The i915_vblank_swap() function schedules an automatic buffer swap
upon receipt of the vertical sync interrupt. Such an operation is
lengthy so it can't be allowed to happen in normal interrupt context,
thus the DRM implements this by scheduling the work in a kernel
softirq-scheduled tasklet. In order for the buffer swap to work
safely, the DRM's central lock must be taken, via a call to
drm_lock_take() located in drivers/char/drm/drm_irq.c within the
function drm_locked_tasklet_func(). The lock-taking logic uses a
non-interrupt-blocking spinlock to implement the manipulations needed
to take the lock. This semantic would be safe if all attempts to use
the spinlock only happen from process context. However this buffer
swap happens from softirq context which is really a form of interrupt
context. Thus we have an unsafe situation, in that
drm_locked_tasklet_func() can block on a spinlock already taken by a
thread in process context which will never get scheduled again because
of the blocked softirq tasklet. This wedges the kernel hard.
To trigger this bug, run a dual-head cloned mode configuration which
uses the i915 drm, then execute an opengl application which
synchronizes buffer swaps against the vertical sync interrupt. In my
testing, a lockup always results after running anywhere from 5 minutes
to an hour and a half. I believe dual-head is needed to really
trigger the problem because then the vertical sync interrupt handling
is no longer predictable (due to being interrupt-sourced from two
different heads running at different speeds). This raises the
probability of the tasklet trying to run while the userspace DRI is
doing things to the GPU (and manipulating the DRM lock).
The fix is to change the relevant spinlock semantics to be the
interrupt-blocking form. After this change I am no longer able to
trigger the lockup; the longest test run so far was 20 hours (test
stopped after that point).
Note: I have examined the places where this spinlock is being
employed; all are reasonably short bounded sequences and should be
suitable for interrupts being blocked without impacting overall kernel
interrupt response latency.
Signed-off-by: Mike Isely <isely@pobox.com>
We now always create a drm_ref_object for user objects and this is then the only
things that holds a reference to the user object. This way unreference on will
destroy the user object when the last drm_ref_object goes way.
Debug print fix in drm_release().
Forgotten local variable init in drm_setversion().
Unnecessary put_user() in drm_addmap_ioctl().
ioctl->cmd check broken in drm_ioctl(); workaround.
The data is now in kernel space, copied in/out as appropriate according to the
This results in DRM_COPY_{TO,FROM}_USER going away, and error paths to deal
with those failures. This also means that XFree86 4.2.0 support for i810 DRM
is lost.
As a fallout, replace filp storage with file_priv storage for "unique
identifier of a client" all over the DRM. There is a 1:1 mapping, so this
should be a noop. This could be a minor performance improvement, as everything
on Linux dereferenced filp to get file_priv anyway, while only the mmap ioctls
went the other direction.
Add refcounting of user waiters to the DRM hardware lock, so that we can use the
DRM_LOCK_CONT flag more conservatively.
Also add a kernel waiter refcount that if nonzero transfers the lock for the kernel context,
when it is released. This is useful when waiting for idle and can be used
for very simple fence object driver implementations for the new memory manager.
It also resolves the AIGLX startup deadlock for the sis and the via drivers.
i810, i830 still require that the hardware lock is really taken so the deadlock remains
for those two. I'm not sure about ffb. Anyone familiar with that code?
understandable: preinit -> load postinit -> (removed) presetup ->
firstopen postsetup -> (removed) open_helper -> open prerelease ->
preclose free_filp_priv -> postclose pretakedown -> lastclose
postcleanup -> unload release -> reclaim_buffers_locked version ->
(removed)
postinit and version were replaced with generic code in the Linux DRM
(drivers now set their version numbers and description in the driver
structure, like on BSD). postsetup wasn't used at all. Fixes the savage
hooks for initializing and tearing down mappings at the right times.
Testing involved at least starting X, running glxgears, killing
glxgears, exiting X, and repeating.
Tested on: FreeBSD (g200, g400, r200, r128) Linux (r200, savage4)
Thomas Hellstrom
Mike Isely
Dave Airlie
Jerome Glisse
Kristian Høgsberg
Pekka Paalanen
Eric Anholt
Dave Airlie
Dave Airlie
Thomas Hellstrom
Eric Anholt
Jon Smirl
Felix Kuehling
Jose Fonseca