Bug 349952

Two apparently unrelated things happened here.  First, syscall 216 was
pre-failed by Valgrind, since it isn't currently supported.  But that doesn't
kill the process.  Secondly, some time later, the process ran into an unimplemented
instruction.

What does your app do when 216 fails?  Does it assume it always succeeds?

You could try with -v --trace-signals=yes to see if you can get a stack trace
at the point where the unimplemented insn happens.

Instruction is MOV r/m64,Sreg which is "Move zero extended 16-bit segment register to r/m64" and so distinctly at the odd end of the spectrum.

Thanks for your answer. 
The resulting crash from unimplemented instruction was caused by remap_file_pages call not executed. That's what I was able to diagnose. The failing program needs a lot of surrounding infrastructure without which it's impossible to run it. Essentially, the program heavily relies on memory mapping and remapping to implement some mechanism. I can compile and run with this mechanism disabled but then it defies the whole purpose of using Valgrind to diagnose mutex contention. 
I wrote a simple program to illustrate the problem, but I guess, you are already aware of it. Input file created: dd if=/dev/zero of=input bs=4kB count=16
Until support for this call is added I can use Valgrind in limited mode to diagnose various issues.

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
    char buf[1024];
    void *base;
    int fd;
    size_t pagesz = sysconf(_SC_PAGE_SIZE);

    if ( argc < 2 ) {
        fprintf(stderr, "Missing file\n");
        return 1;
    }

    printf("Opening file %s\n", argv[1]);

    fd = open(argv[1], O_RDWR);
    if (fd < 0) {
       perror("open");
       return 1;
    }

    base = mmap(0, 4*pagesz, PROT_READ, MAP_SHARED, fd, 0);
    if (base < 0) {
        perror("mmap");
        close(fd);
        return 1;
    }

    memcpy(buf, (char*)base + 2*pagesz, 1024);

    if (remap_file_pages(base, pagesz, 0, 2, 0) < 0) {
        perror("remap_file_pages");
        munmap(base, 4*pagesz);
        close(fd);
        return 1;
    }

    printf("After remap_file_pages %d\n", memcmp(buf, base, 1024));

    munmap(base, 4*pagesz);
    close(fd);

    printf("Before returing from main\n");

    return 0;
}

*** This bug has been marked as a duplicate of bug 309554 ***