On 2021-05-06 5:40 a.m., Daniel Vetter wrote:
On Fri, Apr 30, 2021 at 01:27:37PM -0400, Andrey Grodzovsky wrote:
On 2021-04-30 6:25 a.m., Daniel Vetter wrote:
On Thu, Apr 29, 2021 at 04:34:55PM -0400, Andrey Grodzovsky wrote:
On 2021-04-29 3:05 p.m., Daniel Vetter wrote:
On Thu, Apr 29, 2021 at 12:04:33PM -0400, Andrey Grodzovsky wrote:
On 2021-04-29 7:32 a.m., Daniel Vetter wrote:
On Thu, Apr 29, 2021 at 01:23:19PM +0200, Daniel Vetter wrote:
On Wed, Apr 28, 2021 at 11:12:00AM -0400, Andrey Grodzovsky wrote:
With this calling drm_dev_unplug will flush and block
all in flight IOCTLs
Also, add feature such that if device supports graceful unplug
we enclose entire IOCTL in SRCU critical section.
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
Nope.
The idea of drm_dev_enter/exit is to mark up hw access. Not entire ioctl.
Then I am confused why we have https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Fv5.12%2Fsource%2Fdrivers%2Fgpu%2Fdrm%2Fdrm_ioctl.c%23L826&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Ca0ca5bdab20a4533491c08d91072fe2a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637558908355926609%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=SESZFWQEcQUHGGek8d1cNi9Iwo9XOmXqxg9MieRkxNU%3D&reserved=0
currently in code ?
I forgot about this one, again. Thanks for reminding.
Especially not with an opt-in flag so that it could be shrugged of as a
driver hack. Most of these ioctls should have absolutely no problem
working after hotunplug.
Also, doing this defeats the point since it pretty much guarantees
userspace will die in assert()s and stuff. E.g. on i915 the rough contract
is that only execbuf (and even that only when userspace has indicated
support for non-recoverable hw ctx) is allowed to fail. Anything else
might crash userspace.
Given that as I pointed above we already fail any IOCTls with -ENODEV
when device is unplugged, it seems those crashes don't happen that
often ? Also, in all my testing I don't think I saw a user space crash
I could attribute to this.
I guess it should be ok.
What should be ok ?
Your approach, but not your patch. If we go with this let's just lift it
to drm_ioctl() as the default behavior. No driver opt-in flag, because
that's definitely worse than any other approach because we really need to
get rid of driver specific behaviour for generic ioctls, especially
anything a compositor will use directly.
My reasons for making this work is both less trouble for userspace (did
you test with various wayland compositors out there, not just amdgpu x86
I didn't - will give it a try.
Weston worked without crashes, run the egl tester cube there.
driver?), but also testing.
We still need a bunch of these checks in various places or you'll wait a
very long time for a pending modeset or similar to complete. Being able to
run that code easily after hotunplug has completed should help a lot with
testing.
Plus various drivers already acquired drm_dev_enter/exit and now I wonder
whether that was properly tested or not ...
I guess maybe we need a drm module option to disable this check, so that
we can exercise the code as if the ioctl has raced with hotunplug at the
worst possible moment.
Also atomic is really tricky here: I assume your testing has just done
normal synchronous commits, but anything that goes through atomic can be
done nonblocking in a separate thread. Which the ioctl catch-all here wont
capture.
Yes, async commit was on my mind and thanks for reminding me. Indeed
I forgot this but i planned to scope the entire amdgpu_dm_atomic_tail in
drm_dev_enter/exit. Note that i have a bunch of patches, all name's
starting with 'Scope....' that just methodically put all the background
work items and timers the drivers schedules in drm_dev_enter/exit scope.
This was supposed to be part of the 'Scope Display code' patch.
That's too much. You still have to arrange that the flip completion event
gets sent out. So it's a bit tricky.
In other places the same problem applies, e.g. probe functions need to
make sure they report "disconnected".
I see, well, this is all part of KMS support which I defer for now
anyway. Will tackle it then.
You probably need similar (and very precisely defined) rules for amdgpu.
And those must definitely exclude any shard ioctls from randomly failing
with EIO, because that just kills the box and defeats the point of trying
to gracefully handling hotunplug and making sure userspace has a chance of
survival. E.g. for atomic everything should continue, including flip
completion, but we set all outputs to "disconnected" and send out the
uevent. Maybe crtc enabling can fail too, but that can also be handled
through the async status we're using to signal DP link failures to
userspace.
As I pointed before, because of the complexity of the topic I prefer to
take it step by step and solve first for secondary device use case, not
for primary, display attached device.
