Re: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock_pair(v3)
From: Ming Lei
Date: Sat May 26 2012 - 05:24:40 EST
On Sat, May 26, 2012 at 3:16 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>
> I've still to hear what's wrong with a simple:
>
>
> if (!tty2 || tty == tty2) {
> tty_lock(tty);
> return;
> }
>
> if (tty > tty2)
> swap(tty, tty2);
>
> tty_lock(tty);
> tty_lock_nested(tty2, SINGLE_DEPTH_NESTING);
I remember that the patch may cause kernel hang in
my test. I will test it again to see if it is good.
>
>
> That's a lot more readable than the proposed code.
>
>> @@ -55,8 +64,13 @@ EXPORT_SYMBOL(tty_lock_pair);
>> void __lockfunc tty_unlock_pair(struct tty_struct *tty,
>> struct tty_struct *tty2)
>> {
>> - tty_unlock(tty);
>> - if (tty2 && tty2 != tty)
>> + if (tty < tty2) {
>> tty_unlock(tty2);
>> + tty_unlock(tty);
>> + } else {
>> + tty_unlock(tty);
>> + if (tty2 && tty2 != tty)
>> + tty_unlock(tty2);
>> + }
>> }
>> EXPORT_SYMBOL(tty_unlock_pair);
>
> This is complete crap, unlock order doesn't matter.
You mean that the below is good usage of lock?
LOCK A
LOCK B
UNLOCK A
UNLOCK B
Thanks,
--
Ming Lei
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