linux-kernel - Re: Potential data race in psmouse

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Message-ID: <CACT4Y+aG1zHHhfUmV_vMh-XsDLqwj3oSH54wobrf77=qyxVUhA@mail.gmail.com>
Date:	Fri, 28 Aug 2015 19:34:33 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Andrey Konovalov <andreyknvl@...gle.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Hans de Goede <hdegoede@...hat.com>,
	Mathias Gottschlag <mgottschlag@...il.com>,
	Shailendra Verma <shailendra.capricorn@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	linux-input@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>, ktsan@...glegroups.com
Subject: Re: Potential data race in psmouse_interrupt

Hello,

I am looking at this code in __ps2_command again:

/*
* The reset command takes a long time to execute.
*/
timeout = msecs_to_jiffies(command == PS2_CMD_RESET_BAT ? 4000 : 500);

timeout = wait_event_timeout(ps2dev->wait,
    !(READ_ONCE(ps2dev->flags) & PS2_FLAG_CMD1), timeout);

if (smp_load_acquire(&ps2dev->cmdcnt) &&
    !(smp_load_acquire(&ps2dev->flags) & PS2_FLAG_CMD1)) {
              timeout = ps2_adjust_timeout(ps2dev, command, timeout);
              wait_event_timeout(ps2dev->wait,
                        !(smp_load_acquire(&ps2dev->flags) &
PS2_FLAG_CMD), timeout);
}

if (param)
    for (i = 0; i < receive; i++)
              param[i] = ps2dev->cmdbuf[(receive - 1) - i];


Here are two moments I don't understand:
1. The last parameter of ps2_adjust_timeout is timeout in jiffies (it
is compared against 100ms). However, timeout is assigned to result of
wait_event_timeout, which returns 0 or 1. This does not make sense to
me. What am I missing?
2. This code pays great attention to timeouts, but in the end I don't
see how it handles timeouts. That is, if a timeout is happened, we
still copyout (garbage) from cmdbuf. What am I missing here?

Thank you
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