DragonFlyBSD

Here is a better test program, works for x86-64 or i386. The size of the file depends on how UFS was formatted. The file must be such that the last block is a fragment which covers less than a page. We then issue a write that hits that fragment. The bread() of the buffer brings in the VM page but the VM page's valid bits are not fully set. This then causes the copyin or copyout to fault, creating the double-lock.

/* * Must be one fragment less than a page. Typical UFS filesystems * are configured 1K/8K or 2K/16K. Larger ones tend to be configured * 2K/16K. Use 7168 for 1K/8K and 6144 for 2K/16K * * dd if=/dev/zero of=test bs=6144 count=1 * dd if=/dev/zero of=test bs=7168 count=1
*/
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>

main(int argc, char **argv) {
int fd;
int backup_fd;
char *mmap_region;
char buf⁸¹⁹²;
int i0, i;
ssize_t n;

fd = open(argv[1], O_RDWR);
        mmap_region = mmap(NULL, 16384 * 2,
                 PROT_READ|PROT_WRITE,MAP_PRIVATE,
                fd, 0);
        fprintf(stderr, "%d %p\n", fd, mmap_region);
#if 1
        madvise(mmap_region, 16384 * 2, MADV_RANDOM);
        madvise(mmap_region, 16384 * 2, MADV_INVAL);
#endif
        mmap_region[4] = 'b';
        lseek(fd, 0, SEEK_SET);
        n = write(fd, mmap_region, 4096+2048);
        printf("write n=%zd\n", n);
        munmap(mmap_region, 16384 * 2);
}

Fixed in commit 44480e310a5e2fdec131e9154d62ac8fb0f011a9. However, there are still potential deadlock situations due to the same recursion VM fault issue that have not yet been resolved.