On Sat, May 07, 2022 at 05:11:35PM +0800, cael wrote: > We have met a hang on pty device, the reader was blocking at epoll on > master side, the writer was sleeping at wait_woken inside n_tty_write > on slave side , and the write buffer on tty_port was full, we found > that the reader and writer would never be woken again and block > forever. > > We thought the problem was caused as a race between reader and kworker > as follows: > n_tty_read(reader): | > n_tty_receive_buf_common(kworker): > | > room = N_TTY_BUF_SIZE - (ldata->read_head - tail); > | > room <= 0 > copy_from_read_buf(tty, &b, &nr); | > n_tty_kick_worker(tty); | > | > ldata->no_room = true > > After writing to slave device, writer wakes up kworker to flush data > on tty_port to reader, and the kworker finds that reader has no room > to store data so room <= 0 is met. At this moment, reader consumes all > the data on reader buffer and call n_tty_kick_worker to check > ldata->no_room and finds that there is no need to call > tty_buffer_restart_work to flush data to reader and reader quits > reading. Then kworker sets ldata->no_room = true and quits too. > > If write buffer is not full, writer will wake kworker to flush data > again after following writes, but if writer buffer is full and writer > goes to sleep, kworker will never be woken again and tty device is > blocked. > > We think this problem can be solved with a check for read buffer > inside function n_tty_receive_buf_common, if read buffer is empty and > ldata->no_room is true, this means that kworker has more data to flush > to read buffer, so a call to n_tty_kick_worker is necessary. > > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c > index f9c584244..4e65e2422 100644 > --- a/drivers/tty/n_tty.c > +++ b/drivers/tty/n_tty.c > @@ -1760,6 +1760,8 @@ n_tty_receive_buf_common(struct tty_struct *tty, > const unsigned char *cp, > } else > n_tty_check_throttle(tty); > > + if (!chars_in_buffer(tty)) > + n_tty_kick_worker(tty); > up_read(&tty->termios_rwsem); > > return rcvd; Hi, This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him a patch that has triggered this response. He used to manually respond to these common problems, but in order to save his sanity (he kept writing the same thing over and over, yet to different people), I was created. Hopefully you will not take offence and will fix the problem in your patch and resubmit it so that it can be accepted into the Linux kernel tree. You are receiving this message because of the following common error(s) as indicated below: - Your patch is malformed (tabs converted to spaces, linewrapped, etc.) and can not be applied. Please read the file, Documentation/email-clients.txt in order to fix this. - Your patch does not have a Signed-off-by: line. Please read the kernel file, Documentation/SubmittingPatches and resend it after adding that line. Note, the line needs to be in the body of the email, before the patch, not at the bottom of the patch or in the email signature. - You did not write a descriptive Subject: for the patch, allowing Greg, and everyone else, to know what this patch is all about. Please read the section entitled "The canonical patch format" in the kernel file, Documentation/SubmittingPatches for what a proper Subject: line should look like. If you wish to discuss this problem further, or you have questions about how to resolve this issue, please feel free to respond to this email and Greg will reply once he has dug out from the pending patches received from other developers. thanks, greg k-h's patch email bot
