drivers/acpi/scan.c: Fixup "acquire device_hotplug_lock in acpi_scan_init()"

Let's document why we take the lock here. If we're going to overhaul memory hotplug locking, we'll have to touch many places - this comment will help to clairfy why it was added here.

Cc Rafael J. Wysocki <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Oscar Salvador <[email protected]>

Cc Andrew Morton <[email protected]>

Signed-off-by David Hildenbrand <[email protected]>

---
 drivers/acpi/scan.c | 5 +++++
 1 file changed, 5 insertions(+)

drivers/acpi/scan.c +5 -0

index cbc9d64b48dd..9193f1d46148 100644
@@ -2204,6 +2204,11 @@ int __init acpi_scan_init(void)
 	acpi_gpe_apply_masked_gpes();
 	acpi_update_all_gpes();
 
+	/*
+	 * We end up calling __add_memory(), which expects the
+	 * device_hotplug_lock to be held. Races with userspace and other
+	 * hotplug activities are not really possible - lock for consistency.
+	 */
 	lock_device_hotplug();
 	mutex_lock(&acpi_scan_lock);
 

Signature · 1 line

2.21.0

Quoted 3 lines · David Hildenbrand

Let's document why we take the lock here. If we're going to overhaul memory hotplug locking, we'll have to touch many places - this comment will help to clairfy why it was added here.

And how exactly is "lock for consistency" comment going to help the poor soul touching that code? How do people know that it is safe to remove it? I am not going to repeat my arguments how/why I hate "locking for consistency" (or fun or whatever but a real synchronization reasons) but if you want to help then just explicitly state what should done to remove this lock.

Quoted 5 lines · Michal Hocko

Quoted 3 lines · David Hildenbrand

Let's document why we take the lock here. If we're going to overhaul memory hotplug locking, we'll have to touch many places - this comment will help to clairfy why it was added here.

Quoted 7 lines

And how exactly is "lock for consistency" comment going to help the poor soul touching that code? How do people know that it is safe to remove it? I am not going to repeat my arguments how/why I hate "locking for consistency" (or fun or whatever but a real synchronization reasons) but if you want to help then just explicitly state what should done to remove this lock.

I know that you have a different opinion here. To remove the lock, add_memory() locking has to be changed *completely* to the point where we can drop the lock from the documentation of the function (*whoever knows what we have to exactly change* - and I don't have time to do that *right now*).

Quoted 14 lines · David Hildenbrand

Quoted 5 lines · Michal Hocko

Quoted 3 lines · David Hildenbrand

Let's document why we take the lock here. If we're going to overhaul memory hotplug locking, we'll have to touch many places - this comment will help to clairfy why it was added here.

Quoted 7 lines

And how exactly is "lock for consistency" comment going to help the poor soul touching that code? How do people know that it is safe to remove it? I am not going to repeat my arguments how/why I hate "locking for consistency" (or fun or whatever but a real synchronization reasons) but if you want to help then just explicitly state what should done to remove this lock.

Quoted 5 lines

I know that you have a different opinion here. To remove the lock, add_memory() locking has to be changed *completely* to the point where we can drop the lock from the documentation of the function (*whoever knows what we have to exactly change* - and I don't have time to do that *right now*).

Not really. To remove a lock in this particular path it would be sufficient to add
/*
* Although __add_memory used down the road is documented to
* require lock_device_hotplug, it is not necessary here because
* this is an early code when userspace or any other code path
* cannot trigger hotplug operations.
*/

Quoted 21 lines · Michal Hocko

Quoted 14 lines · David Hildenbrand

Quoted 5 lines · Michal Hocko

Quoted 3 lines · David Hildenbrand

Let's document why we take the lock here. If we're going to overhaul memory hotplug locking, we'll have to touch many places - this comment will help to clairfy why it was added here.

Quoted 7 lines

And how exactly is "lock for consistency" comment going to help the poor soul touching that code? How do people know that it is safe to remove it? I am not going to repeat my arguments how/why I hate "locking for consistency" (or fun or whatever but a real synchronization reasons) but if you want to help then just explicitly state what should done to remove this lock.

Quoted 5 lines

I know that you have a different opinion here. To remove the lock, add_memory() locking has to be changed *completely* to the point where we can drop the lock from the documentation of the function (*whoever knows what we have to exactly change* - and I don't have time to do that *right now*).

Quoted 8 lines

Not really. To remove a lock in this particular path it would be sufficient to add
/*
* Although __add_memory used down the road is documented to
* require lock_device_hotplug, it is not necessary here because
* this is an early code when userspace or any other code path
* cannot trigger hotplug operations.
*/

Okay, let me phrase it like this: Are you 100% (!) sure that we don't need the lock here. I am not - I only know what I documented back then and what I found out - could be that we are forgetting something else the lock protects.

As I already said, I am fine with adding such a comment instead. But I am not convinced that the absence of the lock is 100% safe. (I am 99.99% sure ;) ).

Quoted 31 lines · David Hildenbrand

Quoted 21 lines · Michal Hocko

Quoted 14 lines · David Hildenbrand

Quoted 5 lines · Michal Hocko

Quoted 3 lines · David Hildenbrand

Let's document why we take the lock here. If we're going to overhaul memory hotplug locking, we'll have to touch many places - this comment will help to clairfy why it was added here.

Quoted 7 lines

And how exactly is "lock for consistency" comment going to help the poor soul touching that code? How do people know that it is safe to remove it? I am not going to repeat my arguments how/why I hate "locking for consistency" (or fun or whatever but a real synchronization reasons) but if you want to help then just explicitly state what should done to remove this lock.

Quoted 5 lines

I know that you have a different opinion here. To remove the lock, add_memory() locking has to be changed *completely* to the point where we can drop the lock from the documentation of the function (*whoever knows what we have to exactly change* - and I don't have time to do that *right now*).

Quoted 8 lines

Not really. To remove a lock in this particular path it would be sufficient to add
/*
* Although __add_memory used down the road is documented to
* require lock_device_hotplug, it is not necessary here because
* this is an early code when userspace or any other code path
* cannot trigger hotplug operations.
*/

Quoted 8 lines

Okay, let me phrase it like this: Are you 100% (!) sure that we don't need the lock here. I am not - I only know what I documented back then and what I found out - could be that we are forgetting something else the lock protects.

As I already said, I am fine with adding such a comment instead. But I am not convinced that the absence of the lock is 100% safe. (I am 99.99% sure ;) ).

I am sorry but this is a shiny example of cargo cult programming. You do not add locks just because you are not sure. Locks are protecting data structures not code paths! If it is not clear what is actually protected then that should be explored first before the lock is spread "just to be sure"

Just look here. You have pushed that uncertainty to whoever is going touch this code and guess what, they are going to follow that lead and we are likely to grow the unjustified usage and any further changes will be just harder. I have seen that pattern so many times that it is even not funny. And that's why I pushed back here.

So let me repeat. If the lock is to stay then make sure that the comment actually explains what has to be done to remove it because it is not really required as of now.

The other extreme I saw: People dropping locks because they think they can be smart but end up making developers debug crashes for months (I am not lying).

As I want to move on with this patch and have other stuff to work on, I will adjust the comment you gave and add that instead of the lock.