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SubjectRe: [PATCH] (2.6.20-rt3) PowerPC: convert spinlocks into raw

Benjamin Herrenschmidt wrote:

>>Convert the spinlocks in the PowerPC interrupt related code into the raw ones,
>>also convert the PURR and PMC related spinlocks...

> Care to explain the difference ?

In the realtime preemption mode, spinlocks are implemented via mutexes,
and the kernel may be preempted if the lock it's trying to acquire is already
taken by another process which shouldn't happen in the interrupt (or otherwise
non-preempatble) context. In such context, the "raw" spinlocks must still be
used which are (almost?) the same as the "usual" Linux spinlocks.
IIUC, the notion of the interrupt context" in the realtim (and also in the
low-latency desktop with hardirq preemption) mode is somewhat different form
the usual as the interrupt handlers are executed in their own threads, and so,
out of the interrupt context (that's not an absolute law: the timer interrupts
are still handled the usual way, for example)...
I hope Ingo will correct me where I may be wrong. :-)

> This is related to the -rt stuff ?

Yes, it's CONFIG_PREEMPT_RT specific. That's whey I specified 2.6.20-rt3
in the subject.

> Cheers,
> Ben.

In my turn, I'd like to have your (or anybody's) comments since I have
doubts about the nee of some conversions below which Mark have originally done
(though I limited myself to only adding couple more that seemed really necessary)

>>Signed-off-by: Mark A. Greer <>
>>Signed-off-by: Sergei Shtylyov <>

>>Index: linux-2.6/arch/powerpc/kernel/irq.c
>>--- linux-2.6.orig/arch/powerpc/kernel/irq.c
>>+++ linux-2.6/arch/powerpc/kernel/irq.c
>>@@ -392,7 +392,7 @@ EXPORT_SYMBOL(do_softirq);
>> static LIST_HEAD(irq_hosts);
>>-static spinlock_t irq_big_lock = SPIN_LOCK_UNLOCKED;
>>+static DEFINE_RAW_SPINLOCK(irq_big_lock);
>> static DEFINE_PER_CPU(unsigned int, irq_radix_reader);
>> static unsigned int irq_radix_writer;
>> struct irq_map_entry irq_map[NR_IRQS];

Erm, not very sure about irq_big_lock -- seems to only be used by IRQ
mapping code...

>>Index: linux-2.6/arch/powerpc/kernel/pmc.c
>>--- linux-2.6.orig/arch/powerpc/kernel/pmc.c
>>+++ linux-2.6/arch/powerpc/kernel/pmc.c
>>@@ -52,7 +52,7 @@ static void dummy_perf(struct pt_regs *r
>> }
>> #endif
>>-static DEFINE_SPINLOCK(pmc_owner_lock);
>>+static DEFINE_RAW_SPINLOCK(pmc_owner_lock);
>> static void *pmc_owner_caller; /* mostly for debugging */
>> perf_irq_t perf_irq = dummy_perf;

Not sure about this one -- it seemed to guard reserving/releasing the PMC
hardware (by Oprofile) and only be taken out of the interrupt context...

>>Index: linux-2.6/arch/powerpc/kernel/time.c
>>--- linux-2.6.orig/arch/powerpc/kernel/time.c
>>+++ linux-2.6/arch/powerpc/kernel/time.c
>>@@ -223,7 +223,7 @@ struct cpu_purr_data {
>> int initialized; /* thread is running */
>> u64 tb; /* last TB value read */
>> u64 purr; /* last PURR value read */
>>- spinlock_t lock;
>>+ raw_spinlock_t lock;
>> };
>> static DEFINE_PER_CPU(struct cpu_purr_data, cpu_purr_data);

The PURR spinlock obviously needed to be raw being used by the timer

>>Index: linux-2.6/arch/powerpc/sysdev/mpic.c
>>--- linux-2.6.orig/arch/powerpc/sysdev/mpic.c
>>+++ linux-2.6/arch/powerpc/sysdev/mpic.c
>>@@ -44,7 +44,7 @@
>> static struct mpic *mpics;
>> static struct mpic *mpic_primary;
>>-static DEFINE_SPINLOCK(mpic_lock);
>>+static DEFINE_RAW_SPINLOCK(mpic_lock);
>> #ifdef CONFIG_PPC32 /* XXX for now */

Not sure about this one -- it seemed to only be taken out of the interrupt

>>Index: linux-2.6/include/asm-powerpc/mpic.h
>>--- linux-2.6.orig/include/asm-powerpc/mpic.h
>>+++ linux-2.6/include/asm-powerpc/mpic.h
>>@@ -279,7 +279,7 @@ struct mpic
>> /* The fixup table */
>> struct mpic_irq_fixup *fixups;
>>- spinlock_t fixup_lock;
>>+ raw_spinlock_t fixup_lock;
>> #endif
>> /* Register access method */

This one is also taken by the OpenPIC code in the interrupt context.

WBR, Sergei
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