Atomic Variables
With the advent of multiprocessor systems, the requirement of the programs to maintain the synchronization has become mandatory.
In a multiprocessor systems if a program is being executed on multiple processors with data being shared between the processors then the update of data by one one processor might be affected by an update by another processor.
For eg consider the instruction val = val + 1; This is a good example of a read modify write instruction where the processor read the value of "val" modifies it and write it back. In a uniproessor system this poses no problem because the processor will not let go of the memory bus until the instruction is not completely executed.
But in a multiprocessor system, the memory bus might be released after the read and while one processor updates the value another processor can make use of the bus.
If two processors, processor A and processor B reach this instruction at the same time. Assume the value of val is 3 initially.
Processor A reads the value 3 and lets go of the bus
While processor A updates the val processor B also reads val as 3 and lets the go of the bus
Now processor A updates the new value 4 in the val
Then processor B also updates the value of val as 4.
With 2 increments the value of val should have become 5, but it remains at 4 because both the processors were allowed to read the variable before one of them finished the operation on it.
This simultaneous access can be prevented using semaphores and spinlocks but implementing a semaphore or spinlock for one instruction might be an overkill.
There is another solution in linux kernel called atomic variables.
Atomic variables are the ones on whom the read modify write operation is done as one instruction with out any interruption .
In the above example if val was an atomic variable then processor B would have been allowed access to the variable only after processor A has finished the updating the visible, until then the processor A would have held on to the memory bus.
To make use of the atomic variables the variable needs to be declared as of type atomic_t.
The access to the atomic variables is not through the standard instructions but using special functions listed below.
atomic_t *val Declaration
atomic_read(val): Returns the value of *val
atomic_set(val,i) : Sets *val to i
atomic_add(i,val): adds i to *val
atomic_sub(i,val): Subtracts i from *val
atomic_sub_and_test(i, val) : Subtracts i from *val and returns 1 if the result is zero,
atomic_inc(val) : Adds 1 to *val
atomic_dec(val) : Subtracts 1 from *val
atomic_dec_and_test(val): Subtracts 1 from *val and returns 1 if the result is zero, otherwise it returns 0
atomic_inc_and_test(val): Adds 1 to *v and returns 1 if the result is zero; 0 otherwise
atomic_add_negative(i, val): Adds i to *val and returns 1 if the result is negative, otherwise it returns 0
atomic_inc_return(val): Adds 1 to *val and returns the new value of *val
atomic_dec_return(val): Subtracts 1 from *val and return the new value of *val
atomic_add_return(i, val) : Adds i to *val and returns the new value of *val
atomic_sub_return(i, val): Subtracts i from *val and return the new value of *val
Let us write a small module using one of the functions and see how it gets implemented at the assembly level.
***********************test_atomic.c **********************
#include<linux/kernel.h>
#include<linux/module.h>
atomic_t *test;
int test_init(void)
{
atomic_set(test,2);
atomic_add(2,test);
}
void test_exit(void)
{
}
module_init(test_init);
module_exit(test_exit);
**************************************************************
Save the above code as test_atomic.c, the makefile required to compile the code will be
*****************Makefile*******************************
ifneq ($(KERNELRELEASE),)
obj-m := test_atomic.o
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules
clean:
$(MAKE) -C $(KERNELDIR) M=$(PWD) clean
endif
********************************************************
Now compile the code by running the command make
$ make
This should generate the file test_atomic.ko.
To look at the assembly instructions for this code we can use "objdump" which is a command to read information from object files.
Run the command as follows
$ objdump -d -S test_atomic.ko
test_atomic.o: file format elf32-i386
Disassembly of section .text:
00000000 <init_module>:
0: a1 00 00 00 00 mov 0x0,%eax
5: c7 00 02 00 00 00 movl $0x2,(%eax)
b: a1 00 00 00 00 mov 0x0,%eax
10: f0 83 00 02 lock addl $0x2,(%eax)
14: c3 ret
00000015 <cleanup_module>:
15: c3 ret
The lines after <init_module> in the output of the command is the assembly code for the init function we have written.
