[Bug 112385] [CI][BAT]igt at i915_selftest@live_gem_contexts - incomplete - Unexpected fault, Addr: 0x00000000_3ffc9000, Address space: PPGTT

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=112385

Chris Wilson <chris at chris-wilson.co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #2 from Chris Wilson <chris at chris-wilson.co.uk> ---
The ppGTT fault is worrying, and despite  my initial concern, does not lead to
the incomplete.

commit 331bf90591573dfe6c8e892239713ef9702f1396
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Mon Nov 25 09:43:18 2019 +0000

    drm/i915/gt: Mark the execlists->active as the primary volatile access

    Since we want to do a lockless read of the current active request, and
    that request is written to by process_csb also without serialisation, we
    need to instruct gcc to take care in reading the pointer itself.

    Otherwise, we have observed execlists_active() to report 0x40.

    [ 2400.760381] igt/para-4098    1..s. 2376479300us : process_csb: rcs0
cs-irq head=3, tail=4
    [ 2400.760826] igt/para-4098    1..s. 2376479303us : process_csb: rcs0
csb[4]: status=0x00000001:0x00000000
    [ 2400.761271] igt/para-4098    1..s. 2376479306us : trace_ports: rcs0:
promote { b9c59:2622, b9c55:2624 }
    [ 2400.761726] igt/para-4097    0d... 2376479311us : __i915_schedule: rcs0:
-2147483648->3, inflight:0000000000000040, rq:ffff888208c1e940

    which is impossible!

    The answer is that as we keep the existing execlists->active pointing
    into the array as we copy over that array, the unserialised read may see
    a partial pointer value.

    Fixes: df403069029d ("drm/i915/execlists: Lift process_csb() out of the
irq-off spinlock")
    Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
    Reviewed-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>
    Link:
https://patchwork.freedesktop.org/patch/msgid/20191125094318.1630806-1-chris@chris-wilson.co.uk

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