Linux Process Descriptor

Process and task are used interchangeably.  The process descriptor is implemented in struct task_struct.  The task list is a double linklist of task_struct.

Prior to 2.6 kernel, task_stuct is allocated at the end (max top) of kernel stack.  This allow a quick location of the task_stuct using the stack register.  Since 2.6, the task_struct is now allocated by the slab allocator dynamically in cache.  A new struct thread_info sits at the top end of stack.  The first element in thread_info points to task_struct.  A register-impaired architecture is the only reason to create thread_info.

To look up the task_struct of the current task, kernel uses current macro.  The implementation of current macro is hardware dependent.  In x86, current is calculated by masking the lowest 13-bit of the esp stack pointer.  In PowerPC, the current pointer is stored in a register.