-----Original Message----- From: Andy Shevchenko [mailto:andy.shevchenko@gmail.com] Sent: Thursday, February 11, 2021 3:57 AM To: Song Bao Hua (Barry Song) song.bao.hua@hisilicon.com Cc: luojiaxing luojiaxing@huawei.com; Linus Walleij linus.walleij@linaro.org; Grygorii Strashko grygorii.strashko@ti.com; Santosh Shilimkar ssantosh@kernel.org; Kevin Hilman khilman@kernel.org; open list:GPIO SUBSYSTEM linux-gpio@vger.kernel.org; Linux Kernel Mailing List linux-kernel@vger.kernel.org; linuxarm@openeuler.org Subject: Re: [Linuxarm] Re: [PATCH for next v1 0/2] gpio: few clean up patches to replace spin_lock_irqsave with spin_lock
On Wed, Feb 10, 2021 at 11:50:45AM +0000, Song Bao Hua (Barry Song) wrote:
-----Original Message----- From: Andy Shevchenko [mailto:andy.shevchenko@gmail.com] Sent: Wednesday, February 10, 2021 11:51 PM On Wed, Feb 10, 2021 at 5:43 AM luojiaxing luojiaxing@huawei.com wrote:
On 2021/2/9 17:42, Andy Shevchenko wrote:
...
Between IRQ handler A and IRQ handle A, it's no need for a SLIS.
Right, but it's not the case in the patches you provided.
The code still holds spin_lock. So if two cpus call same IRQ handler, spin_lock makes them spin; and if interrupts are threaded, spin_lock makes two threads run the same handler one by one.
If you run on an SMP system and it happens that spin_lock_irqsave() just immediately after spin_unlock(), you will get into the troubles. Am I mistaken?
Hi Andy, Thanks for your reply.
But I don't agree spin_lock_irqsave() just immediately after spin_unlock() could a problem on SMP. When the 1st cpu releases spinlock by spin_unlock, it has completed its section of accessing the critical data, then 2nd cpu gets the spin_lock. These two CPUs won't have overlap on accessing the same data.
I think this entire activity is a carefully crafted mine field for the future syzcaller and fuzzers alike. I don't believe there are no side effects in a long term on all possible systems and configurations (including forced threaded IRQ handlers).
Also I don't understand why forced threaded IRQ could be a problem. Since IRQ has been a thread, this actually makes the situation much simpler than non-threaded IRQ. Since all threads including those IRQ threads want to hold spin_lock, they won't access the same critical data at the same time either.
I would love to see a better explanation in the commit message of such patches which makes it clear that there are *no* side effects.
People had the same questions before, But I guess the discussion in this commit has led to a better commit log:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
For time being, NAK to the all patches of this kind.
Fair enough, if you expect better explanation, I agree the commit log is too short.
-- With Best Regards, Andy Shevchenko
Thanks Barry