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:

> 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.
>
> <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>

From another thread,

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.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);
> +		alloc = pd; /* defer the free to after the lock */
>  	}
>  	spin_unlock(&pdp->lock);
>  unwind:
> @@ -1558,7 +1559,8 @@ static int gen8_ppgtt_alloc_4lvl(struct i915_address_space *vm,
>  	spin_lock(&pml4->lock);
>  	if (atomic_dec_and_test(&pdp->used)) {
>  		gen8_ppgtt_set_pml4e(pml4, vm->scratch_pdp, pml4e);
> -		free_pd(vm, pdp);
> +		GEM_BUG_ON(alloc);
> +		alloc = pdp; /* defer the free until after the lock */
>  	}
>  	spin_unlock(&pml4->lock);
>  unwind:
> -- 
> 2.20.1
>
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