linux-kernel - Re: [Patch v4] rwsem: fix rwsem_is

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Date:	Tue, 13 Oct 2009 13:34:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Amerigo Wang <amwang@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ben Woodard <bwoodard@...l.gov>,
	David Howells <dhowells@...hat.com>,
	Brian Behlendorf <behlendorf1@...l.gov>
Subject: Re: [Patch v4] rwsem: fix rwsem_is_locked() bugs

On Thu, 8 Oct 2009 05:23:53 -0400
Amerigo Wang <amwang@...hat.com> wrote:

> --- a/include/linux/rwsem-spinlock.h
> +++ b/include/linux/rwsem-spinlock.h
> @@ -71,7 +71,13 @@ extern void __downgrade_write(struct rw_semaphore *sem);
>  
>  static inline int rwsem_is_locked(struct rw_semaphore *sem)
>  {
> -	return (sem->activity != 0);
> +	int ret = 1;
> +
> +	if (spin_trylock_irq(&sem->wait_lock)) {
> +		ret = (sem->activity != 0);
> +		spin_unlock_irq(&sem->wait_lock);
> +	}
> +	return ret;
>  }

a) probably to large to be inlined

b) the function will now cause bugs if called under
   local_irq_disable().  That wasn't the case before.  Fixable via
   spin_lock_irqsave().

In the present kernel there don't appear to be any irqs-off callers. 
There may of course be some out-of-tree ones which will get bitten by
this semantic change.

If we decide to leave this new rule in place then we should add a
WARN_ON(irqs_disabled()) to prevent hitting people with a nasty, subtle
bug.

Methinks that _irqsave() is better.
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