[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

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Stanislav Meduna <stano@meduna.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Bogus pseudo-randomness in  |Bogus pseudo-randomness in
                   |garbage collect             |jffs2 garbage collect

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[Bug 1001591] New: Bogus pseudo-randomness in garbage collect

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           Summary: Bogus pseudo-randomness in garbage collect
           Product: eCos
           Version: CVS
          Platform: All
        OS/Version: All
            Status: UNCONFIRMED
          Severity: major
          Priority: low
         Component: Filesystem support
        AssignedTo: unassigned@bugs.ecos.sourceware.org
        ReportedBy: stano@meduna.org
                CC: ecos-bugs@ecos.sourceware.org
             Class: Advice Request


The jffs2_find_gc_block function in fs/jffs2/current/src/gc.c and
jffs2_mark_node_obsolete in nodemgmt.c try to do pseudo-random stuff by
computing jiffies % 128 and making probability decisions on the result. First I
think that this kind of pseudo-randomness is not really correct for embedded
systems that often have very predictable execution times. 

But what is much worse, compat/linux/current/include/linux/kernel.h defines
jiffies as
  #define jiffies 100

I am no expert at jffs2 and cannot say whether this can lead to major problems
or not, so I'd like someone with better understanding of jffs2 to look at it.

Also check other usage of jiffies - I found one more in
io/usb/eth/slave/current/host/ecos_usbeth.c

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[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

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Jonathan Larmour <jifl@ecoscentric.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jifl@ecoscentric.com
         AssignedTo|unassigned@bugs.ecos.source |jifl@ecoscentric.com
                   |ware.org                    |

--- Comment #1 from Jonathan Larmour <jifl@ecoscentric.com> 2012-05-26 16:46:10 BST ---
(In reply to comment #0)
> The jffs2_find_gc_block function in fs/jffs2/current/src/gc.c and
> jffs2_mark_node_obsolete in nodemgmt.c try to do pseudo-random stuff by
> computing jiffies % 128 and making probability decisions on the result. First I
> think that this kind of pseudo-randomness is not really correct for embedded
> systems that often have very predictable execution times. 
> 
> But what is much worse, compat/linux/current/include/linux/kernel.h defines
> jiffies as
>   #define jiffies 100
> 
> I am no expert at jffs2 and cannot say whether this can lead to major problems
> or not, so I'd like someone with better understanding of jffs2 to look at it.

Yes. It's a problem but not a big one. It just means it loses an element of
pseudo-randomness which means that with certain repetitive usage patterns,
there may be uneven flash wear. I'll fix this to use cyg_current_time() which
is close enough to achieve the same job.

> Also check other usage of jiffies - I found one more in
> io/usb/eth/slave/current/host/ecos_usbeth.c

That file is genuinely for Linux so that's ok.

Jifl

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[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

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--- Comment #2 from Jonathan Larmour <jifl@ecoscentric.com> 2012-05-27 01:52:52 BST ---
Created an attachment (id=1768)
 --> (http://bugs.ecos.sourceware.org/attachment.cgi?id=1768)
Make jiffie use cyg_current_time

I've committed the attached patch to fix this issue.

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[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

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Jonathan Larmour <jifl@ecoscentric.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |NEXTRELEASE

--- Comment #3 from Jonathan Larmour <jifl@ecoscentric.com> 2012-05-27 01:53:31 BST ---
Fixed.

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[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

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Jonathan Larmour <jifl@ecoscentric.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jifl@ecoscentric.com
         AssignedTo|unassigned@bugs.ecos.source |jifl@ecoscentric.com
                   |ware.org                    |

--- Comment #1 from Jonathan Larmour <jifl@ecoscentric.com> 2012-05-26 16:46:10 BST ---
(In reply to comment #0)
> The jffs2_find_gc_block function in fs/jffs2/current/src/gc.c and
> jffs2_mark_node_obsolete in nodemgmt.c try to do pseudo-random stuff by
> computing jiffies % 128 and making probability decisions on the result. First I
> think that this kind of pseudo-randomness is not really correct for embedded
> systems that often have very predictable execution times. 
> 
> But what is much worse, compat/linux/current/include/linux/kernel.h defines
> jiffies as
>   #define jiffies 100
> 
> I am no expert at jffs2 and cannot say whether this can lead to major problems
> or not, so I'd like someone with better understanding of jffs2 to look at it.

Yes. It's a problem but not a big one. It just means it loses an element of
pseudo-randomness which means that with certain repetitive usage patterns,
there may be uneven flash wear. I'll fix this to use cyg_current_time() which
is close enough to achieve the same job.

> Also check other usage of jiffies - I found one more in
> io/usb/eth/slave/current/host/ecos_usbeth.c

That file is genuinely for Linux so that's ok.

Jifl

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[Bug 1001591] Bogus pseudo-randomness in jffs2 garbage collect

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http://bugs.ecos.sourceware.org/show_bug.cgi?id=1001591

Stanislav Meduna <stano@meduna.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Bogus pseudo-randomness in  |Bogus pseudo-randomness in
                   |garbage collect             |jffs2 garbage collect

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