文件锁与NFS文件锁
2011-07-04 21:46 zhenjing 阅读(11876) 评论(2) 编辑 收藏 举报缘起
因项目需要,自行设计一套通用的文件读写锁,要求该机制能用于本地文件系统和NFS文件系统。
内核的文件数据结构
内核中有3个数据结构和文件直接相关,分别是:file descriptor table, file table and i-node table。其中file descriptor table是进程私有的;file table和inode table是独立于进程的。进程内部的文件描述符fd只在该进程有才有意义。File table可在进程内共享,也可在进程间共享。I-node table则在整个系统中共享。
文件描述符fd可由Open,dup和fork等操作创建;file table只能有open创建;i-node table在创建真实文件是创建。
摘自Apue-3.12
摘自Apue-14.3
摘自TLPI-5.4
对TLPI-5.4的图作如下说明:
1) A中的fd 1和20共享file table记录,可认为fd 20复制于fd 1(可也反过来)
2) A和B的fd 2共享file table记录,可认为B由A fork产生,B重定向fd 0和1。
3) File table中的0和86指向相同的文件,这意味着0和86由open相同的文件。
理解这个3个数据结构的关系大体可知道linux是如何管理文件。Stat和fcntl可获取或者修改相关的文件信息,结合这两个接口将有助于理解这3个文件结构。
文件记录锁fcntl
文件记录锁位于i-node table这个结构中,使用PID作为锁拥有者的标识。这使其拥有如下特点:
1) 记录锁采用(PID, start,end)三元组作为锁标识,一个文件可拥有多个记录锁,同一区域只允许有一个记录锁。
2) 当进程终止(正常/不正常),该进程拥有的所有记录锁都将释放。
3) 同一个进程中,指向同一文件(i-node)的fd都可以操作该文件上的记录锁:如释放、修改等。显式调用F_UNLCK和close(fd)都将释放锁,close将释放整个文件中该进程拥有的所有记录锁。
4) 记录锁不被fork的子进程继承(PID不同)。
5) 记录锁的类型转换、改变锁范围等操作均为原子的。
6) 未设置FD_CLOEXEC时,记录锁将被exec后的进程继承(PID相同)。
7) 记录锁对文件打开mode有要求:加读锁要求fd有读权限;加写锁要求fd有写权限。
fcntl的使用示例和测试代码:
#include <stdio.h> #include <string.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> int lock_reg(int, int, int, off_t, int, off_t); #define read_lock(fd, offset, whence, len) \ lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len)) #define readw_lock(fd, offset, whence, len) \ lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len)) #define write_lock(fd, offset, whence, len) \ lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len)) #define writew_lock(fd, offset, whence, len) \ lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len)) #define un_lock(fd, offset, whence, len) \ lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len)) pid_t lock_test(int, int, off_t, int, off_t); #define is_read_lockable(fd, offset, whence, len) \ (lock_test((fd), F_RDLCK, (offset), (whence), (len)) == 0) #define is_write_lockable(fd, offset, whence, len) \ (lock_test((fd), F_WRLCK, (offset), (whence), (len)) == 0) int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len) { struct flock lock; lock.l_type = type; /* F_RDLCK, F_WRLCK, F_UNLCK */ lock.l_start = offset; /* byte offset, relative to l_whence */ lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */ lock.l_len = len; /* #bytes (0 means to EOF) */ return(fcntl(fd, cmd, &lock)); } /* Note: lock_test always success with the obtained lock process */ pid_t lock_test(int fd, int type, off_t offset, int whence, off_t len) { struct flock lock; lock.l_type = type; /* F_RDLCK or F_WRLCK */ lock.l_start = offset; /* byte offset, relative to l_whence */ lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */ lock.l_len = len; /* #bytes (0 means to EOF) */ if (fcntl(fd, F_GETLK, &lock) < 0) perror("fcntl error"); /* printf("F_RDLCK=%d, F_WRLCK=%d, F_UNLCK=%d\n", F_RDLCK, F_WRLCK, F_UNLCK); */ printf(" l_type=%d, l_start=%lu, l_where=%d, l_len=%lu, l_pid=%d\n" , lock.l_type, lock.l_start, lock.l_whence, lock.l_len, lock.l_type); if (lock.l_type == F_UNLCK) return(0); /* false, region isn't locked by another proc */ return(lock.l_pid); /* true, return pid of lock owner */ } /* ======================================================== */ void Usage(const