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Line 87: − + + + − + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + − First, the <code>send_fd</code> and <code>recv_fd</code> routines cannot works using one socket for all tasks -- a descriptor sent to task <code>pid</code> should reach ''this'' task, not some arbitrary one that kernel woke up earlier on data arrival. That said, we have to create one socket per at least task to receive descriptors. − Second, files can be shared in a tricky manner, so that task A may have one file shared with task B and some other file shared with task C. If the "who opens a file" voting selects B and C for respective files, they will have to send descriptors to A with proper coordination with each other. This coordination can be simplified if we create sockets not just per-pid, but per-(pid, fd). This is what CRIU does, and this is how it does that.
How to assign needed file descriptor to a file (edit)
Revision as of 11:10, 5 August 2014
, 11:10, 5 August 2014no edit summary
<pre>
<pre>
int fd, i, pid = getpid();
int fd, i, pid = getpid(), sk;
sk = create_socket();
if (pid == file_opener(fd)) {
if (pid == file_opener(fd)) {
dup2(fd, fd->tgt_fds[i].fd);
dup2(fd, fd->tgt_fds[i].fd);
else
else
send_fd(fd, fd->tgt_fds[i]);
send_fd(fd, fd->tgt_fds[i], sk);
}
}
continue;
continue;
fd = recv_fd();
fd = recv_fd(sk);
dup2(fd, fd->tgt_fds[i].fd);
dup2(fd, fd->tgt_fds[i].fd);
close(fd);
close(fd);
}
}
}
}
close(sk);
</pre>
</pre>
Please, note, that all <code>tgt_fds</code> belonging to some task are opened by different one and then are sent to the real owner in the order they are met in the array. So does the receiver -- it receives the fds in the same order, so this algorithm puts files into proper descriptors.
Please, note, that all <code>tgt_fds</code> belonging to some task are opened by different one and then are sent to the real owner in the order they are met in the array. So does the receiver -- it receives the fds in the same order, so this algorithm puts files into proper descriptors.
There are several interesting things about the above code snippet.
There are several bugs in the above code snippet however :(
First, the <code>send_fd</code> and <code>recv_fd</code> routines cannot works using one socket for all tasks -- a descriptor sent to task <code>pid</code> should reach ''this'' task, not some arbitrary one that kernel woke up earlier on data arrival. That said, we have to create one socket per at least task to receive descriptors. But files can be shared in a tricky manner, so that task A may have one file shared with task B and some other file shared with task C. If the "who opens a file" voting selects B and C for respective files, they will have to send descriptors to A with proper coordination with each other. This coordination can be simplified if we create sockets not just per-pid, but per-(pid, fd). And where to keep all this bunch of sockets? The easiest answer -- in the places where the files they will receive should sit :)
Second, when the file opener calls <code>dup2()</code> it may overwrite the <code>sk</code> descriptor. This is sad, but OK, since we can move the sk into any free place using plain <code>dup()</code> system call.
<pre>
int fd, i, pid = getpid(), sk;
if (pid == file_opener(fd)) {
sk = create_socket();
fd = open_a_file(fd->file);
for (i = 0; i < fd->n_fds; i++) {
if (fd->tgt_fds[i].pid == pid) {
if (sk == fd->tgt_fds[i].fd)
sk = dup(sk);
dup2(fd, fd->tgt_fds[i].fd);
} else
send_fd(fd, fd->tgt_fds[i], sk);
}
close(fd);
close(sk);
} else {
for (i = 0; i < fd->n_fds; i++) {
if (fd->tgt_fds[i].pid != pid)
continue;
sk = create_socket();
dup2(sk, fd->tgt_fds[i].fd);
}
for (i = 0; i < fd->n_fds; i++) {
if (fd->tgt_fds[i].pid != pid)
continue;
fd = recv_fd(fd->tgt_fds[i].fd);
dup2(fd, fd->tgt_fds[i].fd);
close(fd);
}
}
</pre>
[[Category:Under the hood]]
[[Category:Under the hood]]