Re: [PATCH] rtlwifi: rtl8192x: Enabling and disabling hardware interrupts after enabling local irq flags

[Larry Finger]

On 01/19/2017 08:35 AM, Lino Sanfilippo wrote:
> Hi,
>
> altek/rtlwifi/rtl8192ce/hw.c b/drivers/net/wireless/realtek/rtlwifi/rtl8192ce/hw.c
> > index a47be73..143766c4 100644
> > --- a/drivers/net/wireless/realtek/rtlwifi/rtl8192ce/hw.c
> > +++ b/drivers/net/wireless/realtek/rtlwifi/rtl8192ce/hw.c
> > @@ -1306,9 +1306,9 @@ void rtl92ce_enable_interrupt(struct ieee80211_hw *hw)
> >  	struct rtl_priv *rtlpriv = rtl_priv(hw);
> >  	struct rtl_pci *rtlpci = rtl_pcidev(rtl_pcipriv(hw));
> >
> > +	rtlpci->irq_enabled = true;
> >  	rtl_write_dword(rtlpriv, REG_HIMR, rtlpci->irq_mask[0] & 0xFFFFFFFF);
> >  	rtl_write_dword(rtlpriv, REG_HIMRE, rtlpci->irq_mask[1] & 0xFFFFFFFF);
> > -	rtlpci->irq_enabled = true;
> >  }
> >
> >  void rtl92ce_disable_interrupt(struct ieee80211_hw *hw)
> > @@ -1316,9 +1316,9 @@ void rtl92ce_disable_interrupt(struct ieee80211_hw *hw)
> >  	struct rtl_priv *rtlpriv = rtl_priv(hw);
> >  	struct rtl_pci *rtlpci = rtl_pcidev(rtl_pcipriv(hw));
> >
> > +	rtlpci->irq_enabled = false;
> >  	rtl_write_dword(rtlpriv, REG_HIMR, IMR8190_DISABLED);
> >  	rtl_write_dword(rtlpriv, REG_HIMRE, IMR8190_DISABLED);
> > -	rtlpci->irq_enabled = false;
> >  }
>
> AFAIK you also have to use memory barriers here to ensure that
> the concerning instructions are not reordered, and both irq handler
> and process have a consistent perception of irq_enabled, e.g:
>
> rtlpci->irq_enabled = true;
> smp_wmb();
> rtl_write_dword(rtlpriv, REG_HIMR, rtlpci->irq_mask[0] & 0xFFFFFFFF);
>
> and in the irq handler
>
> if (rtlpci->irq_enabled == 0) {
>         smp_rmb();
> 	return ret;
> }

I can see the potential race condition between setting interrupts and setting 
the flag, and I will likely accept Bharat's patch after testing.

I am likely displaying my ignorance regarding instruction reordering, but what 
compiler/cpu combination is likely to move a simple set operation after a call 
to an external routine? Is the smp_wmb() operation really needed? I am also 
unsure of the smp_rmb() call in the interrupt handler. Neither instruction 
should cause any problems, but I'm not sure they are needed.

Larry