linux用户态ipc实现机制

今天来聊聊用户态System V ipc，也就是常说的信号量、消息队列以及共享内存，以信号量为例

内核当中信号量的数据结构，跟命名空间是相关的，给定的进程属于task_struct->nsproxy->ipc_ns指向的命名空间，ipc_namespace结构如下：

struct ipc_namespace {
    refcount_t    count;
    struct ipc_ids    ids[3];
...
};

其中，数组ids当中的每个ipc_ids分别代表当前ipc命名空间下的三种ipc机制0-信号量，1-消息队列，2-共享内存

struct ipc_ids {
    int in_use;
    unsigned short seq;
    struct rw_semaphore rwsem;
    struct idr ipcs_idr;
    int max_idx;
    int last_idx;    /* For wrap around detection */
#ifdef CONFIG_CHECKPOINT_RESTORE
    int next_id;
#endif
    struct rhashtable key_ht;
};

其中，ipcs_idr是将当前类型的ipc的id映射到对应的内核数据结构kern_ipc_perm的基数树，

struct kern_ipc_perm {
    spinlock_t    lock;
    bool        deleted;
    int        id;
    key_t        key;
    kuid_t        uid;
    kgid_t        gid;
    kuid_t        cuid;
    kgid_t        cgid;
    umode_t        mode;
    unsigned long    seq;
    void        *security;

    struct rhash_head khtnode;

    struct rcu_head rcu;
    refcount_t refcount;
};

三种ipc在内核当中的表示的第一个元素，都是kern_ipc_perm，当通过基数树找到对应的kern_ipc_perm的指针时候，直接强转就ok了。比如信号量：

struct sem_array {
    struct kern_ipc_perm    sem_perm;    /* permissions .. see ipc.h */
    time64_t        sem_ctime;    /* create/last semctl() time */
    struct list_head    pending_alter;    /* pending operations */
                        /* that alter the array */
    struct list_head    pending_const;    /* pending complex operations */
                        /* that do not alter semvals */
    struct list_head    list_id;    /* undo requests on this array */
    int            sem_nsems;    /* no. of semaphores in array */
    int            complex_count;    /* pending complex operations */
    unsigned int        use_global_lock;/* >0: global lock required */

    struct sem        sems[];
};

sem_nsems表示当前信号量集合包含信号量的数目，以及在当前信号量上挂着的队列

struct sem_queue {
    struct list_head    list;     /* queue of pending operations */
    struct task_struct    *sleeper; /* this process */
    struct sem_undo        *undo;     /* undo structure */
    struct pid        *pid;     /* process id of requesting process */
    int            status;     /* completion status of operation */
    struct sembuf        *sops;     /* array of pending operations */
    struct sembuf        *blocking; /* the operation that blocked */
    int            nsops;     /* number of operations */
    bool            alter;     /* does *sops alter the array? */
    bool                    dupsop;     /* sops on more than one sem_num */
};

如果内核配置了CONFIG_DETECT_HUNG_TASK，task_struct里面才会有ipc相关成员

struct task_struct{
...
#ifdef CONFIG_SYSVIPC
    struct sysv_sem            sysvsem;
    struct sysv_shm            sysvshm;
#endif
...
};

其中，这俩都是用于当前程序崩溃时，释放相关资源的链表

struct sysv_sem {
    struct sem_undo_list *undo_list;
};