linux-kernel - Re: [driver-core PATCH v5 4/9] driver core: Move async_synchronize

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Message-ID: <1541524943.196084.178.camel@acm.org>
Date:   Tue, 06 Nov 2018 09:22:23 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org
Cc:     linux-nvdimm@...ts.01.org, tj@...nel.org,
        akpm@...ux-foundation.org, linux-pm@...r.kernel.org,
        jiangshanlai@...il.com, rafael@...nel.org, len.brown@...el.com,
        pavel@....cz, zwisler@...nel.org, dan.j.williams@...el.com,
        dave.jiang@...el.com
Subject: Re: [driver-core PATCH v5 4/9] driver core: Move
 async_synchronize_full call

On Tue, 2018-11-06 at 08:18 -0800, Alexander Duyck wrote:
> On Mon, 2018-11-05 at 17:04 -0800, Bart Van Assche wrote:
> > On Mon, 2018-11-05 at 13:11 -0800, Alexander Duyck wrote:
> > > This patch moves the async_synchronize_full call out of
> > > __device_release_driver and into driver_detach.
> > > 
> > > The idea behind this is that the async_synchronize_full call will only
> > > guarantee that any existing async operations are flushed. This doesn't do
> > > anything to guarantee that a hotplug event that may occur while we are
> > > doing the release of the driver will not be asynchronously scheduled.
> > > 
> > > By moving this into the driver_detach path we can avoid potential deadlocks
> > > as we aren't holding the device lock at this point and we should not have
> > > the driver we want to flush loaded so the flush will take care of any
> > > asynchronous events the driver we are detaching might have scheduled.
> > > 
> > > Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> > > ---
> > >  drivers/base/dd.c |    6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > > index 76c40fe69463..e74cefeb5b69 100644
> > > --- a/drivers/base/dd.c
> > > +++ b/drivers/base/dd.c
> > > @@ -975,9 +975,6 @@ static void __device_release_driver(struct device *dev, struct device *parent)
> > >  
> > >  	drv = dev->driver;
> > >  	if (drv) {
> > > -		if (driver_allows_async_probing(drv))
> > > -			async_synchronize_full();
> > > -
> > >  		while (device_links_busy(dev)) {
> > >  			__device_driver_unlock(dev, parent);
> > >  
> > > @@ -1087,6 +1084,9 @@ void driver_detach(struct device_driver *drv)
> > >  	struct device_private *dev_prv;
> > >  	struct device *dev;
> > >  
> > > +	if (driver_allows_async_probing(drv))
> > > +		async_synchronize_full();
> > > +
> > >  	for (;;) {
> > >  		spin_lock(&drv->p->klist_devices.k_lock);
> > >  		if (list_empty(&drv->p->klist_devices.k_list)) {
> > 
> > Have you considered to move that async_synchronize_full() call into
> > bus_remove_driver()? Verifying the correctness of this patch requires to
> > check whether the async_synchronize_full() comes after the
> > klist_remove(&drv->p->knode_bus) call. That verification is easier when
> > the async_synchronize_full() call occurs in bus_remove_driver() instead
> > of in driver_detach().
> 
> I considered it, however it ends up with things being more symmetric to
> have use take care of synchronizing things in driver_detach since after
> this patch set we are scheduling thing asynchronously in driver_attach.
> 
> Also I don't think it would be any great risk simply because calling
> driver_detach with the driver still associated with the bus would be a
> blatent error as it could easily lead to issues where you unbind a
> driver but have it get hotplugged to a device while that is going on.

Thanks for the additional clarification. Since I'm fine with this patch:

Reviewed-by: Bart Van Assche <bvanassche@....org>