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Date: Thu, 26 Mar 2009 12:27:17 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [patch 04/10] qemu: introduce main_loop_break
Message-ID: <20090326122717.GB20866@shareable.org>
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To: qemu-devel@nongnu.org
Cc: Marcelo Tosatti <mtosatti@redhat.com>

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