Create ch09-03-datarace.md

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+# 数据竞争
+
+如果我们通过多个image同时对某一个image进行修改，此时会发生数据竞争。例如其他的image同时对image1进行累加操作，当同时有多个image想要执行该操作时，就会出现错误。
+
+``` fortran
+program main
+   implicit none
+   integer::id,np,i
+   integer::a[*]
+   id = this_image()
+   np = num_images()
+   a=0
+   do i = 1, np
+      a[i]=a[i]+1
+   end do
+   sync all 
+   write(*,*)"this image=",id,"a",a
+end program main
+```
+多次运行就会发现结果并不正确，也不相同
+``` sh
+$ fpm run --compiler="caf" --runner="cafrun -n 4"
+ this image=           2 a           4
+ this image=           3 a           4
+ this image=           4 a           3
+ this image=           1 a           2
+$ fpm run --compiler="caf" --runner="cafrun -n 4"
+ this image=           4 a           3
+ this image=           1 a           2
+ this image=           2 a           4
+ this image=           3 a           3
+```
+所以此处我们需要的`a[i]=a[i]+1`这条语句同时只能被一个image执行。
+
+## `critical`
+通过`critical`块语句，可以直接保证在该区域只有一个image执行
+
+``` fortran
+program main
+   implicit none
+   integer::id,np,i
+   integer::a[*]
+   id = this_image()
+   np = num_images()
+   a=0
+   do i = 1, np
+      critical
+         a[i]=a[i]+1
+      end critical
+   end do
+   sync all 
+   write(*,*)"this image=",id,"a",a
+end program main
+```
+``` sh
+$ fpm run --compiler="caf" --runner="cafrun -n 4"
+ this image=           3 a           4
+ this image=           4 a           4
+ this image=           1 a           4
+ this image=           2 a           4
+```
+## 原子操作
+Fortran内置函数中还提供了原子操作的函数，通过这些函数，可以实现上述功能
+``` fortran
+program main
+   implicit none
+   integer::id,np,i
+   integer::a[*]
+   id = this_image()
+   np = num_images()
+   a=0
+   do i = 1, np
+      call atomic_add(a[i],1)
+   end do
+   sync all 
+   write(*,*)"this image=",id,"a",a
+end program main
+```
+## 锁
+使用`lock`语句也可以实现该功能
+``` fortran
+program main
+   use iso_fortran_env
+   implicit none
+   integer::id,np,i
+   integer::a[*]
+   type(lock_type)::lock_a[*]
+   id = this_image()
+   np = num_images()
+   a=0
+   do i = 1, np
+      lock(lock_a[i])
+      a[i]=a[i]+1
+      unlock(lock_a[i])
+   end do
+   sync all 
+   write(*,*)"this image=",id,"a",a
+end program main
+```
+- 使用`lock`的时候要注意`unlock`
+
+
+