On 18.01.2017 11:48, Fam Zheng wrote: > On Fri, 12/02 03:58, Max Reitz wrote: >> On 31.10.2016 16:38, Fam Zheng wrote: >>> This implements open flag sensible image locking for local file >>> and host device protocol. >>> >>> virtlockd in libvirt locks the first byte, so we start looking at the >>> file bytes from 1. >>> >>> Quoting what was proposed by Kevin Wolf <kw...@redhat.com>, there are >>> four locking modes by combining two bits (BDRV_O_RDWR and >>> BDRV_O_SHARE_RW), and implemented by taking two locks: >>> >>> Lock bytes: >>> >>> * byte 1: I can't allow other processes to write to the image >>> * byte 2: I am writing to the image >>> >>> Lock modes: >>> >>> * shared writer (BDRV_O_RDWR | BDRV_O_SHARE_RW): Take shared lock on >>> byte 2. Test whether byte 1 is locked using an exclusive lock, and >>> fail if so. >> >> Testing whether something is locked would be easier by using F_OFD_GETLK >> instead of actually creating an exclusive lock and then releasing it. > > My attempt to do this shows it doesn't work: fcntl forces the tested lock type > to read lock for some unknown reason, so it cannot be used to test other > shared > lockers.
Do you have a reproducer? The attached quick and dirty test case works for me (compile test.c to test and test2.c to test2), producing the following output (when running ./test): set rd lock: 0, Success get lock: 0, Success; read lock in place set wr lock: -1, Resource temporarily unavailable unset lock: 0, Resource temporarily unavailable === unset lock: 0, Success get lock: 0, Success; unlocked set wr lock: 0, Success unset lock: 0, Success === set wr lock: 0, Success get lock: 0, Success; write lock in place set wr lock: -1, Resource temporarily unavailable unset lock: 0, Resource temporarily unavailable So the l_type field is correctly set after every F_OFD_GETLK query call. Max
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main(void)
{
system("touch foo");
int fd = open("foo", O_RDWR);
int ret = fcntl(fd, F_OFD_SETLK, &(struct flock){ .l_type = F_RDLCK });
printf("set rd lock: %i, %s\n", ret, strerror(errno));
system("./test2");
printf("===\n");
ret = fcntl(fd, F_OFD_SETLK, &(struct flock){ .l_type = F_UNLCK });
printf("unset lock: %i, %s\n", ret, strerror(errno));
system("./test2");
printf("===\n");
ret = fcntl(fd, F_OFD_SETLK, &(struct flock){ .l_type = F_WRLCK });
printf("set wr lock: %i, %s\n", ret, strerror(errno));
system("./test2");
close(fd);
return 0;
}
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
static const char *lock_names[] = {
[F_UNLCK] = "unlocked",
[F_RDLCK] = "read lock in place",
[F_WRLCK] = "write lock in place",
};
int main(void)
{
int fd = open("foo", O_RDWR);
struct flock fl = { .l_type = F_WRLCK };
int ret = fcntl(fd, F_OFD_GETLK, &fl);
printf("get lock: %i, %s; %s\n", ret, strerror(errno), lock_names[fl.l_type]);
ret = fcntl(fd, F_OFD_SETLK, &(struct flock){ .l_type = F_WRLCK });
printf("set wr lock: %i, %s\n", ret, strerror(errno));
close(fd);
return 0;
}
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