Re: [PATCH 2/5] mwifiex: use spinlock for 'mwifiex_processing' in shutdown_drv

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Tue, Oct 25, 2016 at 04:11:14PM +0000, Amitkumar Karwar wrote:
> Hi Dmitry,
> 
> > From: Dmitry Torokhov [mailto:dmitry.torokhov@gmail.com]
> > Sent: Tuesday, October 25, 2016 5:28 AM
> > To: Brian Norris
> > Cc: Amitkumar Karwar; linux-wireless@vger.kernel.org; Cathy Luo; Nishant
> > Sarmukadam
> > Subject: Re: [PATCH 2/5] mwifiex: use spinlock for 'mwifiex_processing'
> > in shutdown_drv
> > 
> > On Mon, Oct 24, 2016 at 12:19:15PM -0700, Brian Norris wrote:
> > > On Mon, Oct 24, 2016 at 07:51:29PM +0530, Amitkumar Karwar wrote:
> > > > This variable is guarded by spinlock at all other places. This patch
> > > > takes care of missing spinlock usage in mwifiex_shutdown_drv().
> > > >
> > > > Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
> > > > ---
> > > >  drivers/net/wireless/marvell/mwifiex/init.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/drivers/net/wireless/marvell/mwifiex/init.c
> > > > b/drivers/net/wireless/marvell/mwifiex/init.c
> > > > index 82839d9..8e5e424 100644
> > > > --- a/drivers/net/wireless/marvell/mwifiex/init.c
> > > > +++ b/drivers/net/wireless/marvell/mwifiex/init.c
> > > > @@ -670,11 +670,14 @@ mwifiex_shutdown_drv(struct mwifiex_adapter
> > > > *adapter)
> > > >
> > > >  	adapter->hw_status = MWIFIEX_HW_STATUS_CLOSING;
> > > >  	/* wait for mwifiex_process to complete */
> > > > +	spin_lock_irqsave(&adapter->main_proc_lock, flags);
> > > >  	if (adapter->mwifiex_processing) {
> > >
> > > I'm not quite sure why we have this check in the first place; if the
> > > main process is still running, then don't we have bigger problems here
> > > anyway? But this is definitely the "right" way to check this flag, so:
> > 
> > No, this is definitely not right way to check it. What exactly does this
> > spinlock protect? It seems that the intent is to make sure we do not
> > call mwifiex_shutdown_drv() while mwifiex_main_process() is executing.
> > It even says above that we are "waiting" for it (but we do not, we may
> > bail out or we may not, depends on luck).
> > 
> > To implement this properly you not only need to take a lock and check
> > the flag, but keep the lock until mwifiex_shutdown_drv() is done, or use
> > some other way to let mwifiex_main_process() know it should not access
> > the adapter while mwifiex_shutdown_drv() is running.
> > 
> > NACK.
> > 
> 
> As I explained in other email, here is the sequence.
> 1) We find mwifiex_processing is true in mwifiex_shitdown_drv(). "-EINPROGRESS" is returned by the function and hw_status state is changed to MWIFIEX_HW_STATUS_CLOSING.
> 2) We wait until main_process is completed.
> 
>                 if (mwifiex_shutdown_drv(adapter) == -EINPROGRESS)
>                         wait_event_interruptible(adapter->init_wait_q,
>                                                  adapter->init_wait_q_woken);
> 
> 3) We have following check at the end of main_process()
> 
> exit_main_proc:
>         if (adapter->hw_status == MWIFIEX_HW_STATUS_CLOSING)
>                 mwifiex_shutdown_drv(adapter);
> 
> 4) Here at the end of mwifiex_shutdown_drv(), we are setting "adapter->init_wait_q_woken" and waking up the thread in point (2)

1. We do not find mwifiex_processing to be true
2. We proceed to try and shut down the driver
3. Interrupt is raised in the meantime, kicking work item
4. mwifiex_main_process() sees that adapter->hw_status is
MWIFIEX_HW_STATUS_CLOSING and jumps to exit_main_proc
5. mwifiex_main_process() calls into mwifiex_shutdown_drv() that is now
racing with another copy of the same.

It seems to me that mwifiex_main_process() is [almost] always used from
a workqueue. Can you instead of sprinkling spinlocks ensure that
mwifiex_shutdown_drv():

1. Inhibits scheduling of mwifiex_main_process()
2. Does cancel_work_sync(...) to ensure that mwifiex_main_process() does
not run
3. Continues shutting down the driver.

Alternatively, do these have to be spinlocks? Can you make them mutexes
and take them for the duration of mwifiex_main_process() and
mwifiex_shutdown_drv() and others, as needed?

Thanks.

-- 
Dmitry