A process is a program in execution. It consists of the executing program code, a set of resources such as open files, internal kernel data, an address space, one or more threads of execution and a data section containing global variables.

Process Descriptors
The kernel stores the processes in a circular doubly linked list called the task list. Each element in the task list is a process discriptor of the type struct task_struct:

struct task_struct {
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
struct thread_info *thread_info;
atomic_t usage;
unsigned long flags; /* per process flags, defined below */
unsigned long ptrace;
int lock_depth; /* BKL lock depth */
int load_weight; /* for niceness load balancing purposes */
int prio, static_prio, normal_prio;
struct list_head run_list;
struct prio_array *array;
unsigned short ioprio;
unsigned int btrace_seq;
unsigned long sleep_avg;
unsigned long long timestamp, last_ran;
unsigned long long sched_time; /* sched_clock time spent running */
enum sleep_type sleep_type;
unsigned long policy;
cpumask_t cpus_allowed;
unsigned int time_slice, first_time_slice;
struct list_head tasks;
/*
* ptrace_list/ptrace_children forms the list of my children
* that were stolen by a ptracer.
*/
struct list_head ptrace_children;
struct list_head ptrace_list;
struct mm_struct *mm, *active_mm;
/* task state */
struct linux_binfmt *binfmt;
long exit_state;
int exit_code, exit_signal;
int pdeath_signal; /* The signal sent when the parent dies */
/* ??? */
unsigned long personality;
unsigned did_exec:1;
pid_t pid;
pid_t tgid;
/*
* pointers to (original) parent process, youngest child, younger sibling,
* older sibling, respectively. (p->father can be replaced with
* p->parent->pid)
*/
struct task_struct *real_parent;
struct task_struct *parent; /* parent process */
/*
* children/sibling forms the list of my children plus the
* tasks I'm ptracing.
*/
struct list_head children; /* list of my children */
struct list_head sibling; /* linkage in my parent's children list */
struct task_struct *group_leader; /* threadgroup leader */
/* PID/PID hash table linkage. */
struct pid_link pids[PIDTYPE_MAX];
struct list_head thread_group;
struct completion *vfork_done; /* for vfork() */
int __user *set_child_tid; /* CLONE_CHILD_SETTID */
int __user *clear_child_tid; /* CLONE_CHILD_CLEARTID */
unsigned long rt_priority;
cputime_t utime, stime;
unsigned long nvcsw, nivcsw; /* context switch counts */
struct timespec start_time;
unsigned long min_flt, maj_flt;
cputime_t it_prof_expires, it_virt_expires;
unsigned long long it_sched_expires;
struct list_head cpu_timers[3];
/* process credentials */
uid_t uid,euid,suid,fsuid;
gid_t gid,egid,sgid,fsgid;
struct group_info *group_info;
kernel_cap_t cap_effective, cap_inheritable, cap_permitted;
unsigned keep_capabilities:1;
struct user_struct *user;
int oomkilladj; /* OOM kill score adjustment (bit shift). */
char comm[TASK_COMM_LEN];
/* file system info */
int link_count, total_link_count;
/* ipc stuff */
struct sysv_sem sysvsem;
/* CPU-specific state of this task */
struct thread_struct thread;
/* filesystem information */
struct fs_struct *fs;
/* open file information */
struct files_struct *files;
/* namespace */
struct namespace *namespace;
/* signal handlers */
struct signal_struct *signal;
struct sighand_struct *sighand;
sigset_t blocked, real_blocked;
sigset_t saved_sigmask;
struct sigpending pending;
unsigned long sas_ss_sp;
size_t sas_ss_size;
int (*notifier)(void *priv);
void *notifier_data;
sigset_t *notifier_mask;
void *security;
struct audit_context *audit_context;
seccomp_t seccomp;
/* Thread group tracking */
u32 parent_exec_id;
u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty, keyrings */
spinlock_t alloc_lock;
/* Protection of the PI data structures: */
spinlock_t pi_lock;
/* journalling filesystem info */
void *journal_info;
/* VM state */
struct reclaim_state *reclaim_state;
struct backing_dev_info *backing_dev_info;
struct io_context *io_context;
unsigned long ptrace_message;
siginfo_t *last_siginfo; /* For ptrace use. */
/*
* current io wait handle: wait queue entry to use for io waits
* If this thread is processing aio, this points at the waitqueue
* inside the currently handled kiocb. It may be NULL (i.e. default
* to a stack based synchronous wait) if its doing sync IO.
*/
wait_queue_t *io_wait;
/* i/o counters(bytes read/written, #syscalls */
u64 rchar, wchar, syscr, syscw;
struct robust_list_head __user *robust_list;
struct list_head pi_state_list;
struct futex_pi_state *pi_state_cache;
atomic_t fs_excl; /* holding fs exclusive resources */
struct rcu_head rcu;
/*
* cache last used pipe for splice
*/
struct pipe_inode_info *splice_pipe;
};


The Task List
The struct task_struct has this member:

struct list_head tasks;

This line shows us that the kernel is using a circular linked list to store the tasks. This means we can use the standard kernel linked list macros and functions to traverse through the complete task list.

Got the task_struct from GDB
the task_struct of current thread can be got from this way:

(gdb) p $esp
$1 = (void *) 0xbffda01c
(gdb) x 0xbffda01c & 0xffffe000