Date: Wed, 27 Aug 2008 12:05:53 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: cmm@us.ibm.com, tytso@mit.edu, sandeen@redhat.com,
       linux-ext4@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com,
       a.p.zijlstra@chello.nl, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -V3 01/11] percpu_counters: make fbc->count read atomic
 on 32 bit architecture
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On Wed, 27 Aug 2008 20:58:26 +0530
"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> fbc->count is of type s64. The change was introduced by
> 0216bfcffe424a5473daa4da47440881b36c1f4 which changed the type
> from long to s64. Moving to s64 also means on 32 bit architectures
> we can get wrong values on fbc->count. Since fbc->count is read
> more frequently and updated rarely use seqlocks. This should
> reduce the impact of locking in the read path for 32bit arch.
> 

So...  yesterday's suggestionm to investigate implementing this at a
lower level wasn't popular?

>  include/linux/percpu_counter.h |   28 ++++++++++++++++++++++++----
>  lib/percpu_counter.c           |   20 ++++++++++----------
>  2 files changed, 34 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 9007ccd..1b711a1 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -6,7 +6,7 @@
>   * WARNING: these things are HUGE.  4 kbytes per counter on 32-way P4.
>   */
>  
> -#include <linux/spinlock.h>
> +#include <linux/seqlock.h>
>  #include <linux/smp.h>
>  #include <linux/list.h>
>  #include <linux/threads.h>
> @@ -16,7 +16,7 @@
>  #ifdef CONFIG_SMP
>  
>  struct percpu_counter {
> -	spinlock_t lock;
> +	seqlock_t lock;
>  	s64 count;
>  #ifdef CONFIG_HOTPLUG_CPU
>  	struct list_head list;	/* All percpu_counters are on a list */
> @@ -53,10 +53,30 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
>  	return __percpu_counter_sum(fbc);
>  }
>  
> -static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> +#if BITS_PER_LONG == 64
> +static inline s64 fbc_count(struct percpu_counter *fbc)
>  {
>  	return fbc->count;
>  }
> +#else
> +/* doesn't have atomic 64 bit operation */
> +static inline s64 fbc_count(struct percpu_counter *fbc)
> +{
> +	s64 ret;
> +	unsigned seq;
> +	do {
> +		seq = read_seqbegin(&fbc->lock);
> +		ret = fbc->count;
> +	} while (read_seqretry(&fbc->lock, seq));
> +	return ret;
> +

Please don't put unneeded blank lines into random places.

> +}
> +#endif

This is now too large to be inlined.

> +static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> +{
> +	return fbc_count(fbc);
> +}

This change means that a percpu_counter_read() from interrupt context
on a 32-bit machine is now deadlockable, whereas it previously was not
deadlockable on either 32-bit or 64-bit.

This flows on to the lib/proportions.c, which uses
percpu_counter_read() and also does spin_lock_irqsave() internally,
indicating that it is (or was) designed to be used in IRQ contexts.

It means that bdi_stat() can no longer be used from interrupt context.

So a whole lot of thought and review and checking is needed here.  It
should all be spelled out in the changelog.  This will be a horridly
rare deadlock, so suitable WARN_ON()s should be added to detect when
callers are vulnerable to it.

Or we make the whole thing irq-safe.

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