Re: [PATCH 2/3] drm/i915/gtt: Defer the free for alloc error paths

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Quoting Matthew Auld (2019-07-04 11:28:18)
>> On 03/07/2019 18:19, Chris Wilson wrote:
>> > 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
>> 
>> holding
>> 
>> > later free to defer it until after we drop the lock.
>> > 
>> > <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]
>> > 
>> > Fixes: 1d1b5490b91c ("drm/i915/gtt: Replace struct_mutex serialisation for allocation")
>> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> > Cc: Matthew Auld <matthew.auld@intel.com>
>> > Cc: Mika Kuoppala <mika.kuoppala@intel.com>
>> > ---
>> >   drivers/gpu/drm/i915/i915_gem_gtt.c | 6 ++++--
>> >   1 file changed, 4 insertions(+), 2 deletions(-)
>> > 
>> > diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
>> > index 9e76347e039e..1065753e86fb 100644
>> > --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
>> > +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
>> > @@ -1489,7 +1489,8 @@ static int gen8_ppgtt_alloc_pdp(struct i915_address_space *vm,
>> >               gen8_ppgtt_set_pdpe(pdp, vm->scratch_pd, pdpe);
>> >               GEM_BUG_ON(!atomic_read(&pdp->used));
>> >               atomic_dec(&pdp->used);
>> > -             free_pd(vm, pd);
>> > +             GEM_BUG_ON(alloc);
>> 
>> Pretty sure it's possible for alloc != NULL at this point...two threads, 
>> one is late to the party, we are unlucky and hit the unwind_pd path. No?
>
> Hmm. I was thinking we would only get here on an alloc failure path, but
> yeah, the BUG_ON was a case for doubt.

Am I staring at the wrong spot then. I thought the atomic_inc(&pd_used)
saves us.

-Mika


>
> Drat.
> -Chris
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