linux-kernel - Re: [PATCH v3] irq/core: synchronize irq

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Date:   Tue, 10 May 2022 13:37:51 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Thomas Pfaff <tpfaff@....com>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v3] irq/core: synchronize irq_thread startup

On Tue, May 10 2022 at 13:34, Thomas Gleixner wrote:
> On Tue, May 10 2022 at 10:43, Thomas Pfaff wrote:
>> It can be done only before the interrupt thread is stopped, and free_irq() 
>> itself is already taking care about synchronizing, no matter if its done by 
>> __synchronize_hardirq() or by synchronize_irq(), like it was before commit 
>> 519cc8652b3a.
>
> No, it does not really take care about it. It can return with
> irq_desc::threads_active > 0 due to the interrupt thread being stopped
> before reaching the thread function. Think about shared interrupts.

Duh. Hit send too fast.

It does matter whether the synchronization is done via
__synchronize_hardirq() or via synchronize_irq(). The latter ensured
that the thread reached the thread function and handled the pending
wakeup _before_ kthread_stop() become true.

So the fix is required to undo the damage created by 519cc8652b3a.

The synchronize_irq() after free_irq() is a completely different
problem.

Thanks,

        tglx