Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH RESEND RFC] driver: core: don't queue device links removal for dt overlays | From | Nuno Sá <> | Date | Wed, 31 Jan 2024 15:55:48 +0100 |
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On Wed, 2024-01-31 at 15:28 +0100, Rafael J. Wysocki wrote: > On Wed, Jan 31, 2024 at 3:18 PM Nuno Sá <noname.nuno@gmail.com> wrote: > > > > On Wed, 2024-01-31 at 14:30 +0100, Rafael J. Wysocki wrote: > > > On Wed, Jan 31, 2024 at 1:20 PM Nuno Sá <noname.nuno@gmail.com> wrote: > > > > > > > > On Tue, 2024-01-23 at 16:40 +0100, Nuno Sa via B4 Relay wrote: > > > > > From: Nuno Sa <nuno.sa@analog.com> > > > > > > > > > > For device links, releasing the supplier/consumer devices references > > > > > happens asynchronously in device_link_release_fn(). Hence, the > > > > > possible > > > > > release of an of_node is also asynchronous. If these nodes were added > > > > > through overlays we have a problem because this does not respect the > > > > > devicetree overlays assumptions that when a changeset is > > > > > being removed in __of_changeset_entry_destroy(), it must hold the last > > > > > reference to that node. Due to the async nature of device links that > > > > > cannot be guaranteed. > > > > > > > > > > Given the above, in case one of the link consumer/supplier is part of > > > > > an overlay node we call directly device_link_release_fn() instead of > > > > > queueing it. Yes, it might take some significant time for > > > > > device_link_release_fn() to complete because of synchronize_srcu() but > > > > > we would need to, anyways, wait for all OF references to be released > > > > > if > > > > > we want to respect overlays assumptions. > > > > > > > > > > Signed-off-by: Nuno Sa <nuno.sa@analog.com> > > > > > --- > > > > > This RFC is a follow up of a previous one that I sent to the > > > > > devicetree > > > > > folks [1]. It got rejected because it was not really fixing the root > > > > > cause of the issue (which I do agree). Please see the link where I > > > > > fully explain what the issue is. > > > > > > > > > > I did also some git blaming and did saw that commit > > > > > 80dd33cf72d1 ("drivers: base: Fix device link removal") introduced > > > > > queue_work() as we could be releasing the last device reference and > > > > > hence > > > > > sleeping which is against SRCU callback requirements. However, that > > > > > same > > > > > commit is now making use of synchronize_srcu() which may take > > > > > significant time (and I think that's the reason for the work item?). > > > > > > > > > > However, given the dt overlays requirements, I'm not seeing any > > > > > reason to not be able to run device_link_release_fn() synchronously if > > > > > we > > > > > detect an OVERLAY node is being released. I mean, even if we come up > > > > > (and I did some experiments in this regard) with some async mechanism > > > > > to > > > > > release the OF nodes refcounts, we still need a synchronization point > > > > > somewhere. > > > > > > > > > > Anyways, I would like to have some feedback on how acceptable would > > > > > this > > > > > be or what else could I do so we can have a "clean" dt overlay > > > > > removal. > > > > > > > > > > I'm also including dt folks so they can give some comments on the new > > > > > device_node_overlay_removal() function. My goal is to try to detect > > > > > when > > > > > an > > > > > overlay is being removed (maybe we could even have an explicit flag > > > > > for > > > > > it?) and only directly call device_link_release_fn() in that case. > > > > > > > > > > [1]: > > > > > https://lore.kernel.org/linux-devicetree/20230511151047.1779841-1-nuno.sa@analog.com/ > > > > > --- > > > > > drivers/base/core.c | 25 ++++++++++++++++++++++++- > > > > > 1 file changed, 24 insertions(+), 1 deletion(-) > > > > > > > > > > diff --git a/drivers/base/core.c b/drivers/base/core.c > > > > > index 14d46af40f9a..31ea001f6142 100644 > > > > > --- a/drivers/base/core.c > > > > > +++ b/drivers/base/core.c > > > > > @@ -497,6 +497,18 @@ static struct attribute *devlink_attrs[] = { > > > > > }; > > > > > ATTRIBUTE_GROUPS(devlink); > > > > > > > > > > +static bool device_node_overlay_removal(struct device *dev) > > > > > +{ > > > > > + if (!dev_of_node(dev)) > > > > > + return false; > > > > > + if (!of_node_check_flag(dev->of_node, OF_DETACHED)) > > > > > + return false; > > > > > + if (!of_node_check_flag(dev->of_node, OF_OVERLAY)) > > > > > + return false; > > > > > + > > > > > + return true; > > > > > +} > > > > > + > > > > > static void device_link_release_fn(struct work_struct *work) > > > > > { > > > > > struct device_link *link = container_of(work, struct > > > > > device_link, > > > > > rm_work); > > > > > @@ -532,8 +544,19 @@ static void devlink_dev_release(struct device > > > > > *dev) > > > > > * synchronization in device_link_release_fn() and if the > > > > > consumer > > > > > or > > > > > * supplier devices get deleted when it runs, so put it into the > > > > > "long" > > > > > * workqueue. > > > > > + * > > > > > + * However, if any of the supplier, consumer nodes is being > > > > > removed > > > > > + * through overlay removal, the expectation in > > > > > + * __of_changeset_entry_destroy() is for the node 'kref' to be 1 > > > > > which > > > > > + * cannot be guaranteed with the async nature of > > > > > + * device_link_release_fn(). Hence, do it synchronously for the > > > > > overlay > > > > > + * case. > > > > > */ > > > > > - queue_work(system_long_wq, &link->rm_work); > > > > > + if (device_node_overlay_removal(link->consumer) || > > > > > + device_node_overlay_removal(link->supplier)) > > > > > + device_link_release_fn(&link->rm_work); > > > > > + else > > > > > + queue_work(system_long_wq, &link->rm_work); > > > > > } > > > > > > > > > > static struct class devlink_class = { > > > > > > > > > > --- > > > > > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d > > > > > change-id: 20240123-fix-device-links-overlays-5422e033a09b > > > > > -- > > > > > > > > > > Thanks! > > > > > - Nuno Sá > > > > > > > > > > > > > Hi Rafael, > > > > > > > > Would be nice to have your feedback on this one or if this is a complete > > > > nack... > > > > I think calling device_link_release_fn() synchronously is ok but I might > > > > be > > > > completely wrong. > > > > > > Well, it sounds like you are expecting me to confirm that what you are > > > doing makes sense, but I cannot do that, because I am not sufficiently > > > familiar with DT overlays. > > > > > > > I'm trying to understand if there's no hidden issue by calling it > > synchronously. > > (don't think there is but this is rather core stuff :)). > > > > From the DT guys, it would be helpful to get feedback on the new > > device_node_overlay_removal() helper I'm introducing. The goal is to just do > > the > > sync release in case we detect a node being removed as a result of an > > overlay > > removal. > > > > > You first need to convince yourself that you are not completely wrong. > > > > I mean, the problem is definitely real and if you see the link I pasted in > > the > > cover, this will all lead to big splats. > > > > > > > > > +Cc Saravan as he should also be very familiar with device_links and see > > > > if > > > > the > > > > above fairly simple solution is sane. > > > > > > > > I also don't want to be pushy as I know you guys are all very busy but > > > > it's > > > > (i > > > > think) the third time I resend the patch :) > > > > > > Sorry about that, I haven't realized that my input is requisite. > > > > > > > Yeah, get_mantainers gives me you and Greg but I think you're the main dev > > on > > dev_links right? > > > > > So the patch not only calls device_link_release_fn() synchronously, > > > but it also calls this function directly and I, personally, wouldn't > > > do at least the latter. > > > > > > > So you mean adding something like adding a new > > > > device_link_release(struct device_link *link) helper > > and either call it synchronously from devlink_dev_release() or > > asynchronously > > from device_link_release_fn()? > > > > I can drop the RFC and send a patch with the above... > > No, IMV devlink_dev_release() needs to be called via > device_link_put_kref(), but it may run device_link_release_fn() > directly if the link is marked in a special way or something like > this.
Sorry, I'm not totally getting this. I'm directly calling device_link_release_fn() from devlink_dev_release(). We should only get into devlink_dev_release() after all the references are dropped right (being it the release callback for the link class)?
device_node_overlay_removal() is my way to see if the link "is marked in a special way" as you put it. This checks if one of the supplier/consumer is marked as an OVERLAY and if it's being removed (I think that OF_DETACHED tells us that but some feedback from DT guys will be helpful).
Alternatively, I could just check the OF_OVERLAY flag when the link is being created and have a new variable in struct device_link to flag the synchronous release. Disadvantage is that in this way even a sysfs unbind or module unload (without necessarily removing the overly) would lead to a synchronous release which can actually make sense (now that I think about it). Because if someone does some crazy thing like "echo device > unbind" and then removes the overlay we could still hit the overly removal path before device_link_release_fn() completed.
- Nuno Sá
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