char* program) { fprintf(stderr, "Usage: %s read/read2/write/rd-wr/wr-rd/fork/dup file time\n", program); exit(-1); } int main(int argc, char *argv[]) { if(argc < 3){ Usage(argv[0]); } int interval = 10; if(argc > 3) { interval = atoi(argv[3]); } printf("PID=%d\n", getpid()); char* path = argv[2]; if( strcmp(argv[1], "read") == 0 ) { int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } printf("read lock try\n"); /* read lock entire file */ if( read_lock(fd, 0, SEEK_SET, 0) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock success.\n"); printf("Read Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); sleep(interval); exit(0); } else if( strcmp(argv[1], "read2") == 0 ){ int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } printf("read lock try\n"); /* read lock entire file */ if( read_lock(fd, 0, SEEK_SET, 0) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock success.\n"); printf("Read Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); if( read_lock(fd, 0, SEEK_SET, 1) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock 2 success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "write") == 0) { int fd = open(path, O_WRONLY ); printf("file: %s, fd=%d\n", path, fd); if(fd == -1) { perror("open"); exit(0); } printf("write lock try\n"); /* write lock entire file */ if( write_lock(fd, 0, SEEK_SET, 0) < 0){ perror("write_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); printf("Read Lock by pid=%d\n", lock_test(fd, F_RDLCK, 0, SEEK_SET, 0 )); exit(0); } printf("write lock success.\n"); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); int ret = write(fd, path, strlen(path) ); if(ret != strlen(path)){ perror("write"); } sleep(interval); close(fd); exit(0); } else if( strcmp(argv[1], "rd-wr") == 0){ int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* read lock entire file */ if( read_lock(fd, 0, SEEK_SET, 0) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); /* if( un_lock(fd, 0, SEEK_SET, 0) < 0){ perror("un-lock:"); } */ if( write_lock(fd, 0, SEEK_SET, 1) < 0){ perror("write_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("write lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); sleep(interval); exit(0); } else if( strcmp(argv[1], "wr-rd") == 0){ int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd, 0, SEEK_SET, 0) < 0){ perror("write_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("write lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); /* if( un_lock(fd, 0, SEEK_SET, 0) < 0){ perror("un-lock:"); } */ if( read_lock(fd, 0, SEEK_SET, 0) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); sleep(interval); exit(0); } else if( strcmp(argv[1], "fork") == 0){ int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd, 0, SEEK_SET, 0) < 0){ perror("write_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("write lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); pid_t pid; if ((pid = fork()) < 0) { perror("un-lock:"); exit(1); } else if(pid == 0){ sleep(1); printf("New PID=%d\n", getpid()); if( read_lock(fd, 0, SEEK_SET, 0) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); sleep(interval); exit(0); } else{ exit(1); } } else if( strcmp(argv[1], "dup") == 0) { int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd, 0, SEEK_SET, 0) < 0){ perror("write_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("write lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd, F_WRLCK, 0, SEEK_SET, 0 )); int fd2 = 0; if( (fd2=dup(fd)) < 0){ perror("dup:"); exit(1); } close(fd); if( read_lock(fd2, 0, SEEK_SET, 0) < 0){ perror("read_lock: "); printf("Write Lock by pid=%d\n", lock_test(fd2, F_WRLCK, 0, SEEK_SET, 0 )); exit(0); } printf("read lock success.