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SubjectRe: start_kernel(): bug: interrupts were enabled early
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Wed, 31 Mar 2010, H. Peter Anvin wrote:
>>
>> The obvious way to fix this would be to use
>> spin_lock_irqsave..spin_lock_irqrestore in __down_read as well as in the
>> other locations; I don't have a good feel for what the cost of doing so
>> would be, though. On x86 it's fairly expensive simply because the only
>> way to save the state is to push it on the stack, which the compiler
>> doesn't deal well with, but this code isn't used on x86.
>

[...]

> So making the slow-path do the spin_[un]lock_irq{save,restore}() versions
> sounds like the right thing. It won't be a performance issue: it _is_ the
> slow-path, and we're already doing the expensive part (the spinlock itself
> and the irq thing).
>
> So ACK on the idea. Who wants to write the trivial patch and test it?

OK, I'll bite since I was seeing boot-time hangs on ARM (TI OMAP3) due
to this. Patch below.

Kevin


From 7baff4008353bbfd2a2e2a4da22b87bc4efa4194 Mon Sep 17 00:00:00 2001
From: Kevin Hilman <khilman@deeprootsystems.com>
Date: Wed, 7 Apr 2010 11:52:46 -0700
Subject: [PATCH] rwsem generic spinlock: use IRQ save/restore spinlocks

rwsems can be used with IRQs disabled, particularily in early boot
before IRQs are enabled. Currently the spin_unlock_irq() usage in the
slow-patch will unconditionally enable interrupts and cause problems
since interrupts are not yet initialized or enabled.

This patch uses save/restore versions of IRQ spinlocks in the slowpath
to ensure interrupts are not unintentionally disabled.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
---
lib/rwsem-spinlock.c | 14 ++++++++------
1 files changed, 8 insertions(+), 6 deletions(-)
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
index ccf95bf..ffc9fc7 100644
--- a/lib/rwsem-spinlock.c
+++ b/lib/rwsem-spinlock.c
@@ -143,13 +143,14 @@ void __sched __down_read(struct rw_semaphore *sem)
{
struct rwsem_waiter waiter;
struct task_struct *tsk;
+ unsigned long flags;

- spin_lock_irq(&sem->wait_lock);
+ spin_lock_irqsave(&sem->wait_lock, flags);

if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->activity++;
- spin_unlock_irq(&sem->wait_lock);
+ spin_unlock_irqrestore(&sem->wait_lock, flags);
goto out;
}

@@ -164,7 +165,7 @@ void __sched __down_read(struct rw_semaphore *sem)
list_add_tail(&waiter.list, &sem->wait_list);

/* we don't need to touch the semaphore struct anymore */
- spin_unlock_irq(&sem->wait_lock);
+ spin_unlock_irqrestore(&sem->wait_lock, flags);

/* wait to be given the lock */
for (;;) {
@@ -209,13 +210,14 @@ void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
{
struct rwsem_waiter waiter;
struct task_struct *tsk;
+ unsigned long flags;

- spin_lock_irq(&sem->wait_lock);
+ spin_lock_irqsave(&sem->wait_lock, flags);

if (sem->activity == 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->activity = -1;
- spin_unlock_irq(&sem->wait_lock);
+ spin_unlock_irqrestore(&sem->wait_lock, flags);
goto out;
}

@@ -230,7 +232,7 @@ void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
list_add_tail(&waiter.list, &sem->wait_list);

/* we don't need to touch the semaphore struct anymore */
- spin_unlock_irq(&sem->wait_lock);
+ spin_unlock_irqrestore(&sem->wait_lock, flags);

/* wait to be given the lock */
for (;;) {
--
1.7.0.2


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