Re: Repost: Bug with select?

From: Manfred Spraul <manfred@colorfullife.com>
To: Eli Barzilay <eli@barzilay.org>, linux-kernel@vger.kernel.org
Cc: viro@parcelfarce.linux.theplanet.co.uk
Subject: Re: Repost: Bug with select?
Date: Sun, 27 Jul 2003 21:29:57 +0200	[thread overview]
Message-ID: <3F242835.9060001@colorfullife.com> (raw)

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Hi Eli,

The problem is normal_poll in drivers/char/n_tty.c:

 > if (tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS)
 >                mask |= POLLOUT | POLLWRNORM;

It assumes that a following write will succeed if less than 256 bytes 
are in the write buffer right now. This assumption is wrong for 
con_write_room: if the console is stopped, it returns 0 bytes buffer 
size (con_write_room()). Dito for pty_write_room.

The attached patch fixes your test case, but I don't understand tty 
devices good enough to guarantee anything.

--
    Manfred

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--- 2.5/drivers/char/n_tty.c	2003-07-05 09:13:01.000000000 +0200
+++ build-2.5/drivers/char/n_tty.c	2003-07-27 20:44:58.000000000 +0200
@@ -1251,7 +1251,8 @@
 		else
 			tty->minimum_to_wake = 1;
 	}
-	if (tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS)
+	if (tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS &&
+			tty->driver->write_room(tty) > 0)
 		mask |= POLLOUT | POLLWRNORM;
 	return mask;
 }