\n"); printf("Lock by pid=%d\n", lock_test(fd2, F_WRLCK, 0, SEEK_SET, 0 )); sleep(interval); exit(0); } else { printf("Unknown action!\n"); } return 0; }
文件锁flock
文件锁位于file table这个数据结构中,一个文件可有多个file table entry(多次open),一个entry有且只有一个文件锁(共享/独占)。文件锁有如下特点:
1)每个file table entry有且只有一个文件锁,fork和dup后的fd指向同一个file table entry,故拥有同一个文件锁。这意味着文件锁可被fork后的子进程继承。
2)显式调用LOCK_UN将释放该file table entry的文件锁;当所有指向同一file table entry的fd关闭后,该file table entry将被释放,其上的文件锁也将随之释放。
3)一个进程中多次打开同一文件将创建多个file table entry,这意味着这些entry的文件锁是独立的。
4)一个文件只能有一个文件锁,锁类型可转换,flock不保证锁类型转换是原子操作。
5)未设置FD_CLOEXEC时,记录锁将被exec后的进程继承(file entry未释放)。
6)文件锁对文件的读写权限没有要求。
flock的使用示例和测试代码:
#include <stdio.h> #include <string.h> #include <unistd.h> #include <stdlib.h> /* #include <fcntl.h> */ #include <sys/file.h> #define read_lock(fd) \ flock((fd), LOCK_SH|LOCK_NB) #define readw_lock(fd) \ flock((fd), LOCK_SH) #define write_lock(fd) \ flock((fd), LOCK_EX|LOCK_NB) #define writew_lock(fd, offset, whence, len) \ flock((fd), LOCK_EX) #define un_lock(fd) \ flock((fd), LOCK_UN) /* ======================================================== */ void Usage(const char* program) { fprintf(stderr, "Usage: %s read/read2/write/rd-wr/wr-rd/fork/dup/unlink file time\n", program); exit(-1); } int main(int argc, char *argv[]) { if(argc < 3){ Usage(argv[0]); } int interval = 10; if(argc > 3) { interval = atoi(argv[3]); } printf("PID=%d\n", getpid()); char* path = argv[2]; if( strcmp(argv[1], "read") == 0 ) { int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } printf("read lock try...\n"); /* read lock entire file */ if( read_lock(fd) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "read2") == 0 ){ int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } printf("read lock try...\n"); /* read lock entire file */ if( read_lock(fd) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); if( read_lock(fd) < 0){ perror("read_lock: "); exit(0); } printf("read lock 2 success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "write") == 0) { int fd = open(path, O_WRONLY ); printf("file: %s, fd=%d\n", path, fd); if(fd == -1) { perror("open"); exit(0); } printf("write lock try...\n"); /* write lock entire file */ if( write_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); int ret = write(fd, path, strlen(path) ); if(ret != strlen(path)){ perror("write"); } sleep(interval); close(fd); exit(0); } else if( strcmp(argv[1], "rd-wr") == 0){ int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* read lock entire file */ if( read_lock(fd) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); /* if( un_lock(fd) < 0){ perror("un-lock:"); } */ if( write_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "wr-rd") == 0){ int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); /* if( un_lock(fd, 0, SEEK_SET, 0) < 0){ perror("un-lock:"); } */ if( read_lock(fd) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "fork") == 0){ int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); pid_t pid; if ((pid = fork()) < 0) { perror("fork:"); exit(1); } else if(pid == 0){ sleep(1); printf("New PID=%d\n", getpid()); if( read_lock(fd) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); fd = open(path, O_RDWR); /* O_RDWR */ sleep(interval); exit(0); } else{ sleep(interval); exit(1); } } else if( strcmp(argv[1], "dup") == 0) { int fd = open(path, O_RDWR); /* O_RDWR */ if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); int fd2 = 0; if( (fd2=dup(fd)) < 0){ perror("dup:"); exit(1); } close(fd); if( read_lock(fd2) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "open") == 0) { int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( write_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); int fd2 = open(path, O_WRONLY); if(fd2 < 0){ perror("open:"); exit(1); } if( read_lock(fd2) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "open2") == 0) { int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( read_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("write lock success.\n"); int fd2 = open(path, O_WRONLY); if(fd2 < 0){ perror("open:"); exit(1); } if( write_lock(fd2) < 0){ perror("read_lock: "); exit(0); } printf("read lock success.\n"); sleep(interval); exit(0); } else if( strcmp(argv[1], "unlink") == 0) { int fd = open(path, O_RDONLY); if(fd == -1) { perror("open"); exit(0); } /* write lock entire file */ if( read_lock(fd) < 0){ perror("write_lock: "); exit(0); } printf("read lock success.\n"); int fd2 = open(path, O_WRONLY); if(fd2 < 0){ perror("open:"); exit(1); } unlink(path); sleep(interval); exit(0); } else { printf("Unknown action!\n"); } return 0; }
NFS文件锁-fcntl
NFS3支持文件记录fcntl和O_EXCL创建文件,但不支持文件锁。NFS的文件锁是通过Network Lock Manager(lockd后台进程)这个独立程序实现的,NFS server本身依旧是无状态的,所有的锁状态均由NLM维护。从NFS4开始,锁协议融入NFS自身的协议中,锁状态由NFS server维护,这意味这NFS4是有状态的,未简化锁设计,采用租赁锁。
文件读写锁
双文件读写锁机制
双文件读写锁机制采用目标文件(datafile)和“众所周知”的临时写文件(datafile.lock),依赖于新文件的原子创建来保证进程间的互斥写操作。其本质:新文件的原子创建模拟写锁,保证有且只有一个进程能够进行写操作;多进程直接读数据,并依赖操作系统保证打开后的文件在fd关闭之前一直可读的技巧保证读不会因文件删除而中断。
缺点:1) 不支持部分修改;2)某个进程拥有“读锁”,无法保证后续进程也将拿到“读锁”(文件被移除)。3)临时文件(锁文件)遗留问题。如果临时文件(datafile.lock)未能在写操作终止后(正常/不正常)被移除,那么将阻塞其他写进程。这个问题很棘手。可通过atexit()或者顶层脚本部分解决,但两者都很难避免删除的文件不是另一个写进程新创建的临时文件。
双文件读写锁机制的程序流程如下:
改进的双文件读写锁机制
改进的双文件读写锁机制采用文件硬链接数来模拟“读锁”,硬链接大于1表示有进程拥有目标文件的读锁。
改进的双文件机制和实际的“读写锁”吻合,只有当“读锁”全部被释放后,进程才能拿到“写锁”。但是仍然无法解决“锁文件遗留问题”。
上面的流程可进一步简化,既然建立link必不可少,可让读进程直接读link文件,通过link()的原子操作保证获取“读锁”的原子性,从而简化读流程。
替代的类文件锁技术
open(file, O_CREAT | O_EXCL,...) plus unlink(file)
注释:
open(file, O_CREAT | O_EXCL,...)获取锁;unlink(file)释放锁。
处理遗留锁文件的一个较可靠的办法:将PID写入锁文件;原子创建失败后,读取PID,kill(pid,0)检查PID是否有效,若无效,删除后,重新原子创建。(删除和创建存在“竞态”,无法避免!)
link(file, lockfile) plus unlink(lockfile)
用法:link和unlink均为原子的。每个需要加锁的进程,先创建一个临时文件,并link一个“众所周知”的“锁文件”到该目标文件。Link成功获取锁;unlink释放锁。
open(file, O_CREAT | O_TRUNC | O_WRONLY, 0) plus unlink(file)
用法:当使用O_TRUNC | O_WRONLY去打开一个已存在的文件,若该进程不具有写权限时,open将失败。
总结
依赖于“锁文件”的替代文件锁机制都无法真正可靠地解决“锁文件”遗留问题。这在实际工程项目会成为一个很棘手的问题。在未找到可靠解决“锁文件”遗留问题的方案之前,还是尽可能地使用系统自带的文件锁机制(flock/fcntl)。
参考书籍:
高级UNIX环境编程
The Linux Programming Interface
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作者:zhenjing.chen
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