Subject: Re: [PATCH] powerpc/pseries: Avoid context switch in EEH reset if required

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 20 Jan 2015 10:28:16 +0100

On Mon, 2015-01-19 at 09:47 +1100, Gavin Shan wrote:
> On pseries platform, the EEH reset backend pseries_eeh_reset() can
> be called in atomic context as follows. For this case, we should
> call udelay() instead of msleep() to avoid context switching.
> 
>      drivers/scsi/ipr.c::ipr_reset_slot_reset_done()
>      drivers/pci/pci.c::pci_set_pcie_reset_state()
>      arch/powerpc/kernel/eeh.c::pcibios_set_pcie_reset_state()
>      arch/powerpc/platforms/pseries/eeh_pseries.c::pseries_eeh_reset()

It's not acceptable to introduce multi-millisecond delays at interrupt
time. In fact, we should generally not use udelay in such context.

I understand that this is an exceptional error handling case but it's
still not right.

Are there many other users of pci_set_pcie_reset_state() at interrupt
time ? Can we have a discussion with the PCI folks as to whether that
should be legal or not ?

I'm tempted to require that it's made illegal.

Ben.

> Signed-off-by: Gavin Shan <[email-masked]>
> Tested-by: Wen Xiong<[email-masked]>
> ---
>  arch/powerpc/platforms/pseries/eeh_pseries.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/eeh_pseries.c b/arch/powerpc/platforms/pseries/eeh_pseries.c
> index a6c7e19..67623a3 100644
> --- a/arch/powerpc/platforms/pseries/eeh_pseries.c
> +++ b/arch/powerpc/platforms/pseries/eeh_pseries.c
> @@ -503,8 +503,7 @@ static int pseries_eeh_get_state(struct eeh_pe *pe, int *state)
>   */
>  static int pseries_eeh_reset(struct eeh_pe *pe, int option)
>  {
> -	int config_addr;
> -	int ret;
> +	int config_addr, delay, ret;
>  
>  	/* Figure out PE address */
>  	config_addr = pe->config_addr;
> @@ -528,9 +527,14 @@ static int pseries_eeh_reset(struct eeh_pe *pe, int option)
>  	/* We need reset hold or settlement delay */
>  	if (option == EEH_RESET_FUNDAMENTAL ||
>  	    option == EEH_RESET_HOT)
> -		msleep(EEH_PE_RST_HOLD_TIME);
> +		delay = EEH_PE_RST_HOLD_TIME;
> +	else
> +		delay = EEH_PE_RST_SETTLE_TIME;
> +
> +	if (in_atomic())
> +		udelay(delay * 1000);
>  	else
> -		msleep(EEH_PE_RST_SETTLE_TIME);
> +		msleep(delay);
>  
>  	return ret;
>  }