Re: start_kernel(): bug: interrupts were enabled early

["Américo Wang" <xiyou.wangcong@gmail.com>]

On Wed, Apr 07, 2010 at 12:09:17PM -0700, Kevin Hilman wrote:
>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>

This looks reasonable and fine for me.

Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>

Thanks.


>---
> 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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