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Re: [Qemu-devel] [patch 04/10] qemu: introduce main_loop_break
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] [patch 04/10] qemu: introduce main_loop_break |
Date: |
Thu, 26 Mar 2009 12:27:17 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Marcelo Tosatti wrote:
> Use a pipe to signal pending work for the iothread.
> +void main_loop_break(void)
> +{
> + uint64_t value = 1;
> + char buffer[8];
> + size_t offset = 0;
> +
> + if (io_thread_fd == -1)
> + return;
> +
> + memcpy(buffer, &value, sizeof(value));
> +
> + while (offset < 8) {
> + ssize_t len;
> +
> + len = write(io_thread_fd, buffer + offset, 8 - offset);
> + if (len == -1 && errno == EINTR)
> + continue;
> +
> + if (len <= 0)
> + break;
> +
> + offset += len;
> + }
> +
> + if (offset != 8)
> + fprintf(stderr, "failed to notify io thread\n");
1: Why do you write 8 bytes instead of 1 byte? If you're thinking of
passing a value at some point, you could change it later when it's
needed. But beware that requiring the pipe to hold all values
written is what made Netscape 4 lock up on too-new Linux kernels
some 10 years ago :-)
2: Do you know that writes <= PIPE_BUF are atomic so you don't need
the offset calculation?
> +/* Used to break IO thread out of select */
> +static void io_thread_wakeup(void *opaque)
> +{
> + int fd = (unsigned long)opaque;
> + char buffer[8];
> + size_t offset = 0;
> +
> + while (offset < 8) {
> + ssize_t len;
> +
> + len = read(fd, buffer + offset, 8 - offset);
> + if (len == -1 && errno == EINTR)
> + continue;
> +
> + if (len <= 0)
> + break;
> +
> + offset += len;
> + }
> +}
You should read until the pipe is empty, in case more than one signal
was raised and called main_loop_break() between calls to
io_thread_wait().
Since reads <= PIPE_BUF are atomic too, the easiest way to do that is
try to read at least one more byte than you wrote per
main_loop_break() call, and if you don't get that many, you've emptied
the pipe.
> +static void setup_iothread_fd(void)
> +{
> + int fds[2];
> +
> + if (pipe(fds) == -1) {
> + fprintf(stderr, "failed to create iothread pipe");
> + exit(0);
> + }
> +
> + qemu_set_fd_handler2(fds[0], NULL, io_thread_wakeup, NULL,
> + (void *)(unsigned long)fds[0]);
> + io_thread_fd = fds[1];
> +}
To avoid deadlock in perverse conditions where lots of signals call
main_loop_break() enough to fill the pipe before it's read, set the
write side non-blocking.
To use the trick of reading more bytes than you expect to detect EOF
without an extra read, set the read side non-blocking too.
-- Jamie
- [Qemu-devel] [patch 02/10] qemu: mutex/thread/cond wrappers, (continued)
[Qemu-devel] [patch 05/10] qemu: separate thread for io, Marcelo Tosatti, 2009/03/25
[Qemu-devel] [patch 07/10] qemu: handle reset/poweroff/shutdown in iothread, Marcelo Tosatti, 2009/03/25
[Qemu-devel] [patch 08/10] qemu: pause and resume cpu threads, Marcelo Tosatti, 2009/03/25
[Qemu-devel] [patch 09/10] qemu: handle vmstop from cpu context, Marcelo Tosatti, 2009/03/25
[Qemu-devel] [patch 10/10] qemu: basic kvm iothread support, Marcelo Tosatti, 2009/03/25