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SubjectRe: Question about rtc_lock
On Sat, 2001-10-06 at 11:24, Jonathan Lundell wrote:
> rtc_interrupt(), you mean.


> Even if there weren't current interrupt code doing CMOS accesses, it
> would seem prudent to assume that there might be eventually, the
> RTC/NVRAM being a multi-purpose shared resource.

I'm not concerned about an irq handler (present or future)
interfering with us as we write to the CMOS RAM. What I'm
concerned about is getting a rtc interrupt while we hold rtc_lock,
with deadlock being the result (since rtc_interrupt will spin on
the lock).

Either (1) we need to change these spinlocks to _irq, or (2) we
need to know that this bit of code runs only with irqs disabled.
My question is: Is it (1) or (2)?

Or is it (3) Thomas Hood is failing to understand something here?

Assuming the answer is (1), I append a patch that changes the
spinlock calls to _irqsave versions.


The patch:
--- linux-2.4.10-ac5-fix/arch/i386/kernel/bootflag.c_PREV Fri Oct 5 23:20:43 2001
+++ linux-2.4.10-ac5-fix/arch/i386/kernel/bootflag.c Sat Oct 6 23:15:33 2001
@@ -81,26 +81,30 @@

static void __init sbf_write(u8 v)
+ unsigned long flags;
if(sbf_port != -1)
v &= ~(1<<7);

- spin_lock(&rtc_lock);
+ spin_lock_irqsave(&rtc_lock, flags);
CMOS_WRITE(v, sbf_port);
- spin_unlock(&rtc_lock);
+ spin_unlock_irqrestore(&rtc_lock, flags);

static u8 __init sbf_read(void)
u8 v;
+ unsigned long flags;
if(sbf_port == -1)
return 0;
- spin_lock(&rtc_lock);
+ spin_lock_irqsave(&rtc_lock, flags);
v = CMOS_READ(sbf_port);
- spin_unlock(&rtc_lock);
+ spin_unlock_irqrestore(&rtc_lock, flags);
return v;

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