Two in one go:
- it is allowed to call dma_fence_wait() while holding a
dma_resv_lock(). This is fundamental to how eviction works with ttm,
so required.
- it is allowed to call dma_fence_wait() from memory reclaim contexts,
specifically from shrinker callbacks (which i915 does), and from mmu
notifier callbacks (which amdgpu does, and which i915 sometimes also
does, and probably always should, but that's kinda a debate). Also
for stuff like HMM we really need to be able to do this, or things
get real dicey.
Consequence is that any critical path necessary to get to a
dma_fence_signal for a fence must never a) call dma_resv_lock nor b)
allocate memory with GFP_KERNEL. Also by implication of
dma_resv_lock(), no userspace faulting allowed. That's some supremely
obnoxious limitations, which is why we need to sprinkle the right
annotations to all relevant paths.
The one big locking context we're leaving out here is mmu notifiers,
added in
commit 23b68395c7c78a764e8963fc15a7cfd318bf187f
Author: Daniel Vetter <daniel.vetter@xxxxxxxx>
Date: Mon Aug 26 22:14:21 2019 +0200
mm/mmu_notifiers: add a lockdep map for invalidate_range_start/end
that one covers a lot of other callsites, and it's also allowed to
wait on dma-fences from mmu notifiers. But there's no ready-made
functions exposed to prime this, so I've left it out for now.
v2: Also track against mmu notifier context.
v3: kerneldoc to spec the cross-driver contract. Note that currently
i915 throws in a hard-coded 10s timeout on foreign fences (not sure
why that was done, but it's there), which is why that rule is worded
with SHOULD instead of MUST.
Also some of the mmu_notifier/shrinker rules might surprise SoC
drivers, I haven't fully audited them all. Which is infeasible anyway,
we'll need to run them with lockdep and dma-fence annotations and see
what goes boom.
v4: A spelling fix from Mika
v5: #ifdef for CONFIG_MMU_NOTIFIER. Reported by 0day. Unfortunately
this means lockdep enforcement is slightly inconsistent, it won't spot
GFP_NOIO and GFP_NOFS allocations in the wrong spot if
CONFIG_MMU_NOTIFIER is disabled in the kernel config. Oh well.
v5: Note that only drivers/gpu has a reasonable (or at least
historical) excuse to use dma_fence_wait() from shrinker and mmu
notifier callbacks. Everyone else should either have a better memory
manager model, or better hardware. This reflects discussions with
Jason Gunthorpe.
Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Cc: Felix Kuehling <Felix.Kuehling@xxxxxxx>
Cc: kernel test robot <lkp@xxxxxxxxx>
Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxx> (v4)
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
Cc: Thomas Hellstrom <thomas.hellstrom@xxxxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
Cc: linux-rdma@xxxxxxxxxxxxxxx
Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
Documentation/driver-api/dma-buf.rst | 6 ++++
drivers/dma-buf/dma-fence.c | 46 ++++++++++++++++++++++++++++
drivers/dma-buf/dma-resv.c | 8 +++++
include/linux/dma-fence.h | 1 +
4 files changed, 61 insertions(+)
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 05d856131140..f8f6decde359 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -133,6 +133,12 @@ DMA Fences
.. kernel-doc:: drivers/dma-buf/dma-fence.c
:doc: DMA fences overview
+DMA Fence Cross-Driver Contract
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-fence.c
+ :doc: fence cross-driver contract
+
DMA Fence Signalling Annotations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 0005bc002529..af1d8ea926b3 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -64,6 +64,52 @@ static atomic64_t dma_fence_context_counter = ATOMIC64_INIT(1);
* &dma_buf.resv pointer.
*/
+/**
+ * DOC: fence cross-driver contract
+ *
+ * Since &dma_fence provide a cross driver contract, all drivers must follow the
+ * same rules:
+ *
+ * * Fences must complete in a reasonable time. Fences which represent kernels
+ * and shaders submitted by userspace, which could run forever, must be backed
+ * up by timeout and gpu hang recovery code. Minimally that code must prevent
+ * further command submission and force complete all in-flight fences, e.g.
+ * when the driver or hardware do not support gpu reset, or if the gpu reset
+ * failed for some reason. Ideally the driver supports gpu recovery which only
+ * affects the offending userspace context, and no other userspace
+ * submissions.
+ *
+ * * Drivers may have different ideas of what completion within a reasonable
+ * time means. Some hang recovery code uses a fixed timeout, others a mix
+ * between observing forward progress and increasingly strict timeouts.
+ * Drivers should not try to second guess timeout handling of fences from
+ * other drivers.
+ *
+ * * To ensure there's no deadlocks of dma_fence_wait() against other locks
+ * drivers should annotate all code required to reach dma_fence_signal(),
+ * which completes the fences, with dma_fence_begin_signalling() and
+ * dma_fence_end_signalling().
+ *
+ * * Drivers are allowed to call dma_fence_wait() while holding dma_resv_lock().
+ * This means any code required for fence completion cannot acquire a
+ * &dma_resv lock. Note that this also pulls in the entire established
+ * locking hierarchy around dma_resv_lock() and dma_resv_unlock().
+ *
+ * * Drivers are allowed to call dma_fence_wait() from their &shrinker
+ * callbacks. This means any code required for fence completion cannot
+ * allocate memory with GFP_KERNEL.
+ *
+ * * Drivers are allowed to call dma_fence_wait() from their &mmu_notifier
+ * respectively &mmu_interval_notifier callbacks. This means any code required
+ * for fence completeion cannot allocate memory with GFP_NOFS or GFP_NOIO.
+ * Only GFP_ATOMIC is permissible, which might fail.
+ *
+ * Note that only GPU drivers have a reasonable excuse for both requiring
+ * &mmu_interval_notifier and &shrinker callbacks at the same time as having to
+ * track asynchronous compute work using &dma_fence. No driver outside of
+ * drivers/gpu should ever call dma_fence_wait() in such contexts.
+ */
+
static const char *dma_fence_stub_get_name(struct dma_fence *fence)
{
return "stub";
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index e7d7197d48ce..0e6675ec1d11 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -36,6 +36,7 @@
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/sched/mm.h>
+#include <linux/mmu_notifier.h>
/**
* DOC: Reservation Object Overview
@@ -116,6 +117,13 @@ static int __init dma_resv_lockdep(void)
if (ret == -EDEADLK)
dma_resv_lock_slow(&obj, &ctx);
fs_reclaim_acquire(GFP_KERNEL);
+#ifdef CONFIG_MMU_NOTIFIER
+ lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
+ __dma_fence_might_wait();
+ lock_map_release(&__mmu_notifier_invalidate_range_start_map);
+#else
+ __dma_fence_might_wait();
+#endif
fs_reclaim_release(GFP_KERNEL);
ww_mutex_unlock(&obj.lock);
ww_acquire_fini(&ctx);
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 3f288f7db2ef..09e23adb351d 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -360,6 +360,7 @@ dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep)
#ifdef CONFIG_LOCKDEP
bool dma_fence_begin_signalling(void);
void dma_fence_end_signalling(bool cookie);
+void __dma_fence_might_wait(void);
#else
static inline bool dma_fence_begin_signalling(void)
{
--
2.27.0
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