linux-kernel - RE: [PATCH 1/2] pci: host: pci-hyperv: Replace GFP_ATOMIC with GFP_KERNEL in hv_pci

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Message-ID: <DM5PR2101MB1030145AB251BF2F31FBA4BEDCD50@DM5PR2101MB1030.namprd21.prod.outlook.com>
Date:   Sun, 18 Mar 2018 22:40:55 +0000
From:   "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
CC:     "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] pci: host: pci-hyperv: Replace GFP_ATOMIC with
 GFP_KERNEL in hv_pci_onchannelcallback

> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org <linux-kernel-owner@...r.kernel.org> On Behalf
> Of Jia-Ju Bai
> Sent: Sunday, March 18, 2018 7:53 AM
> To: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang <haiyangz@...rosoft.com>; Stephen
> Hemminger <sthemmin@...rosoft.com>; bhelgaas@...gle.com
> Cc: devel@...uxdriverproject.org; linux-pci@...r.kernel.org; linux-kernel@...r.kernel.org;
> Jia-Ju Bai <baijiaju1990@...il.com>
> Subject: [PATCH 1/2] pci: host: pci-hyperv: Replace GFP_ATOMIC with GFP_KERNEL in
> hv_pci_onchannelcallback
> 
> hv_pci_onchannelcallback() is not called in atomic context.
> 
> The call chain ending up at hv_pci_onchannelcallback() is:
> [1] hv_pci_onchannelcallback() <- hv_pci_probe()
> hv_pci_probe() is only set as ".probe" in hv_driver
> structure "hv_pci_drv".
> 
> Despite never getting called from atomic context,

Not true.   hv_pci_probe() registers hv_pci_onchannelcallback() as
a callback function that is invoked from the VMbus interrupt handler.
So GFP_ATOMIC is appropriate.

Michael

> hv_pci_onchannelcallback() calls kmalloc with GFP_ATOMIC,
> which waits busily for allocation.
> GFP_ATOMIC is not necessary and can be replaced with GFP_KERNEL
> to avoid busy waiting.
> 
> This is found by a static analysis tool named DCNS written by myself.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>  drivers/pci/host/pci-hyperv.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/host/pci-hyperv.c b/drivers/pci/host/pci-hyperv.c
> index 0fe3ea1..c5c8a99 100644
> --- a/drivers/pci/host/pci-hyperv.c
> +++ b/drivers/pci/host/pci-hyperv.c
> @@ -1887,7 +1887,7 @@ static void hv_pci_onchannelcallback(void *context)
>  	struct pci_dev_incoming *dev_message;
>  	struct hv_pci_dev *hpdev;
> 
> -	buffer = kmalloc(bufferlen, GFP_ATOMIC);
> +	buffer = kmalloc(bufferlen, GFP_KERNEL);
>  	if (!buffer)
>  		return;
> 
> @@ -1899,7 +1899,7 @@ static void hv_pci_onchannelcallback(void *context)
>  			kfree(buffer);
>  			/* Handle large packet */
>  			bufferlen = bytes_recvd;
> -			buffer = kmalloc(bytes_recvd, GFP_ATOMIC);
> +			buffer = kmalloc(bytes_recvd, GFP_KERNEL);
>  			if (!buffer)
>  				return;
>  			continue;
> --
> 1.9.1