atomic operations

atomic operations

[Shraddha Kamat]

what is the relation between atomic operations and memory alignment ?

I read from UTLK that "an unaligned memory access is not atomic" 

please explain me , I am not able to get the relationship between 
memory alignment and atomicity of the operation.

atomic operations

[richard -rw- weinberger]

On Sun, Feb 24, 2013 at 10:42 AM, Shraddha Kamat <sh2008ka@gmail.com> wrote:
> what is the relation between atomic operations and memory alignment ?
>
> I read from UTLK that "an unaligned memory access is not atomic"
>
> please explain me , I am not able to get the relationship between
> memory alignment and atomicity of the operation.

Not all CPUs support unaligned memory access, such an access may cause a fault
which needs to be fixed by the kernel...

-- 
Thanks,
//richard

atomic operations

[Peter Teoh]

in simple terms, any operation, in terms assembly instructions, which can
be executed in ONE instruction, is "atomic", because, just like an atom, it
cannot be broken up into parts.   any instructions that is longer than one,
for eg, TWO instruction, is NOT atomic, because in BETWEEN the first and
2nd instruction, something like an interrupt can come in, and affect the
values of the operand when it is passed from instruction one to second
instruction.  To save me from reiteration:

http://www.ibm.com/developerworks/library/pa-dalign/ (search for
"atomicity").

http://stackoverflow.com/questions/381244/purpose-of-memory-alignment

http://lwn.net/Articles/260832/

http://www.songho.ca/misc/alignment/dataalign.html

http://www.cis.upenn.edu/~palsetia/cit595s08/Lectures08/alignmentOrdering.pdf

Essentially, atomicity and non-alignment become problematic when u tried to
to read using non-byte addressing mode with non-aligned address.

On Sun, Feb 24, 2013 at 5:42 PM, Shraddha Kamat <sh2008ka@gmail.com> wrote:

> what is the relation between atomic operations and memory alignment ?
>
> I read from UTLK that "an unaligned memory access is not atomic"
>
> please explain me , I am not able to get the relationship between
> memory alignment and atomicity of the operation.
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
Regards,
Peter Teoh
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atomic operations

[Peter Teoh]

Another good article on atomicty and data sizes:

http://www.ibm.com/developerworks/library/pa-atom/

On Sun, Feb 24, 2013 at 8:50 PM, Peter Teoh <htmldeveloper@gmail.com> wrote:

> in simple terms, any operation, in terms assembly instructions, which can
> be executed in ONE instruction, is "atomic", because, just like an atom, it
> cannot be broken up into parts.   any instructions that is longer than one,
> for eg, TWO instruction, is NOT atomic, because in BETWEEN the first and
> 2nd instruction, something like an interrupt can come in, and affect the
> values of the operand when it is passed from instruction one to second
> instruction.  To save me from reiteration:
>
> http://www.ibm.com/developerworks/library/pa-dalign/ (search for
> "atomicity").
>
> http://stackoverflow.com/questions/381244/purpose-of-memory-alignment
>
> http://lwn.net/Articles/260832/
>
> http://www.songho.ca/misc/alignment/dataalign.html
>
>
> http://www.cis.upenn.edu/~palsetia/cit595s08/Lectures08/alignmentOrdering.pdf
>
> Essentially, atomicity and non-alignment become problematic when u tried
> to to read using non-byte addressing mode with non-aligned address.
>
> On Sun, Feb 24, 2013 at 5:42 PM, Shraddha Kamat <sh2008ka@gmail.com>wrote:
>
>> what is the relation between atomic operations and memory alignment ?
>>
>> I read from UTLK that "an unaligned memory access is not atomic"
>>
>> please explain me , I am not able to get the relationship between
>> memory alignment and atomicity of the operation.
>>
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>
>
>
> --
> Regards,
> Peter Teoh
>



-- 
Regards,
Peter Teoh
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atomic operations

[Valdis.Kletnieks at vt.edu]

On Sun, 24 Feb 2013 11:50:14 +0100, richard -rw- weinberger said:
> On Sun, Feb 24, 2013 at 10:42 AM, Shraddha Kamat <sh2008ka@gmail.com> wrote:
> > what is the relation between atomic operations and memory alignment ?
> >
> > I read from UTLK that "an unaligned memory access is not atomic"
> >
> > please explain me , I am not able to get the relationship between
> > memory alignment and atomicity of the operation.
>
> Not all CPUs support unaligned memory access, such an access may cause a fault
> which needs to be fixed by the kernel...

