CS 378 (Spring 2003)

Transcription

1 Department of Computer Sciences THE UNIVERSITY OF TEXAS AT AUSTIN CS 378 (Spring 2003) Linux Kernel Programming Yongguang Zhang Copyright 2003, Yongguang Zhang

2 This Lecture Interprocess Communications Signal Pipe Questions? Spring Yongguang Zhang 2

3 Signal in Linux Signal: sent to a process to notify an event Long standing Unix abstraction A well-known number of signals (SIGXXX) Usually, a process will call a user-space function (signal-handler) in response to a signal Process can change how it handle certain signal Use a different signal-handler function (user-provided) Use kernel default (set signal handler to 0x0) Ignore the signal (set signal handler to 0x1) Block the signal (will be pending until unblocked later) Spring Yongguang Zhang 3

4 Signal Handling Each task maintains data structure: An array of signal handler (one per signal type) A list of pending signals A mask of blocked signals Pending signals Signals are pending if they are posted to a task but not yet processed When is signal processed? When return from interrupt or exception (including system call) Only one signal per type in pending list Spring Yongguang Zhang 4

5 Signal Information in Task Struct Fields in task_struct structure int sigpending: whether there are pending signals struct signal_struct *sig: vector of signal handlers sigset_t blocked: a bitmask of blocked signals struct sigpending pending: a list of pending signals Major fields in struct signal_struct struct k_sigaction action[_nsig]; Signal handler from task struct task->sig->action[x].sa.sa_handler Spring Yongguang Zhang 5

6 Signal Handler Data Structure Major field of struct sigpending struct sigqueue *head: first element in the list sigset_t signal: bitmask of all signals in the list Fields of struct sigqueue struct sigqueue *next: next element in the list siginfo_t info: information about this pending signal Major fields of struct siginfo (siginfo_t) si_signo: signal number si_code: who raised the signal (user? Kernel? timer?) Information specific to each type of signals (see include/asm-i386/siginfo.h) Spring Yongguang Zhang 6

7 Signal Generation From kernel Function send_sig_info(), send_sig(),... From another user process System call kill() sys_kill() eventually calls send_sig_info() Function send_sig_info(sig,info,t) Source code: kernel/signal.c Check STOP/CONT signal Check if the signal action is ignore Deliver signal to task: call deliver_signal(sig,info,t) Spring Yongguang Zhang 7

8 Signal Delivery Function deliver_signal(sig,info,t) Add signal to taks t's pending list send_signal(sig,info,&t->pending) If the signal is not blocked, call signal_wake_up(t) Function send_signal(sig,info,signals) Allocate memory for new list element (from slab cache) Fill in the signal info and add to the list (signals) Update the pending signal bitmask (signals->signal) Function signal_wake_up(t) t->sigpending = 1; if (t->state & TASK_INTERRUPTIBLE) wake_up_process(t); Spring Yongguang Zhang 8

9 Signal Action When process returns from interrupts/exceptions Before resuming excution in user-space Check pending signals (task->sigpending), if set call do_signal() Function do_signal() Dequeue each signal in the pending list (except those blocked) Find the corresponding handler Process ignored or default handler Context switch to user-space (for handler) and back Spring Yongguang Zhang 9

10 Analogy Signal to process is like interrupts to kernel Real-time Signal? Linux does not support true real-time signal Real-time signal in linux is just to allow more than one signal per type in the pending signal list Spring Yongguang Zhang 10

11 Pipe Communication between two processes Pipe: a pair of file descriptors, one for read, one for write Parent create a pipe, fork, then communicate with child Named pipe Similar to pipe, but persistent in normal file system Also called fifo in Linux kernel Linux support for pipe: pipefs A special file system type for pipe (and named pipe) Kernel mounted, but no mount point Spring Yongguang Zhang 11

12 Pipe File System See fs/pipe.c Dentry operation: varible pipefs_dentry_operations File operations: variable read_pipe_fops, write_pipe_fops, read_fifo_fops, write_fifo_fops,... Register the pipefs file system DECLARE_FSTYPE(pipe_fs_type, pipefs, pipefs_read_super, FS_NOMOUNT); init_pipe_fs() err = register_filesystem(&pipe_fs_type); pipe_mnt = kern_mount(&pipe_fs_type); Spring Yongguang Zhang 12

13 Creating a Pipe System call pipe() sys_pipe() calls do_pipe() Function do_pipe() (in fs/pipe.c) f1 = get_empty_filp(); f2 = get_empty_filp(); inode = get_pipe_inode(); dentry = d_alloc(pipe_mnt->mnt_sb->s_root,...); dentry->d_op = &pipefs_dentry_operations; d_add(dentry, inode); f1->f_dentry = f2->f_dentry = dget(dentry); f1->f_flags = O_RDONLY; 1->f_op = &read_pipe_fops; f2->f_flags = O_WRONLY; f2->f_op = &write_pipe_fops; Spring Yongguang Zhang 13

14 Pipe Read/Write Inode has a semaphore and (limited) buffer pipe_write() down_interruptible(pipe_sem(*inode)) If no more space, add to wait queue PIPE_WAIT(*inode) Fill in the buffer: pipebuf = PIPE_BASE(*inode)+PIPE_END(*inode); copy_from_user(pipebuf, buf, chars); wake_up_interruptible(pipe_wait(*indoe)) up(pipe_sem(*inode) pipe_read(): Similar Spring Yongguang Zhang 14

15 Summary Poll: Which book is better (in term of helping linux kernel programming) Spring Yongguang Zhang 15