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From: Bjorn Andersson <bjorn.andersson@linaro.org>
To: Arnaud POULIQUEN <arnaud.pouliquen@st.com>
Cc: Andy Gross <agross@kernel.org>, Ohad Ben-Cohen <ohad@wizery.com>,
	linux-arm-msm@vger.kernel.org, linux-remoteproc@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Mathieu Poirier <mathieu.poirier@linaro.org>
Subject: Re: [PATCH v4 1/4] remoteproc: Traverse rproc_list under RCU read lock
Date: Tue, 10 Mar 2020 09:20:29 -0700
Message-ID: <20200310162029.GI264362@yoga> (raw)
In-Reply-To: <87a14705-186d-01a4-e8a5-1844dab4ea14@st.com>

On Tue 10 Mar 06:41 PDT 2020, Arnaud POULIQUEN wrote:

> Hi Bjorn,
> 
> 
> On 3/10/20 7:38 AM, Bjorn Andersson wrote:
> > In order to be able to traverse the mostly read-only rproc_list without
> > locking during panic migrate traversal to be done under rcu_read_lock().
> > 
> > Mutual exclusion for modifications of the list continues to be handled
> > by the rproc_list_mutex and a synchronization point is added before
> > releasing objects that are popped from the list.
> > 
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> > ---
> > 
> > Change v3:
> > - New patch
> > 
> >  drivers/remoteproc/remoteproc_core.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
> > index 097f33e4f1f3..f0a77c30c6b1 100644
> > --- a/drivers/remoteproc/remoteproc_core.c
> > +++ b/drivers/remoteproc/remoteproc_core.c
> > @@ -1854,8 +1854,8 @@ struct rproc *rproc_get_by_phandle(phandle phandle)
> >  	if (!np)
> >  		return NULL;
> >  
> > -	mutex_lock(&rproc_list_mutex);
> > -	list_for_each_entry(r, &rproc_list, node) {
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(r, &rproc_list, node) {
> >  		if (r->dev.parent && r->dev.parent->of_node == np) {
> >  			/* prevent underlying implementation from being removed */
> >  			if (!try_module_get(r->dev.parent->driver->owner)) {
> > @@ -1868,7 +1868,7 @@ struct rproc *rproc_get_by_phandle(phandle phandle)
> >  			break;
> >  		}
> >  	}
> > -	mutex_unlock(&rproc_list_mutex);
> > +	rcu_read_unlock();
> >  
> >  	of_node_put(np);
> >  
> > @@ -1925,7 +1925,7 @@ int rproc_add(struct rproc *rproc)
> >  
> >  	/* expose to rproc_get_by_phandle users */
> >  	mutex_lock(&rproc_list_mutex);
> > -	list_add(&rproc->node, &rproc_list);
> > +	list_add_rcu(&rproc->node, &rproc_list);
> >  	mutex_unlock(&rproc_list_mutex);
> >  
> >  	return 0;
> > @@ -2140,9 +2140,12 @@ int rproc_del(struct rproc *rproc)
> >  
> >  	/* the rproc is downref'ed as soon as it's removed from the klist */
> >  	mutex_lock(&rproc_list_mutex);
> > -	list_del(&rproc->node);
> > +	list_del_rcu(&rproc->node);
> >  	mutex_unlock(&rproc_list_mutex);
> i'm not familiar with rcu but as rproc_panic_handler can be called in interrupt context, 
> does mutex should be replaced by a spinlock?
> 

Code traversing the list doesn't need to hold a lock, because the
rculist implementation ensures that the list itself is always
consistent.

Updates however can not be done concurrently, so that's why we're
maintaining this lock - which can be a mutex, because it now only
protects modifications.

And then the last piece is to guarantee that a node is not freed while
it's being accessed by the code traversing the list. This is ensured by
the synchronize_rcu() call below, which makes sure that no code holding
a rcu_read_lock() is still traversing the list.

Regards,
Bjorn

> Regards,
> Arnaud
> >  
> > +	/* Ensure that no readers of rproc_list are still active */
> > +	synchronize_rcu();
> > +
> >  	device_del(&rproc->dev);
> >  
> >  	return 0;
> > 

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-10  6:38 [PATCH v4 0/4] remoteproc: Panic handling Bjorn Andersson
2020-03-10  6:38 ` [PATCH v4 1/4] remoteproc: Traverse rproc_list under RCU read lock Bjorn Andersson
2020-03-10 13:41   ` Arnaud POULIQUEN
2020-03-10 16:20     ` Bjorn Andersson [this message]
2020-03-23 21:52   ` Mathieu Poirier
2020-03-10  6:38 ` [PATCH v4 2/4] remoteproc: Introduce "panic" callback in ops Bjorn Andersson
2020-03-23 22:29   ` Mathieu Poirier
2020-03-23 22:33   ` Mathieu Poirier
2020-03-10  6:38 ` [PATCH v4 3/4] remoteproc: qcom: q6v5: Add common panic handler Bjorn Andersson
2020-03-23 22:36   ` Mathieu Poirier
2020-03-10  6:38 ` [PATCH v4 4/4] remoteproc: qcom: Introduce panic handler for PAS and ADSP Bjorn Andersson
2020-03-23 22:45   ` Mathieu Poirier

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