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

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Message-ID: <87bkwgi28x.wl-maz@kernel.org>
Date:   Mon, 02 May 2022 12:48:46 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Thomas Pfaff <tpfaff@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>, <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH v3] irq/core: synchronize irq_thread startup

On Mon, 02 May 2022 12:28:29 +0100,
Thomas Pfaff <tpfaff@....com> wrote:
> 
> From: Thomas Pfaff <tpfaff@....com>
> 
> While running
> "while /bin/true; do setserial /dev/ttyS0 uart none;
> setserial /dev/ttyS0 uart 16550A; done"
> on a kernel with threaded irqs, setserial is hung after some calls.
> 
> setserial opens the device, this will install an irq handler if the uart is
> not none, followed by TIOCGSERIAL and TIOCSSERIAL ioctls.
> Then the device is closed. On close, synchronize_irq() is called by
> serial_core.
> 
> If the close comes too fast, the irq_thread does not really start,
> it is terminated immediately without going into irq_thread().
> But an interrupt might already been handled by
> irq_default_primary_handler(), going to __irq_wake_thread() and
> incrementing threads_active.
> If this happens, synchronize_irq() will hang forever, because the
> irq_thread is already dead, and threads_active will never be decremented.
> 
> The fix is to make sure that the irq_thread is really started
> during __setup_irq().
> 
> Signed-off-by: Thomas Pfaff <tpfaff@....com>

Reviewed-by: Marc Zyngier <maz@...nel.org>

	M.

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