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Date: Fri, 10 Mar 2006 15:45:24 -0800
From: Andrew Morton <akpm@osdl.org>
To: ebiederm@xmission.com (Eric W. Biederman)
Cc: oleg@tv-sign.ru, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] pidhash: Refactor the pid hash table.
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ebiederm@xmission.com (Eric W. Biederman) wrote:
>
> +fastcall void put_pid(struct pid *pid)
> +{
> +	if (!pid)
> +		return;
> +	if ((atomic_read(&pid->count) == 1) ||
> +	     atomic_dec_and_test(&pid->count))
> +		kmem_cache_free(pid_cachep, pid);
> +}

This looks odd.  It's an RCU callback so it's asynchronous.  It doesn't
take any locks, so if anyone else can have a ref on this thing then the
refcount can change at any time.

And both sides of the || are basically equivalent.  Perhaps you meant &&. 
But I'm more worried by the apparent raciness?