On Tue, Jun 14, 2022 at 02:18:42PM +0300, Andrey Ryabinin wrote:
> This reverts commit 9b938c7262e4 ("chardev: clear O_NONBLOCK on SCM_RIGHTS
> file descriptors").
> File descriptor passed to QEMU via 'getfd' QMP command always
> changed to blocking mode. Instead of that, change blocking mode by QEMU
> file descriptors users when necessary, e.g. like migration.
>
> We need to preserve the state of the file descriptor in case it's still
> used by an external process and before the QEMU itself started
> using it.
>
> E.g. our local migration scenario with TAP networking looks like this:
> 1. Create TAP devices and pass file descriptors to source QEMU
> 2. Launch destination QEMU (-incoming defer) and pass same descriptors to it.
> 3. Start migration
>
> In such scenario setting blocking state at stage (2) will hang source QEMU
> since TAP fd suddenly become blocking.Is it possible to add a special flag or API for preserving the O_NONBLOCK open flag? That way the rest of QEMU could continue to safely reset the flag while the tap fd passing code would explicitly ask for the O_NONBLOCK open flag to be preserved. That seems safer but I haven't checked whether it's possible to do this. > > Signed-off-by: Andrey Ryabinin <a...@yandex-team.com> > --- > chardev/char-socket.c | 3 --- > io/channel-socket.c | 3 --- > migration/fd.c | 2 ++ > 3 files changed, 2 insertions(+), 6 deletions(-) > > diff --git a/chardev/char-socket.c b/chardev/char-socket.c > index dc4e218eeb6..c9592fb5836 100644 > --- a/chardev/char-socket.c > +++ b/chardev/char-socket.c > @@ -310,9 +310,6 @@ static ssize_t tcp_chr_recv(Chardev *chr, char *buf, > size_t len) > continue; > } > > - /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */ > - qemu_socket_set_block(fd); > - > #ifndef MSG_CMSG_CLOEXEC > qemu_set_cloexec(fd); > #endif > diff --git a/io/channel-socket.c b/io/channel-socket.c > index dc9c165de11..8b9679460dc 100644 > --- a/io/channel-socket.c > +++ b/io/channel-socket.c > @@ -479,9 +479,6 @@ static void qio_channel_socket_copy_fds(struct msghdr > *msg, > continue; > } > > - /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */ > - qemu_socket_set_block(fd); > - > #ifndef MSG_CMSG_CLOEXEC > qemu_set_cloexec(fd); > #endif > diff --git a/migration/fd.c b/migration/fd.c > index 6f2f50475f4..793fffeb169 100644 > --- a/migration/fd.c > +++ b/migration/fd.c > @@ -60,6 +60,8 @@ void fd_start_incoming_migration(const char *fdname, Error > **errp) > return; > } > > + qemu_socket_set_block(fd); > + > trace_migration_fd_incoming(fd); > > ioc = qio_channel_new_fd(fd, errp); > -- > 2.35.1 >
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