linux-kernel - Re: [PATCH 01/15] kmemleak: Add the base support

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Message-ID: <84144f020811302315p2ba9fb55g4319ad12a7fa4347@mail.gmail.com>
Date:	Mon, 1 Dec 2008 09:15:33 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Catalin Marinas" <catalin.marinas@....com>
Cc:	linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...e.hu>
Subject: Re: [PATCH 01/15] kmemleak: Add the base support

Hi Catalin,

On Sat, Nov 29, 2008 at 12:43 PM, Catalin Marinas
<catalin.marinas@....com> wrote:
> +/*
> + * Insert a pointer into the pointer hash table.
> + */
> +static inline void create_object(unsigned long ptr, size_t size, int ref_count)
> +{

[...]

> +       if (ptr < min_addr)
> +               min_addr = ptr;
> +       if (ptr + size > max_addr)
> +               max_addr = ptr + size;
> +       /*
> +        * Update the boundaries before inserting the object in the
> +        * prio search tree.
> +        */
> +       smp_mb();

I'm not sure I understand the purpose of this memory barrier. As soon
as some other CPU acquires object_tree_lock, updates to the boundaries
will be visible due to the implicit memory barriers in locking
functions (see Documentation/memory-barrier.txt for details).

However, I'm wondering why this isn't a smp_wmb() and..

> +/*
> + * Scan a block of memory (exclusive range) for pointers and move
> + * those found to the gray list.
> + */
> +static void scan_block(void *_start, void *_end, struct memleak_object *scanned)
> +{
> +       unsigned long *ptr;
> +       unsigned long *start = PTR_ALIGN(_start, BYTES_PER_WORD);
> +       unsigned long *end = _end - (BYTES_PER_WORD - 1);
> +
> +       for (ptr = start; ptr < end; ptr++) {

...why don't we have the pairing smp_rmb() here before we read
min_addr and max_addr?

> +
> +               /*
> +                * The boundaries check doesn't need to be precise
> +                * (hence no locking) since orphan objects need to
> +                * pass a scanning threshold before being reported.
> +                */
> +               if (pointer < min_addr || pointer >= max_addr)
> +                       continue;

                Pekka
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