Re: [PATCH 1/2] mt76: mt7921: fix a leftover race in runtime-pm

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> Lorenzo Bianconi <lorenzo@kernel.org> writes:
> 
> > Fix a possible race in mt7921_pm_power_save_work() if rx/tx napi
> > schedules ps_work and we are currently accessing device register
> > on a different cpu.
> >
> > Fixes: 1d8efc741df8 ("mt76: mt7921: introduce Runtime PM support")
> > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > ---
> >  drivers/net/wireless/mediatek/mt76/mt7921/mac.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> > index defef3496246..0744f6e42ba3 100644
> > --- a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> > +++ b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> > @@ -1553,6 +1553,14 @@ void mt7921_pm_power_save_work(struct work_struct *work)
> >  	    dev->fw_assert)
> >  		goto out;
> >  
> > +	if (mutex_is_locked(&dev->mt76.mutex))
> > +		/* if mt76 mutex is held we should not put the device
> > +		 * to sleep since we are currently accessing device
> > +		 * register map. We need to wait for the next power_save
> > +		 * trigger.
> > +		 */
> > +		goto out;
> 
> This looks fishy to me. What protects the case when ps_work is run first
> and at the same time another cpu starts accessing the registers?
> 
> Do note that I didn't check the code, so I might be missing something.

before accessing chip registers, we run mt7921_mutex_acquire() so we grab mt76
mutex and run mt76_connac_pm_wake(). In mt76_connac_pm_wake() we cancel
ps_work, so it is not possible to access regs while mt7921_pm_power_save_work() is
running. The only leftover case is the other way around, i.e. if we
schedule mt7921_pm_power_save_work while we are already reading/writing chip
regs. This is only possible when mt7921_pm_power_save_work is scheduled by
rx_napi and this patch is fixing the latter case. Agree?

Regards,
Lorenzo

> 
> -- 
> https://patchwork.kernel.org/project/linux-wireless/list/
> 
> https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches

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