Yeah makes sense. But then I think the right patch is to roll this out for
all drivers, properly justified with existing code. Not behind a driver
flag, because with all these different compositors the last thing we want
is a proliferation of driver-specific behaviour. That's imo the worst
option of all of them and needs to be avoided.
So this kind of patch would be acceptable to you if I unconditionally
scope the drm_ioctl with drm_dev_enter/exit without the driver flag ?
I am worried to break other drivers with this, see patch https://nam11.safelinks.protection.outlook.com/?url=https:%2F%2Fcgit.freedesktop.org%2F~agrodzov%2Flinux%2Fcommit%2F%3Fh%3Ddrm-misc-next%26id%3Df0c593f35b22ca5bf60ed9e7ce2bf2b80e6c68c6&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Ca0ca5bdab20a4533491c08d91072fe2a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637558908355926609%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=62f4gdl3lQH0ap58HTyv47zxALjaa5Td%2BysskR83rig%3D&reserved=0
Before setting drm_dev_unplug I go through a whole process of signalling
all possible fences in the system which some one some where might be
waiting on. My concern is that in the absence of HW those fences won't
signal and so unless I signal them myself srcu_synchrionize in
drm_dev_unplug will hang waiting for any such code scoped by
drm_dev_enter/exit.
Uh right. I forgot about this.
Which would kinda mean the top level scope is maybe not the best idea, and
perhaps we should indeed drill it down. But then the testing issue
definitely gets a lot worse.
So what if we'd push that drm_dev_is_unplugged check down into ioctls?
Then we can make a case-by case decision whether it should be converted to
drm_dev_enter/exit, needs to be pushed down further into drivers (due to
fence wait issues) or other concerns?
Also I guess we need to have a subsystem wide rule on whether you need to
force complete all fences before you call drm_dev_unplug, or afterwards.
I don't see how you can handle it afterwards. If a thread is stuck in
dma_fence_wait in non interruptible wait (any kernel thread) and with no
timeout there is nothing you can do to stop the wait. Any such code
scopped with drm_dev_enter/exit will cause a hang in drm_dev_unplug.
The only way then is to preemptively force signal all such fences before
calling drm_dev_unplug - as I do in the above mentioned patch.
Yeah, which is why I don't think top-level drm_dev_enter/exit is a good
idea.
If we have mixed behaviour on this there will be disappointment. And since
hotunplug and dma_fence completion are both userspace visible that
inconsistency might have bigger impact.
This is all very tricky indeed :-/
btw for the "gradual pushing drm_dev_enter into ioctl" approach, if we go
with that: We could do the same trick we've done for DRM_UNLOCKED:
- drm_dev_enter/exit is called for any ioctl that has not set the
DRM_HOTUNPLUG_SAFE flag
- for drm core ioctls we push them into all ioctls and decide how to
handle/where (with the aim to have the least amount of code flow
different during hotunplug vs after hotunplug has finished, to reduce
testing scope)
- then we make DRM_HOTUNPLUG_SAFE the implied default
This would have us left with render ioctls, and I think the defensive
assumption there is that they're all hotunplug safe. We might hang on a
fence wait, but that's fixable, and it's better than blowing up on a
use-after-free security bug.
Thoughts?
I don't fully see a difference between the approach described above and
the full drill down to each driver and even within the driver, to the HW
back-ends - what criteria I would use to decide if for a given IOCTL i
scope with drm_dev_enter/exit at the highest level while for another
i go all the way down ? If we would agree that signaling the fences
preemptively before engaging drm_dev_unplug is generically the right
approach maybe we can then scope drm_ioctl unconditionally with
drm_dev_enter/exit and then for each driver go through the same process
I do for amdgpu - writing driver specific function which takes care of
all the fences. We could then just create a drm callback which would
be called from drm_ioctl before drm_dev_unplug is called.
So I see the appeal of just nuking all the fences, but I'm not sure that's
a good plan. We've done this in the old i915 gpu reset code too, and the
issue is it's defacto inverting the locking. But also the hw is truly
gone, so it also makes sense.
The problem is a bit roll-out, if we state that dma_fence_wait is allowed
with a drm_dev_enter/exit, then all drivers need to force-retire their
fences.
The other option would be that we require that dma_fence_wait is _not_
allowed in drm_dev_enter/exit, and that therefore these areas must be
marked up more fine-grained to avoid deadlocks. I like this more from the
testing aspect (it makes it easier to be reasonable sure your code handles
concurrent hotunplug), but also it's pretty easy to validate with the
dma_fence lockdep annotations we have I think.
