All pinning must be done prior to i915_request_create, to avoid
timeline->mutex inversions.

Here we slightly abuse the context_barrier_task stages to utilise the
'skip' decision as an opportunity to acquire the pin on the new ppgtt.
Consider it s/skip/prepare/. At the moment, we only have on user of
context_barrier_task, so it might be worth breaking it down for the
specific task of set-vm and refactor it later if we find a second
purpose.

<4> [402.377487] WARNING: possible circular locking dependency detected
<4> [402.377493] 5.4.0-rc8-CI-CI_DRM_7491+ #1 Tainted: G     U
<4> [402.377497] ------------------------------------------------------
<4> [402.377502] gem_exec_parall/2506 is trying to acquire lock:
<4> [402.377507] ffff888403cdac70 (&kernel#2){+.+.}, at: i915_request_create+0x16/0x1c0 [i915]
<4> [402.377593]
but task is already holding lock:
<4> [402.377597] ffff88835efad550 (&ppgtt->pin_mutex){+.+.}, at: gen6_ppgtt_pin+0x4d/0x110 [i915]
<4> [402.377660]
which lock already depends on the new lock.

<4> [402.377664]
the existing dependency chain (in reverse order) is:
<4> [402.377668]
-> #1 (&ppgtt->pin_mutex){+.+.}:
<4> [402.377674]        __mutex_lock+0x9a/0x9d0
<4> [402.377713]        gen6_ppgtt_pin+0x4d/0x110 [i915]
<4> [402.377756]        emit_ppgtt_update+0x1dc/0x370 [i915]
<4> [402.377801]        context_barrier_task+0x176/0x310 [i915]
<4> [402.377844]        ctx_setparam+0x400/0xb10 [i915]
<4> [402.377886]        i915_gem_context_setparam_ioctl+0xc8/0x160 [i915]
<4> [402.377891]        drm_ioctl_kernel+0xa7/0xf0
<4> [402.377895]        drm_ioctl+0x2e1/0x390
<4> [402.377899]        do_vfs_ioctl+0xa0/0x6f0
<4> [402.377903]        ksys_ioctl+0x35/0x60
<4> [402.377906]        __x64_sys_ioctl+0x11/0x20
<4> [402.377910]        do_syscall_64+0x4f/0x210
<4> [402.377914]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
<4> [402.377917]
-> #0 (&kernel#2){+.+.}:
<4> [402.377923]        __lock_acquire+0x1328/0x15d0
<4> [402.377926]        lock_acquire+0xa7/0x1c0
<4> [402.377930]        __mutex_lock+0x9a/0x9d0
<4> [402.377977]        i915_request_create+0x16/0x1c0 [i915]
<4> [402.378013]        intel_engine_flush_barriers+0x4c/0x100 [i915]
<4> [402.378062]        i915_ggtt_pin+0x7d/0x130 [i915]
<4> [402.378108]        gen6_ppgtt_pin+0x9c/0x110 [i915]
<4> [402.378148]        ring_context_pin+0x2e/0xc0 [i915]
<4> [402.378183]        __intel_context_do_pin+0x6b/0x190 [i915]
<4> [402.378226]        i915_gem_do_execbuffer+0x180c/0x26b0 [i915]
<4> [402.378268]        i915_gem_execbuffer2_ioctl+0x11b/0x460 [i915]
<4> [402.378272]        drm_ioctl_kernel+0xa7/0xf0
<4> [402.378275]        drm_ioctl+0x2e1/0x390
<4> [402.378279]        do_vfs_ioctl+0xa0/0x6f0
<4> [402.378282]        ksys_ioctl+0x35/0x60
<4> [402.378286]        __x64_sys_ioctl+0x11/0x20
<4> [402.378289]        do_syscall_64+0x4f/0x210
<4> [402.378292]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
<4> [402.378295]
other info that might help us debug this:

<4> [402.378299]  Possible unsafe locking scenario:

<4> [402.378302]        CPU0                    CPU1
<4> [402.378305]        ----                    ----
<4> [402.378307]   lock(&ppgtt->pin_mutex);
<4> [402.378310]                                lock(&kernel#2);
<4> [402.378314]                                lock(&ppgtt->pin_mutex);
<4> [402.378317]   lock(&kernel#2);
<4> [402.378320]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206105527.1130413-4-chris@chris-wilson.co.uk