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Date: Mon, 21 May 2012 19:31:03 +0800
From: Ming Lei <ming.lei@...onical.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock_pair
On Fri, May 18, 2012 at 9:57 AM, Ming Lei <ming.lei@...onical.com> wrote:
> On Fri, May 18, 2012 at 2:48 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Thu, 2012-05-17 at 11:28 -0700, Greg Kroah-Hartman wrote:
>>> > +static void __lockfunc tty_lock_nest_lock(struct tty_struct *tty,
>>> > + struct tty_struct *tty2)
>>>
>>> Duplicating tty_lock() just for this one issue seems wrong and prone to
>>> error, don't you think?
>>>
>>> > +{
>>> > + if (tty->magic != TTY_MAGIC) {
>>> > + printk(KERN_ERR "L Bad %p\n", tty);
>>> > + WARN_ON(1);
>>> > + return;
>>> > + }
>>> > + tty_kref_get(tty);
>>> > + mutex_lock_nest_lock(&tty->legacy_mutex, &tty2->legacy_mutex);
>>
>> Yeah, its completely broken, even the lockdep annotation is the wrong
>> one.
>>
>> Something like the (completely untested) below patch is the 'right' way.
>
> Yes, it should be, but the warning is still triggered with the patch, also
> it is worse, kernel hanged during boot.
>
>> ---
>> drivers/tty/tty_mutex.c | 35 ++++++++++++++++++++---------------
>> 1 file changed, 20 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/tty/tty_mutex.c b/drivers/tty/tty_mutex.c
>> index 69adc80..587330b 100644
>> --- a/drivers/tty/tty_mutex.c
>> +++ b/drivers/tty/tty_mutex.c
>> @@ -10,7 +10,7 @@
>> * Getting the big tty mutex.
>> */
>>
>> -void __lockfunc tty_lock(struct tty_struct *tty)
>> +static void __lockfunc tty_lock_nested(struct tty_struct *tty, int subclass)
>> {
>> if (tty->magic != TTY_MAGIC) {
>> printk(KERN_ERR "L Bad %p\n", tty);
>> @@ -18,7 +18,12 @@ void __lockfunc tty_lock(struct tty_struct *tty)
>> return;
>> }
>> tty_kref_get(tty);
>> - mutex_lock(&tty->legacy_mutex);
>> + mutex_lock_nested(&tty->legacy_mutex, subclass);
>> +}
>> +
>> +void __lockfunc tty_lock(struct tty_struct *tty)
>> +{
>> + tty_lock_nested(tty, 0);
>> }
>> EXPORT_SYMBOL(tty_lock);
>>
>> @@ -38,25 +43,25 @@ EXPORT_SYMBOL(tty_unlock);
>> * Getting the big tty mutex for a pair of ttys with lock ordering
>> * On a non pty/tty pair tty2 can be NULL which is just fine.
>> */
>> -void __lockfunc tty_lock_pair(struct tty_struct *tty,
>> - struct tty_struct *tty2)
>> +void __lockfunc tty_lock_pair(struct tty_struct *tty1, struct tty_struct *tty2)
>> {
>> - if (tty < tty2) {
>> - tty_lock(tty);
>> - tty_lock(tty2);
>> - } else {
>> - if (tty2 && tty2 != tty)
>> - tty_lock(tty2);
>> - tty_lock(tty);
>> + if (!tty2 || tty1 == tty2) {
>> + tty_lock(tty1);
>> + return;
>> }
>> +
>> + if (tty2 < tty1)
>> + swap(tty1, tty2);
>
> That is too crazy.
>
>> +
>> + tty_lock(tty1);
>> + tty_lock_nested(tty2, SINGLE_DEPTH_NESTING);
>> }
>> EXPORT_SYMBOL(tty_lock_pair);
>>
>> -void __lockfunc tty_unlock_pair(struct tty_struct *tty,
>> - struct tty_struct *tty2)
>> +void __lockfunc tty_unlock_pair(struct tty_struct *tty1, struct tty_struct *tty2)
>> {
>> - tty_unlock(tty);
>> - if (tty2 && tty2 != tty)
>> + tty_unlock(tty1);
>> + if (tty2 && tty2 != tty1)
>> tty_unlock(tty2);
>> }
>> EXPORT_SYMBOL(tty_unlock_pair);
>>
>
> So how about the below one(tested OK)?
Looks no objections, I will prepare a formal one later, :-)
>
> diff --git a/drivers/tty/tty_mutex.c b/drivers/tty/tty_mutex.c
> index 69adc80..fecf592 100644
> --- a/drivers/tty/tty_mutex.c
> +++ b/drivers/tty/tty_mutex.c
> @@ -10,7 +10,8 @@
> * Getting the big tty mutex.
> */
>
> -void __lockfunc tty_lock(struct tty_struct *tty)
> +static void __lockfunc tty_lock_nested(struct tty_struct *tty,
> + int subclass)
> {
> if (tty->magic != TTY_MAGIC) {
> printk(KERN_ERR "L Bad %p\n", tty);
> @@ -18,7 +19,12 @@ void __lockfunc tty_lock(struct tty_struct *tty)
> return;
> }
> tty_kref_get(tty);
> - mutex_lock(&tty->legacy_mutex);
> + mutex_lock_nested(&tty->legacy_mutex, subclass);
> +}
> +
> +void __lockfunc tty_lock(struct tty_struct *tty)
> +{
> + tty_lock_nested(tty, 0);
> }
> EXPORT_SYMBOL(tty_lock);
>
> @@ -43,11 +49,14 @@ void __lockfunc tty_lock_pair(struct tty_struct *tty,
> {
> if (tty < tty2) {
> tty_lock(tty);
> - tty_lock(tty2);
> + tty_lock_nested(tty2, SINGLE_DEPTH_NESTING);
> } else {
> - if (tty2 && tty2 != tty)
> + int nested = 0;
> + if (tty2 && tty2 != tty) {
> tty_lock(tty2);
> - tty_lock(tty);
> + nested = SINGLE_DEPTH_NESTING;
> + }
> + tty_lock_nested(tty, nested);
> }
> }
> EXPORT_SYMBOL(tty_lock_pair);
Thanks,
--
Ming Lei
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