Messages in this thread | | | Subject | Re: tty->low_latency + irq context | From | Hollis Blanchard <> | Date | Tue, 02 Jan 2007 11:17:11 -0600 |
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On Tue, 2006-12-26 at 01:08 +0059, Jiri Slaby wrote: > Hi! > > * tty_flip_buffer_push - terminal > * @tty: tty to push > * > * Queue a push of the terminal flip buffers to the line discipline. This > * function must not be called from IRQ context if tty->low_latency is set. > > But some drivers (mxser, nozomi, hvsi...) sets low_latency to 1 in _open and > calls tty_flip_buffer_push in isr or in functions, which are called from isr. > Is the comment correct or the drivers?
The comment would be true if tty_flip_buffer_push() attempted to block with tty->low_latency set, but it doesn't AFAICS. One possibility for deadlock is if the tty->buf.lock spinlock is taken on behalf of a user process...
> Moreover, hvsi says: > tty->low_latency = 1; /* avoid throttle/tty_flip_buffer_push race */
That was a long time ago, but the race is something like this: * data is received, enough to completely fill the tty buffer * tty_flip_buffer_push() schedules flush_to_ldisc() * before flush_to_ldisc() runs, more data is received * flush_to_ldisc() truncates the incoming data (look for tty->receive_room)
I don't see how this is supposed to work in general. I suppose most PC-standard char drivers are not capable of overflowing a tty buffer before the host can empty it. I wasn't comfortable with hoping for that condition in my driver.
Setting "low_latency" ensures that throttle will be called immediately if the tty buffer is filled, avoiding the race.
-- Hollis Blanchard IBM Linux Technology Center
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