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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Akinobu Mita <akinobu.mita@gmail.com>
Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org
Subject: Re: [PATCH 1/5] lib: introduce call_once()
Date: Tue, 11 Mar 2008 23:41:37 +1100	[thread overview]
Message-ID: <200803112341.38005.nickpiggin@yahoo.com.au> (raw)
In-Reply-To: <20080310145704.GA6396@APFDCB5C>

On Tuesday 11 March 2008 01:57, Akinobu Mita wrote:

> +static inline int call_once(struct once_control *once_control,
> +			    int (*init_rouine)(void))
> +{
> +	return likely(once_control->done) ? 0
> +			: call_once_slow(once_control, init_rouine);
> +}
> +
> +#endif /* __LINUX_ONCE_H */
> Index: 2.6-rc/lib/once.c
> ===================================================================
> --- /dev/null
> +++ 2.6-rc/lib/once.c
> @@ -0,0 +1,18 @@
> +#include <linux/module.h>
> +#include <linux/once.h>
> +
> +int call_once_slow(struct once_control *once_control, int
> (*init_rouine)(void)) +{
> +	int err = 0;
> +
> +	mutex_lock(&once_control->lock);
> +	if (!once_control->done) {
> +		err = init_rouine();
> +		if (!err)
> +			once_control->done = 1;
> +	}
> +	mutex_unlock(&once_control->lock);
> +
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(call_once_slow);

The store "once_control->done = 1" can become visible before
init_routine has finished. The code after calling call_once may
also speculatively load some memory before the load of
once_control->done completes, so you can likewise have a data
race that way too.

To fix this, you need smp_wmb after init_rouine(), and probably
smp_mb() in the fastpath after the check but before returning.

Basically any time you have this situation where you're touching
a shared variable without using locks, then you're vastly
increasing the complexity of the code, and so you must have a
good reason for it.

So acquiring the mutex unconditionally would be the best way to
go, unless you're calling this a lot in fastpaths (in which case
I would say you should probably rework your code)

Thanks,
Nick


      parent reply	other threads:[~2008-03-11 12:42 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-10 14:57 Akinobu Mita
2008-03-10 15:00 ` [PATCH 2/5] idr: use call_once() Akinobu Mita
2008-03-10 15:01   ` [PATCH 3/5] hugetlbfs: " Akinobu Mita
2008-03-10 15:03     ` [PATCH 4/5] shmem: " Akinobu Mita
2008-03-10 15:05       ` [PATCH 5/5] tiny-shmem: " Akinobu Mita
2008-03-10 22:15       ` [PATCH 4/5] shmem: " Hugh Dickins
2008-03-11 12:29         ` Akinobu Mita
2008-03-11 13:41           ` Hugh Dickins
2008-03-10 15:29 ` [PATCH 1/5] lib: introduce call_once() Joe Perches
2008-03-11 12:17   ` Akinobu Mita
2008-03-11  3:48 ` Andrew Morton
2008-03-11  4:10   ` Nick Piggin
2008-03-11  4:21     ` Andrew Morton
2008-03-11 12:27       ` Akinobu Mita
2008-03-11 17:35         ` Andrew Morton
2008-03-11 18:56           ` Joe Perches
2008-03-11 19:11             ` Andrew Morton
2008-03-15  4:01           ` Akinobu Mita
2008-03-11 12:41 ` Nick Piggin [this message]

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