If we look at the 4th line of the assebly instructions we notice an instruction that starts with the keyword "lock" and then followed by "addl".
This instruction corresponds to the function "atomic_add". The lock keyword makes sure that the memory bus remains locked as long as the variable is not read modified and written back.
Similarly any of the atomic instructions are executed with the lock keyword making sure that the read modify write happens with out any interruptions.
VI editor tips for programmers - 3
In this post we will look at a few settings that are useful while programming.
Syntax coloring
While typing a program, it is always helpful if various colors are used to higlight different keywords and variables of a program. This
can be acheived by running the command
:syntax on
While in the command mode in the vi editor.
The screenshots below show the difference.
With out syntax highlighting
With syntax highlighting
Without numbering
With Numbering and syntax enabled.
Line numbers
A line number can be added to every line in the file by running the command
:set nu
or
:set number
The screenshots below depict the working of this command
VI editor tips for programmers -2
Playing around with line numbers
In the last post, VI editor tips for programmers -1 , we saw how VI editor helps in keeping track of the braces in a program.
In this post let us look at a few tips that relate to line numbers in a program.
Very often a compilation error points out to a line where some thing is wrong for eg :
*****************hello.c****************
#include<stdio.h>
main ()
{
printf("Hello world");
if(1) {
printf("Hello again"));
}
else {
printf("Hello once more");
}
}
****************************************
If we compile the above program using gcc compiler as follows
$ cc hello.c
We get the error
test.c: In function ‘main’:
test.c:6: error: expected ‘;’ before ‘)’ token
test.c:6: error: expected statement before ‘)’ token
The error thrown also speciifes the line number in which there is possible error. From the above error we know there might be an error in the line no 6.
Now open the file using vi
$ vi test.c
To go to the sixth line we could either move with the arrow keys looking at the bottom right of the editor for the line numbers being displayed.
But if the code is too big and error is 100th or the 1000th line then just using arrow keys of pgup, pgdwn keys are not convienient.
To move to any line number in the file directly , go to command mode (by pressing esc) and type
:"number"
That is in our case, to go to the 6th line we will have to type
:6
and press enter. The cursor should automaticlly move to the sixth line.
Another way of doing this is to pass the line number while opening the file itself .
$ vi test.c +6
The file opens with the cursor directly placed at the sixth line.
Moving with in the file by specific number of lines can also be done with out using the arrow keys.
To move two lines above/before the current line just do the following in the command mode
:-2
To move by two lines below/after the current line
:+2
Hope the tips makes programming and debugging much easier.
In the last post, VI editor tips for programmers -1 , we saw how VI editor helps in keeping track of the braces in a program.
In this post let us look at a few tips that relate to line numbers in a program.
Very often a compilation error points out to a line where some thing is wrong for eg :
*****************hello.c****************
#include<stdio.h>
main ()
{
printf("Hello world");
if(1) {
printf("Hello again"));
}
else {
printf("Hello once more");
}
}
****************************************
If we compile the above program using gcc compiler as follows
$ cc hello.c
We get the error
test.c: In function ‘main’:
test.c:6: error: expected ‘;’ before ‘)’ token
test.c:6: error: expected statement before ‘)’ token
The error thrown also speciifes the line number in which there is possible error. From the above error we know there might be an error in the line no 6.
Now open the file using vi
$ vi test.c
To go to the sixth line we could either move with the arrow keys looking at the bottom right of the editor for the line numbers being displayed.
But if the code is too big and error is 100th or the 1000th line then just using arrow keys of pgup, pgdwn keys are not convienient.
To move to any line number in the file directly , go to command mode (by pressing esc) and type
:"number"
That is in our case, to go to the 6th line we will have to type
:6
and press enter. The cursor should automaticlly move to the sixth line.
Another way of doing this is to pass the line number while opening the file itself .
$ vi test.c +6
The file opens with the cursor directly placed at the sixth line.
Moving with in the file by specific number of lines can also be done with out using the arrow keys.
To move two lines above/before the current line just do the following in the command mode
:-2
To move by two lines below/after the current line
:+2
Hope the tips makes programming and debugging much easier.
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