Linux select 机制深入分析
/* According to POSIX.1-2001 */ #include <sys/select.h> /* According to earlier standards */ #include <sys/time.h> #include <sys/types.h> #include <unistd.h> int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); void FD_CLR(int fd, fd_set *set); int FD_ISSET(int fd, fd_set *set); void FD_SET(int fd, fd_set *set); void FD_ZERO(fd_set *set); #include <sys/select.h> int pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, const struct timespec *timeout, const sigset_t *sigmask);
/* * SYSCALL_DEFINE5宏的作用就是将其转成系统调用的常见形式, * asmlinkage long sys_select(int n, fd_set __user *inp, fd_set __user *outp,fd_set __user *exp, struct timeval __user *tvp); */ SYSCALL_DEFINE5(select, int, n, fd_set __user *, inp, fd_set __user *, outp, fd_set __user *, exp, struct timeval __user *, tvp) { struct timespec end_time, *to = NULL; struct timeval tv; int ret; if (tvp) {//如果设置了超时阈值 if (copy_from_user(&tv, tvp, sizeof(tv))) return -EFAULT; to = &end_time; // 从timeval(秒 微秒)转换为(秒 纳秒) 继而建立超时 if (poll_select_set_timeout(to, tv.tv_sec + (tv.tv_usec / USEC_PER_SEC), (tv.tv_usec % USEC_PER_SEC) * NSEC_PER_USEC)) return -EINVAL; } // 核心工作 ret = core_sys_select(n, inp, outp, exp, to); //core_sys_select处理的fd_set 接下来更新timeout的值 ret = poll_select_copy_remaining(&end_time, tvp, 1, ret); return ret; } /* * We can actually return ERESTARTSYS instead of EINTR, but I'd * like to be certain this leads to no problems. So I return * EINTR just for safety. * * Update: ERESTARTSYS breaks at least the xview clock binary, so * I'm trying ERESTARTNOHAND which restart only when you want to. */ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp, fd_set __user *exp, struct timespec *end_time) { // poll.h :fd_set_bits包装了6个long *,代表三个描述表集的值-结果 fd_set_bits fds; void *bits; int ret, max_fds; unsigned int size; struct fdtable *fdt; /* Allocate small arguments on the stack to save memory and be faster * 先是预分配256B的空间 大多数情况下能够满足需要 特殊情况在下面会分配空间 */ long stack_fds[SELECT_STACK_ALLOC/sizeof(long)]; ret = -EINVAL; if (n < 0) goto out_nofds; /* max_fds can increase, so grab it once to avoid race */ rcu_read_lock(); // 获得打开文件描述符表(指针析取) fdt = files_fdtable(current->files); max_fds = fdt->max_fds; rcu_read_unlock(); if (n > max_fds) n = max_fds;//参数修正 /* * 现在要监视的描述符个数个size*8个对于每一个都需要6个位来标示 * 它是否可以读写异常并且把结果写在res_in res_out res_exp中 * 所以构成了下面的内存布局(见图1) */ size = FDS_BYTES(n); bits = stack_fds; if (size > sizeof(stack_fds) / 6) { /* Not enough space in on-stack array; must use kmalloc */ ret = -ENOMEM; bits = kmalloc(6 * size, GFP_KERNEL); if (!bits) goto out_nofds; } fds.in = bits; fds.out = bits + size; fds.ex = bits + 2*size; fds.res_in = bits + 3*size; fds.res_out = bits + 4*size; fds.res_ex = bits + 5*size; // 从用户空间得到这些fd sets if ((ret = get_fd_set(n, inp, fds.in)) || (ret = get_fd_set(n, outp, fds.out)) || (ret = get_fd_set(n, exp, fds.ex))) goto out; // 初始化这些结果参数为0 zero_fd_set(n, fds.res_in); zero_fd_set(n, fds.res_out); zero_fd_set(n, fds.res_ex); // 到这里 一切准备工作都就绪了..... ret = do_select(n, &fds, end_time); if (ret < 0) goto out; if (!ret) { ret = -ERESTARTNOHAND; if (signal_pending(current)) goto out; ret = 0; } // do_select正确返回后 通过copy_to_user将fds中的描述符就绪结果参数 // 反馈到用户空间 if (set_fd_set(n, inp, fds.res_in) || set_fd_set(n, outp, fds.res_out) || set_fd_set(n, exp, fds.res_ex)) ret = -EFAULT; out: if (bits != stack_fds) kfree(bits); out_nofds: return ret; } // select 的核心工作 int do_select(int n, fd_set_bits *fds, struct timespec *end_time) { ktime_t expire, *to = NULL; struct poll_wqueues table; poll_table *wait; int retval, i, timed_out = 0; unsigned long slack = 0; unsigned int busy_flag = net_busy_loop_on() ? POLL_BUSY_LOOP : 0; unsigned long busy_end = 0; // 得到Select要监测的最大的描述符值 rcu_read_lock(); retval = max_select_fd(n, fds); rcu_read_unlock(); if (retval < 0) return retval; n = retval; poll_initwait(&table); wait = &table.pt; // 定时器值(秒 纳秒)为0的话标示不等待 if (end_time && !end_time->tv_sec && !end_time->tv_nsec) { wait->_qproc = NULL; timed_out = 1; } if (end_time && !timed_out) slack = select_estimate_accuracy(end_time); // 下面会用到这个变量统计就绪的描述符个数 所以先清0 retval = 0; for (;;) { unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp; bool can_busy_loop = false; inp = fds->in; outp = fds->out; exp = fds->ex; rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex; for (i = 0; i < n; ++rinp, ++routp, ++rexp) { unsigned long in, out, ex, all_bits, bit = 1, mask, j; unsigned long res_in = 0, res_out = 0, res_ex = 0; in = *inp++; out = *outp++; ex = *exp++; all_bits = in | out | ex; // 要一次轮询这些这些位图 定位到某个有我们关心的fd的区间 // 否则以32bits步长前进 if (all_bits == 0) { i += BITS_PER_LONG; continue; } // 当前这个区间有我们关心的fd 所以深入细节追踪(图2) for (j = 0; j < BITS_PER_LONG; ++j, ++i, bit <<= 1) { struct fd f; if (i >= n) break; if (!(bit & all_bits)) continue; // 如果发现了当前区间的某一个bit为1 则说明对应的fd需要我们处理 // 此时此刻的i正是文件描述符值 f = fdget(i); if (f.file) { const struct file_operations *f_op; f_op = f.file->f_op; mask = DEFAULT_POLLMASK; //具体到文件操作结果中的poll函数指针 对于 if (f_op->poll) { wait_key_set(wait, in, out, bit, busy_flag); mask = (*f_op->poll)(f.file, wait);// TODO } // 上面的fdget增加了file引用计数 所以这里恢复 fdput(f); /* 判断关注的描述符是否就绪 就绪的话就更新到结果参数中 * 并且增加就绪个数 */ if ((mask & POLLIN_SET) && (in & bit)) { res_in |= bit; retval++; wait->_qproc = NULL; } if ((mask & POLLOUT_SET) && (out & bit)) { res_out |= bit; retval++; wait->_qproc = NULL; } if ((mask & POLLEX_SET) && (ex & bit)) { res_ex |= bit; retval++; wait->_qproc = NULL; } /* got something, stop busy polling * 停止忙循环 */ if (retval) { can_busy_loop = false; busy_flag = 0; /* * only remember a returned * POLL_BUSY_LOOP if we asked for it */ } else if (busy_flag & mask) can_busy_loop = true; } } // 这一轮的区间遍历完之后 更新结果参数 if (res_in) *rinp = res_in; if (res_out) *routp = res_out; if (res_ex) *rexp = res_ex; /* 进行一次调度 允许其他进程运行 * 后面有等待队列唤醒 */ cond_resched(); } // 一轮轮询之后 wait->_qproc = NULL; // 如果有描述符就绪 或者设置了超时 或者有待处理信号 则退出这个死循环 if (retval || timed_out || signal_pending(current)) break; if (table.error) { retval = table.error; break; } /* only if found POLL_BUSY_LOOP sockets && not out of time */ if (can_busy_loop && !need_resched()) { if (!busy_end) { busy_end = busy_loop_end_time(); continue; } if (!busy_loop_timeout(busy_end)) continue; } busy_flag = 0; /* 如果设置超时 并且这是首次循环(to==NULL) */ if (end_time && !to) { // 从timespec转化为ktime类型(64位的有符号值) expire = timespec_to_ktime(*end_time); to = &expire; } /*设置该进程状态TASK_INTERRUPTIBLE 睡眠直到超时 * 返回到这里后进程 TASK_RUNNING */ if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE, to, slack)) timed_out = 1; } // 释放该poll wait queue poll_freewait(&table); return retval; }
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