linux-kernel - Re: [PATCH 0/1] SIGWINCH problem with terminal apps still alive

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Date:	Fri, 10 Oct 2008 05:32:38 +0200
From:	Adam Tlałka <atlka@...gda.pl>
To:	Adam Tlałka <atlka@...gda.pl>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	torvalds@...l.org
Subject: Re: [PATCH 0/1] SIGWINCH problem with terminal apps still alive

Welcome,

Fri, 10 Oct 2008 03:12:34 +0200 - Adam Tlałka <atlka@...gda.pl>:
> now we have 2.6.26.6 kernel and still terminal resize leads to
> undesired effects. It is very inconvenient to wait for 2.6.27 for
> corrections.

Very funny, I've posted my patch just before 2.6.27 appeared so now it
seems obsolete. Only argument for it now is the cleaner and more
optimized code. Why?
Now we have two distant places where we use ws and tty->winsize
variables:

(from drivers/char/tty_io.c:tty_do_resize())

	mutex_lock(&real_tty->termios_mutex);
=>      if (!memcmp(ws, &tty->winsize, sizeof(*ws)))
                goto done;
        /* Get the PID values and reference them so we can
           avoid holding the tty ctrl lock while sending signals */
        spin_lock_irqsave(&tty->ctrl_lock, flags);
        pgrp = get_pid(tty->pgrp);
        rpgrp = get_pid(real_tty->pgrp);
        spin_unlock_irqrestore(&tty->ctrl_lock, flags);

        if (pgrp)
                kill_pgrp(pgrp, SIGWINCH, 1);
        if (rpgrp != pgrp && rpgrp)
                kill_pgrp(rpgrp, SIGWINCH, 1);

        put_pid(pgrp);
        put_pid(rpgrp);

=>      tty->winsize = *ws;
=>      real_tty->winsize = *ws;
done:
	mutex_unlock(&real_tty->termios_mutex);
 
Rearanged code

	mutex_lock(&real_tty->termios_mutex);
=>      if (!memcmp(ws, &tty->winsize, sizeof(*ws)))
                goto done;
=>	tty->winsize = *ws;
=>	real_tty->winsize = *ws;

        /* Get the PID values and reference them so we can
           avoid holding the tty ctrl lock while sending signals */
        spin_lock_irqsave(&tty->ctrl_lock, flags);
        pgrp = get_pid(tty->pgrp);
        rpgrp = get_pid(real_tty->pgrp);
        spin_unlock_irqrestore(&tty->ctrl_lock, flags);

        if (pgrp)
                kill_pgrp(pgrp, SIGWINCH, 1);
        if (rpgrp != pgrp && rpgrp)
                kill_pgrp(rpgrp, SIGWINCH, 1);

        put_pid(pgrp);
        put_pid(rpgrp);

done:
	mutex_unlock(&real_tty->termios_mutex);

is more logical and grouping acces to the same variable in one place
mean that gcc can better optimize outputed machine code. So it should
be a bit faster.
If some app gets SIGWINCH and will be waiting on this mutex so in case
of rearanged code it will be waiting shorter because variables setting
is done before signal generation.

Regards

-- 
Adam Tlałka       mailto:atlka@...gda.pl    ^v^ ^v^ ^v^
System  & Network Administration Group       - - - ~~~~~~
Computer Center, Gdańsk University of Technology, Poland
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