to patch.
Please do not block this bugfix just because I haven't been able to
conclusively determine whether the buggy-without-my-patch workaround
should be entirely removed, and just because I do not fix every other
bug in the same area.
In particular, the fact that the detection of UART_BUG_TXEN remains
buggy is not a reason to block a patch which makes UART_BUG_TXEN's
effects correct.
As you can see `perfect code' is not yet attainable because the people
who originally implemented UART_BUG_TXEN don't seem to be around right
now to ask, and we don't have a 100% clear description of the buggy
behaviour it is trying to detect, and so we don't know whether it can
safely be removed.
Ian.
> 1. It is definitely correct and does not introduce any new bugs.
Right ;)
> 2. It makes the existing bug definitely go away. That is, after my
> patch is applied the code drives the UART correctly - that is,
> according to the specification and in a manner guaranteed to be
The UART hardware and specifications diverge - often quite spectacularly.
> 4. Any fix which does not involve completely removing UART_BUG_TXEN
> will need my change.
Or the locking fix
It was described as a band aid so I treated it as such. Regardless of the
right thing to do long term its not a 2.6.29 candidate at this point but
certainly something that wants looking into in 2.6.30