There's a more subtle issue - an unaligned access can be split across a cache
line boundary, requiring 2 separate memory accesses to do the read or write.
This can result in CPU A fetching the first half of the variable, CPU B
updating both halves, and then A fetching the second half of the now updated
variable.. This can bite you even on CPUs that support unaligned accesses.

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atomic operations

[Kumar amit mehta]

On Sun, Feb 24, 2013 at 08:53:20PM +0800, Peter Teoh wrote:
> Another good article on atomicty and data sizes:
> 
> http://www.ibm.com/developerworks/library/pa-atom/
> 
> On Sun, Feb 24, 2013 at 8:50 PM, Peter Teoh <htmldeveloper@gmail.com> wrote:
> 
> > in simple terms, any operation, in terms assembly instructions, which can
> > be executed in ONE instruction, is "atomic", because, just like an atom, it
> > cannot be broken up into parts.   any instructions that is longer than one,
> > for eg, TWO instruction, is NOT atomic, because in BETWEEN the first and
> > 2nd instruction, something like an interrupt can come in, and affect the
> > values of the operand when it is passed from instruction one to second
> > instruction.  To save me from reiteration:
> >
> > http://www.ibm.com/developerworks/library/pa-dalign/ (search for
> > "atomicity").
> >
> > http://stackoverflow.com/questions/381244/purpose-of-memory-alignment
> >
> > http://lwn.net/Articles/260832/
> >
> > http://www.songho.ca/misc/alignment/dataalign.html
> >
> >
> > http://www.cis.upenn.edu/~palsetia/cit595s08/Lectures08/alignmentOrdering.pdf
> >
> > Essentially, atomicity and non-alignment become problematic when u tried
> > to to read using non-byte addressing mode with non-aligned address.
> >
> > On Sun, Feb 24, 2013 at 5:42 PM, Shraddha Kamat <sh2008ka@gmail.com>wrote:
> >
> >> what is the relation between atomic operations and memory alignment ?
> >>
> >> I read from UTLK that "an unaligned memory access is not atomic"
> >>
> >> please explain me , I am not able to get the relationship between
> >> memory alignment and atomicity of the operation.
It seems for this same reason, every objects of a struct page is organized as 
double word.
<snip from linux/mm_types.h>
 * The objects in struct page are organized in double word blocks in
 * order to allows us to use atomic double word operations on portions
 * of struct page. That is currently only used by slub but the arrangement
 * allows the use of atomic double word operations on the flags/mapping
 * and lru list pointers also.
<snip from linux/mm_types.h>

-Amit

atomic operations

[Arun KS]

Hi Peter,

On Sun, Feb 24, 2013 at 6:20 PM, Peter Teoh <htmldeveloper@gmail.com> wrote:
> in simple terms, any operation, in terms assembly instructions, which can be
> executed in ONE instruction, is "atomic", because, just like an atom, it
> cannot be broken up into parts.   any instructions that is longer than one,
> for eg, TWO instruction, is NOT atomic, because in BETWEEN the first and 2nd
> instruction, something like an interrupt can come in, and affect the values
> of the operand when it is passed from instruction one to second instruction.

Nice explanation.

Thanks,
Arun
> To save me from reiteration:
>
> http://www.ibm.com/developerworks/library/pa-dalign/ (search for
> "atomicity").
>
> http://stackoverflow.com/questions/381244/purpose-of-memory-alignment
>
> http://lwn.net/Articles/260832/
>
> http://www.songho.ca/misc/alignment/dataalign.html
>
> http://www.cis.upenn.edu/~palsetia/cit595s08/Lectures08/alignmentOrdering.pdf
>
> Essentially, atomicity and non-alignment become problematic when u tried to
> to read using non-byte addressing mode with non-aligned address.
>
> On Sun, Feb 24, 2013 at 5:42 PM, Shraddha Kamat <sh2008ka@gmail.com> wrote:
>>
>> what is the relation between atomic operations and memory alignment ?
>>
>> I read from UTLK that "an unaligned memory access is not atomic"
>>
>> please explain me , I am not able to get the relationship between
>> memory alignment and atomicity of the operation.
>>
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>
>
>
> --
> Regards,
> Peter Teoh
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>