Quoting Chris Wilson (2019-02-11 15:45:30)
> If we drop the engine lock, we may run execlists_dequeue which may free
> the priolist. Therefore if we ever drop the execution lock on the
> engine, we have to discard our cache and refetch the priolist to ensure
> we do not use a stale pointer.
>
> [ 506.418935] [IGT] gem_exec_whisper: starting subtest contexts-priority
> [ 593.240825] general protection fault: 0000 [#1] SMP
> [ 593.240863] CPU: 1 PID: 494 Comm: gem_exec_whispe Tainted: G U 5.0.0-rc6+ #100
> [ 593.240879] Hardware name: /NUC6CAYB, BIOS AYAPLCEL.86A.0029.2016.1124.1625 11/24/2016
> [ 593.240965] RIP: 0010:__i915_schedule+0x1fe/0x320 [i915]
> [ 593.240981] Code: 48 8b 0c 24 48 89 c3 49 8b 45 28 49 8b 75 20 4c 89 3c 24 48 89 46 08 48 89 30 48 8b 43 08 48 89 4b 08 49 89 5d 20 49 89 45 28 <48> 89 08 45 39 a7 b8 03 00 00 7d 44 45 89 a7 b8 03 00 00 49 8b 85
> [ 593.240999] RSP: 0018:ffffc90000057a60 EFLAGS: 00010046
> [ 593.241013] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8882582d7870 RCX: ffff88826baba6f0
> [ 593.241026] RDX: 0000000000000000 RSI: ffff8882582d6e70 RDI: ffff888273482194
> [ 593.241049] RBP: ffffc90000057a68 R08: ffff8882582d7680 R09: ffff8882582d7840
> [ 593.241068] R10: 0000000000000000 R11: ffffea00095ebe08 R12: 0000000000000728
> [ 593.241105] R13: ffff88826baba6d0 R14: ffffc90000057a40 R15: ffff888273482158
> [ 593.241120] FS: 00007f4613fb3900(0000) GS:ffff888277a80000(0000) knlGS:0000000000000000
> [ 593.241133] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 593.241146] CR2: 00007f57d3c66a84 CR3: 000000026e2b6000 CR4: 00000000001406e0
> [ 593.241158] Call Trace:
> [ 593.241233] i915_schedule+0x1f/0x30 [i915]
> [ 593.241326] i915_request_add+0x1a9/0x290 [i915]
> [ 593.241393] i915_gem_do_execbuffer+0x45f/0x1150 [i915]
> [ 593.241411] ? init_object+0x49/0x80
> [ 593.241425] ? ___slab_alloc.constprop.91+0x4b8/0x4e0
> [ 593.241491] ? i915_gem_execbuffer2_ioctl+0x99/0x380 [i915]
> [ 593.241563] ? i915_gem_execbuffer_ioctl+0x270/0x270 [i915]
> [ 593.241629] i915_gem_execbuffer2_ioctl+0x1bb/0x380 [i915]
> [ 593.241705] ? i915_gem_execbuffer_ioctl+0x270/0x270 [i915]
> [ 593.241724] drm_ioctl_kernel+0x81/0xd0
> [ 593.241738] drm_ioctl+0x1a7/0x310
> [ 593.241803] ? i915_gem_execbuffer_ioctl+0x270/0x270 [i915]
> [ 593.241819] ? __update_load_avg_se+0x1c9/0x240
> [ 593.241834] ? pick_next_entity+0x7e/0x120
> [ 593.241851] do_vfs_ioctl+0x88/0x5d0
> [ 593.241880] ksys_ioctl+0x35/0x70
> [ 593.241894] __x64_sys_ioctl+0x11/0x20
> [ 593.241907] do_syscall_64+0x44/0xf0
> [ 593.241924] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [ 593.241940] RIP: 0033:0x7f4615ffe757
> [ 593.241952] Code: 00 00 90 48 8b 05 39 a7 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 09 a7 0c 00 f7 d8 64 89 01 48
> [ 593.241970] RSP: 002b:00007ffc1030ddf8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [ 593.241984] RAX: ffffffffffffffda RBX: 00007ffc10324420 RCX: 00007f4615ffe757
> [ 593.241997] RDX: 00007ffc1030e220 RSI: 0000000040406469 RDI: 0000000000000003
> [ 593.242010] RBP: 00007ffc1030e220 R08: 00007f46160c9208 R09: 00007f46160c9240
> [ 593.242022] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000040406469
> [ 593.242038] R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
> [ 593.242058] Modules linked in: i915 intel_gtt drm_kms_helper prime_numbers
>
> Fixes: a02eb975be78 ("drm/i915/execlists: Cache the priolist when rescheduling")
> Testcase: igt/gem_exec_whisper/contexts-priority # rare!
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Michał Winiarski <michal.winiarski@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_scheduler.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
> index d01683167c77..177505b9c509 100644
> --- a/drivers/gpu/drm/i915/i915_scheduler.c
> +++ b/drivers/gpu/drm/i915/i915_scheduler.c
> @@ -340,6 +340,8 @@ static void __i915_schedule(struct i915_request *rq,
>
> engine = sched_lock_engine(node, engine);
> lockdep_assert_held(&engine->timeline.lock);
> + if (last != engine) /* if we drop the lock, refetch priolist */
> + last = NULL;
Alternatively (or perhaps future tweaking) would be to pass in a
cache_struct that we memset inside sched_lock_engine(0.
-Chris
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