They key question as I see it - is it ok for a device to be unplugged
while it's driver has anywhere in it's code a dma_fence_wait
waiting for work completion from this device. The answers seems to me
is no, the HW is gone, this fence will never signal and so you will be
left with indefinitely hanged code thread with all it's unreleased
resources. If i am correct in the above statement then avoiding scoping
code with drm_dev_enter/exit because a dma_fence_wait might be there in
the middle
just hides the problem. Also, then the only solution for each driver
wanting to support hot-unplug is to force retire all it's HW
fences once it's notified of device removal.
A third reasons for not requiring force-retiring of dma_fence before
drm_dev_unplug is the races: Before drm_dev_unplug you haven't stopped new
fences from happening, but until you've stopped new fences it's hard to
guarantee they're all retired. How do you solve this currently.
See amdgpu_finilize_device_fences in
https://patchwork.ozlabs.org/project/linux-pci/patch/20210428151207.1212258-20-andrey.grodzovsky@xxxxxxx/
I think the steps described there answer your
concern here.
Finally there's still hangcheck and all that, so if we go with forbidding
dma_fence_wait from within drm_dev_enter/exit sections, then drivers don't
need to have additional tricky code to force-retire fences. TDR will take
care already (albeit with maybe a slightly annoying long timeout, which
we can shorten to "time out everything immediately" after drm_dev_unplug).
I am not aware of TDR handlers that do it today, at least we don't,
we don't check that if device is gone let's instead of resetting the device
and resubmit jobs just force retire all the HW fences. In any case, this
can and i think should be done in pci remove callback because this is
the place that supposed to handle device extraction. I for example in
amdgpu_finilize_device_fences just block all TDRs from taking place as
first step in the process. If other drivers want to force retire fences
in their TDR handlers they still need to block and wait for all such
TDRs in their pci_remove handler.
What we definitely can't have is half the drivers doing it one way, and
the other half the other way. So your driver flag to wrap the ioctl
optionally in a drm_dev_enter/exit path is a no-go still I think.
I guess my tldr; is: I definitely see how your current approach gives
quicker results for amdgpu right now, but long term I'm seeing more
positives on the other one. At least I expect less special cases due to
hotunplug with that.
As i expressed my viewpoint above - seems to me any driver in need to
support hot-unplug must force retire it's fences because of need to
unblock all dma_fence waits and so it will not be a special case.
Andrey
Cheers, Daniel
Andrey
It is unfortunately even more work until we've reached the goal, but I
think it's safest and most flexible approach overall.
Cheers, Daniel
Andrey
Cheers, Daniel
I guess we should clarify this in the hotunplug doc?
Agree
To clarify: I'm not against throwing an ENODEV at userspace for ioctl that
really make no sense, and where we're rather confident that all properly
implemented userspace will gracefully handle failures. Like a modeset, or
opening a device, or trying to import a dma-buf or stuff like that which
can already fail in normal operation for any kind of reason.
But stuff that never fails, like GETRESOURCES ioctl, really shouldn't fail
after hotunplug.
As I pointed above, this a bit confuses me given that we already do
blanker rejection of IOCTLs if device is unplugged.
Well I'm confused about this too :-/
And then there's the middle ground, like doing a pageflip or buffer flush,
which I guess some userspace might handle, but risky to inflict those
consequences on them. atomic modeset is especially fun since depending
what you're doing it can be both "failures expected" and "failures not
really expected in normal operation".
Also, this really should be consistent across drivers, not solved with a
driver flag for every possible combination.
If you look at the current hotunplug kms drivers, they have
drm_dev_enter/exit sprinkled in specific hw callback functions because of
the above problems. But maybe it makes sense to change things in a few
cases. But then we should do it across the board.
So as I understand your preferred approach is that I scope any back_end, HW
specific function with drm_dev_enter/exit because that where MMIO
access takes place. But besides explicit MMIO access thorough
register accessors in the HW back-end there is also indirect MMIO access
taking place throughout the code in the driver because of various VRAM
BOs which provide CPU access to VRAM through the VRAM BAR. This kind of
access is spread all over in the driver and even in mid-layers such as
TTM and not limited to HW back-end functions. It means it's much harder
to spot such places to surgically scope them with drm_dev_enter/exit and
also that any new such code introduced will immediately break hot unplug
because the developers can't be expected to remember making their code
robust to this specific use case. That why when we discussed internally
what approach to take to protecting code with drm_dev_enter/exit we
opted for using the widest available scope.
The thing is, you kinda have to anyway. There's enormous amounts of
asynchronous processing going on. E.g. nonblocking atomic commits also do
ttm unpinning and fun stuff like that, which if you sync things wrong can
happen way late. So the door for bad fallout is wide open :-(
I'm not sure where the right tradeoff is to make sure we catch them all,
and can make sure with testing that we've indeed caught them all.
-Daniel
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