Re: atomic operations
Hi Peter, On Sun, Feb 24, 2013 at 6:20 PM, Peter Teoh 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 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@kernelnewbies.org >> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > > > > -- > Regards, > Peter Teoh > > ___ > Kernelnewbies mailing list > Kernelnewbies@kernelnewbies.org > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: atomic operations
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 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 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. * 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. -Amit ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: atomic operations
On Sun, 24 Feb 2013 11:50:14 +0100, richard -rw- weinberger said: > On Sun, Feb 24, 2013 at 10:42 AM, Shraddha Kamat 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. pgpaOFflPKynw.pgp Description: PGP signature ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: atomic operations
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 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@kernelnewbies.org > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > -- Regards, Peter Teoh ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: atomic operations
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 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 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@kernelnewbies.org >> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies >> > > > > -- > Regards, > Peter Teoh > -- Regards, Peter Teoh ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: atomic operations
On Sun, Feb 24, 2013 at 10:42 AM, Shraddha Kamat 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 ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
atomic operations
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@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
