Quoting Chris Wilson (2019-07-03 13:32:36)
> If we hit an error while allocating the page tables, we have to unwind
> the incomplete updates, and wish to free the unused pd. However, we are
> not allowed to be hoding the spinlock at that point, and so must use the
> later free to defer it until after we drop the lock.
Just in case it's not obvious why,
<3> [414.363795] BUG: sleeping function called from invalid context at drivers/gpu/drm/i915/i915_gem_gtt.c:472
<3> [414.364167] in_atomic(): 1, irqs_disabled(): 0, pid: 3905, name: i915_selftest
<4> [414.364406] 3 locks held by i915_selftest/3905:
<4> [414.364408] #0: 0000000034fe8aa8 (&dev->mutex){....}, at: device_driver_attach+0x18/0x50
<4> [414.364415] #1: 000000006bd8a560 (&dev->struct_mutex){+.+.}, at: igt_ctx_exec+0xb7/0x410 [i915]
<4> [414.364476] #2: 000000003dfdc766 (&(&pd->lock)->rlock){+.+.}, at: gen8_ppgtt_alloc_pdp+0x448/0x540 [i915]
<3> [414.364529] Preemption disabled at:
<4> [414.364530] [<0000000000000000>] 0x0
<4> [414.364696] CPU: 0 PID: 3905 Comm: i915_selftest Tainted: G U 5.2.0-rc7-CI-CI_DRM_6403+ #1
<4> [414.364698] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.1-0-g8891697-prebuilt.qemu-project.org 04/01/2014
<4> [414.364699] Call Trace:
<4> [414.364704] dump_stack+0x67/0x9b
<4> [414.364708] ___might_sleep+0x167/0x250
<4> [414.364777] vm_free_page+0x24/0xc0 [i915]
<4> [414.364852] free_pd+0xf/0x20 [i915]
<4> [414.364897] gen8_ppgtt_alloc_pdp+0x489/0x540 [i915]
<4> [414.364946] gen8_ppgtt_alloc_4lvl+0x8e/0x2e0 [i915]
<4> [414.364992] ppgtt_bind_vma+0x2e/0x60 [i915]
<4> [414.365039] i915_vma_bind+0xe8/0x2c0 [i915]
<4> [414.365088] __i915_vma_do_pin+0xa1/0xd20 [i915]
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