linux-kernel - Re: [RFC PATCH 5/6] pci: Protect the enable/disable state of pci

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Date:   Fri, 17 Aug 2018 10:09:50 +0200 (CEST)
From:   Marta Rybczynska <mrybczyn@...ray.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        Hari Vyas <hari.vyas@...adcom.com>,
        Ray Jui <ray.jui@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Jens Axboe <axboe@...nel.dk>, Lukas Wunner <lukas@...ner.de>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Pierre-Yves Kerbrat <pkerbrat@...ray.eu>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/6] pci: Protect the enable/disable state of
 pci_dev using the state mutex



----- On 17 Aug, 2018, at 06:49, Benjamin Herrenschmidt benh@...nel.crashing.org wrote:

> This protects enable/disable operations using the state mutex to
> avoid races with, for example, concurrent enables on a bridge.
> 
> The bus hierarchy is walked first before taking the lock to
> avoid lock nesting (though it would be ok if we ensured that
> we always nest them bottom-up, it is better to just avoid the
> issue alltogether, especially as we might find cases where
> we want to take it top-down later).
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>


> 
> static void pci_enable_bridge(struct pci_dev *dev)
> {
> 	struct pci_dev *bridge;
> -	int retval;
> +	int retval, enabled;
> 
> 	bridge = pci_upstream_bridge(dev);
> 	if (bridge)
> 		pci_enable_bridge(bridge);
> 
> -	if (pci_is_enabled(dev)) {
> -		if (!dev->is_busmaster)
> -			pci_set_master(dev);
> +	/* Already enabled ? */
> +	pci_dev_state_lock(dev);
> +	enabled = pci_is_enabled(dev);
> +	if (enabled && !dev->is_busmaster)
> +		pci_set_master(dev);
> +	pci_dev_state_unlock(dev);
> +	if (enabled)
> 		return;
> -	}
> 

This looks complicated too me especially with the double locking. What do you
think about a function doing that check that used an unlocked version of
pcie_set_master?

Like:

        dev_state_lock(dev);
        enabled = pci_is_enabled(dev);
        if (enabled &&  !dev->is_busmaster)
                pci_set_master_unlocked();
        pci_dev_state_unlock(dev);

BTW If I remember correctly the code today can set bus mastering multiple
times without checking if already done.

Marta