This is a note to let you know that I've just added the patch titled

drm/i915: Fix ref->mutex deadlock in i915_active_wait()

to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summ...

The filename of the patch is: drm-i915-fix-ref-mutex-deadlock-in-i915_active_wait.patch and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From sultan@kerneltoast.com Fri Apr 10 11:07:34 2020

From: Sultan Alsawaf sultan@kerneltoast.com Date: Tue, 7 Apr 2020 00:18:09 -0700 Subject: drm/i915: Fix ref->mutex deadlock in i915_active_wait() To: Greg KH greg@kroah.com Cc: stable@vger.kernel.org, Jani Nikula jani.nikula@linux.intel.com, Joonas Lahtinen joonas.lahtinen@linux.intel.com, Rodrigo Vivi rodrigo.vivi@intel.com, David Airlie airlied@linux.ie, Daniel Vetter daniel@ffwll.ch, Chris Wilson chris@chris-wilson.co.uk, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Sultan Alsawaf sultan@kerneltoast.com Message-ID: 20200407071809.3148-1-sultan@kerneltoast.com

From: Sultan Alsawaf sultan@kerneltoast.com

The following deadlock exists in i915_active_wait() due to a double lock on ref->mutex (call chain listed in order from top to bottom): i915_active_wait(); mutex_lock_interruptible(&ref->mutex); <-- ref->mutex first acquired i915_active_request_retire(); node_retire(); active_retire(); mutex_lock_nested(&ref->mutex, SINGLE_DEPTH_NESTING); <-- DEADLOCK

Fix the deadlock by skipping the second ref->mutex lock when active_retire() is called through i915_active_request_retire().

Note that this bug only affects 5.4 and has since been fixed in 5.5. Normally, a backport of the fix from 5.5 would be in order, but the patch set that fixes this deadlock involves massive changes that are neither feasible nor desirable for backporting [1][2][3]. Therefore, this small patch was made to address the deadlock specifically for 5.4.

[1] 274cbf20fd10 ("drm/i915: Push the i915_active.retire into a worker") [2] 093b92287363 ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree") [3] 750bde2fd4ff ("drm/i915: Serialise with remote retirement")

Fixes: 12c255b5dad1 ("drm/i915: Provide an i915_active.acquire callback") Cc: stable@vger.kernel.org # 5.4.x Signed-off-by: Sultan Alsawaf sultan@kerneltoast.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- drivers/gpu/drm/i915/i915_active.c | 27 +++++++++++++++++++++++---- drivers/gpu/drm/i915/i915_active.h | 4 ++-- 2 files changed, 25 insertions(+), 6 deletions(-)

--- a/drivers/gpu/drm/i915/i915_active.c +++ b/drivers/gpu/drm/i915/i915_active.c @@ -120,13 +120,17 @@ static inline void debug_active_assert(s

#endif

+#define I915_ACTIVE_RETIRE_NOLOCK BIT(0) + static void __active_retire(struct i915_active *ref) { struct active_node *it, *n; struct rb_root root; bool retire = false; + unsigned long bits;

+ ref = ptr_unpack_bits(ref, &bits, 2); lockdep_assert_held(&ref->mutex);

/* return the unused nodes to our slabcache -- flushing the allocator */ @@ -138,7 +142,8 @@ __active_retire(struct i915_active *ref) retire = true; }

- mutex_unlock(&ref->mutex); + if (!(bits & I915_ACTIVE_RETIRE_NOLOCK)) + mutex_unlock(&ref->mutex); if (!retire) return;

@@ -155,13 +160,18 @@ __active_retire(struct i915_active *ref) static void active_retire(struct i915_active *ref) { + struct i915_active *ref_packed = ref; + unsigned long bits; + + ref = ptr_unpack_bits(ref, &bits, 2); GEM_BUG_ON(!atomic_read(&ref->count)); if (atomic_add_unless(&ref->count, -1, 1)) return;

/* One active may be flushed from inside the acquire of another */ - mutex_lock_nested(&ref->mutex, SINGLE_DEPTH_NESTING); - __active_retire(ref); + if (!(bits & I915_ACTIVE_RETIRE_NOLOCK)) + mutex_lock_nested(&ref->mutex, SINGLE_DEPTH_NESTING); + __active_retire(ref_packed); }

static void @@ -170,6 +180,14 @@ node_retire(struct i915_active_request * active_retire(node_from_active(base)->ref); }

+static void +node_retire_nolock(struct i915_active_request *base, struct i915_request *rq) +{ + struct i915_active *ref = node_from_active(base)->ref; + + active_retire(ptr_pack_bits(ref, I915_ACTIVE_RETIRE_NOLOCK, 2)); +} + static struct i915_active_request * active_instance(struct i915_active *ref, struct intel_timeline *tl) { @@ -421,7 +439,8 @@ int i915_active_wait(struct i915_active break; }

- err = i915_active_request_retire(&it->base, BKL(ref)); + err = i915_active_request_retire(&it->base, BKL(ref), + node_retire_nolock); if (err) break; } --- a/drivers/gpu/drm/i915/i915_active.h +++ b/drivers/gpu/drm/i915/i915_active.h @@ -309,7 +309,7 @@ i915_active_request_isset(const struct i */ static inline int __must_check i915_active_request_retire(struct i915_active_request *active, - struct mutex *mutex) + struct mutex *mutex, i915_active_retire_fn retire) { struct i915_request *request; long ret; @@ -327,7 +327,7 @@ i915_active_request_retire(struct i915_a list_del_init(&active->link); RCU_INIT_POINTER(active->request, NULL);

- active->retire(active, request); + retire(active, request);

return 0; }

Patches currently in stable-queue which might be from sultan@kerneltoast.com are

queue-5.4/drm-i915-fix-ref-mutex-deadlock-in-i915_active_wait.patch