[Intel-gfx] [PATCH 1/3] drm/i915: Hold an object reference whilst we shrink it
Daniel Vetter
daniel at ffwll.ch
Thu Sep 5 12:16:13 CEST 2013
On Wed, Sep 04, 2013 at 10:45:50AM +0100, Chris Wilson wrote:
> Whilst running the shrinker, we need to hold a reference as we unbind
> the objects, or else we may end up waiting for and retiring requests,
> which in turn may result in this object being freed.
>
> This is very similar to the eviction code which also has to be very
> careful to keep a reference to its objects as it retires and unbinds
> them.
>
> Another similarity, that Ben pointed out, is that as we may call
> retire-requests, the unbound_list is outside of our control. We must
> only process a single element of that list at a time, that is we can not
> rely on the "safe" next pointer being valid after a call to
> i915_vma_unbind().
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
> IP: [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915]
> PGD 758d3067 PUD ac0d6067 PMD 0
> Oops: 0000 [#1] SMP
> Modules linked in: dm_mod snd_hda_codec_realtek iTCO_wdt iTCO_vendor_support pcspkr snd_hda_intel i2c_i801 snd_hda_codec snd_hwdep snd_pcm snd_page_alloc snd_timer snd lpc_ich mfd_core soundcore battery ac option usb_wwan usbserial uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core videodev i915 video button drm_kms_helper drm acpi_cpufreq mperf freq_table
> CPU: 1 PID: 16835 Comm: fbo-maxsize Not tainted 3.11.0-rc7_nightlytop_8fdad4_20130902_+ #7977
> task: ffff8800712106d0 ti: ffff880028e4a000 task.ti: ffff880028e4a000
> RIP: 0010:[<ffffffffa0082892>] [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915]
> RSP: 0018:ffff880028e4b9e8 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff880145734000 RCX: ffff880145735328
> RDX: ffff8801457353fc RSI: 0000000000000000 RDI: ffff88007597cc00
> RBP: ffff88007597cc00 R08: 0000000000000001 R09: ffff88014f257f00
> R10: ffffea0001d65f00 R11: 0000000000bba60b R12: ffff880149e5b000
> R13: ffff880145734001 R14: ffff88007597ccc8 R15: ffff88007597cc00
> FS: 00007ff5bc919740(0000) GS:ffff88014f240000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000008 CR3: 0000000028f4c000 CR4: 00000000001407e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Stack:
> 0000000000000000 ffff88007597cc00 ffff8801440d6840 0000000000000000
> ffff880145734000 ffffffffa007c854 0000000000000010 ffff88007597c900
> 0000000000018000 00000000004a1201 ffff88007597cc60 ffffffffa007d183
> Call Trace:
> [<ffffffffa007c854>] ? i915_vma_unbind+0xe2/0x1d1 [i915]
> [<ffffffffa007d183>] ? __i915_gem_shrink+0xf1/0x162 [i915]
> [<ffffffffa007d2ee>] ? i915_gem_object_get_pages_gtt+0xfa/0x303 [i915]
> [<ffffffffa00795f4>] ? i915_gem_object_get_pages+0x54/0x89 [i915]
> [<ffffffffa007cbda>] ? i915_gem_object_pin+0x238/0x5ce [i915]
> [<ffffffff812cba5f>] ? __sg_page_iter_next+0x2b/0x58
> [<ffffffffa0082056>] ? gen6_ppgtt_insert_entries+0xf2/0x114 [i915]
> [<ffffffffa007fe4b>] ? i915_gem_execbuffer_reserve_vma.isra.13+0x79/0x18d [i915]
> [<ffffffffa008017c>] ? i915_gem_execbuffer_reserve+0x21d/0x347 [i915]
> [<ffffffffa0080bfb>] ? i915_gem_do_execbuffer.isra.17+0x4f3/0xe61 [i915]
> [<ffffffffa00795f4>] ? i915_gem_object_get_pages+0x54/0x89 [i915]
> [<ffffffffa007e405>] ? i915_gem_pwrite_ioctl+0x743/0x7a5 [i915]
> [<ffffffffa0081a46>] ? i915_gem_execbuffer2+0x15e/0x1e4 [i915]
> [<ffffffffa000e20d>] ? drm_ioctl+0x2a5/0x3c4 [drm]
> [<ffffffffa00818e8>] ? i915_gem_execbuffer+0x37f/0x37f [i915]
> [<ffffffff816f64c0>] ? __do_page_fault+0x3ab/0x449
> [<ffffffff810be3da>] ? do_mmap_pgoff+0x2b2/0x341
> [<ffffffff810e49be>] ? vfs_ioctl+0x1e/0x31
> [<ffffffff810e5194>] ? do_vfs_ioctl+0x3ad/0x3ef
> [<ffffffff810e5224>] ? SyS_ioctl+0x4e/0x7e
> [<ffffffff816f88d2>] ? system_call_fastpath+0x16/0x1b
> Code: 52 0c a0 48 c7 c6 22 30 0d a0 31 c0 e8 ef 00 f9 ff bf c6 a7 00 00 e8 90 5d 24 e1 f6 85 13 01 00 00 10 75 44 48 8b 85 18 01 00 00 <8b> 50 08 48 8b 30 49 8b 84 24 88 02 00 00 48 89 c7 48 81 c7 98
> RIP [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915]
> RSP <ffff880028e4b9e8>
> CR2: 0000000000000008
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68171
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: stable at vger.kernel.org
> ---
> drivers/gpu/drm/i915/i915_gem.c | 31 +++++++++++++++++++++++++------
> 1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index fdeecae..c31e0b2 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1695,6 +1695,7 @@ static long
> __i915_gem_shrink(struct drm_i915_private *dev_priv, long target,
> bool purgeable_only)
> {
> + struct list_head still_bound_list;
> struct drm_i915_gem_object *obj, *next;
> long count = 0;
>
> @@ -1709,23 +1710,41 @@ __i915_gem_shrink(struct drm_i915_private *dev_priv, long target,
> }
> }
>
> - list_for_each_entry_safe(obj, next, &dev_priv->mm.bound_list,
> - global_list) {
> + /* As we may completely rewrite the bound list whilst unbinding
> + * (due to retiring requests) we have to strictly process only
> + * one element of the list at the time, and recheck the list
> + * on every iteration.
> + */
> + INIT_LIST_HEAD(&still_bound_list);
> + while (count < target && !list_empty(&dev_priv->mm.bound_list)) {
> struct i915_vma *vma, *v;
>
> + obj = list_first_entry(&dev_priv->mm.bound_list,
> + typeof(*obj), global_list);
> + list_move_tail(&obj->global_list, &still_bound_list);
> +
> if (!i915_gem_object_is_purgeable(obj) && purgeable_only)
> continue;
>
> + /* Hold a reference whilst we unbind this object, as we may
> + * end up waiting for and retiring requests, which may result
> + * in this object being freed.
> + *
> + * Note that the shrinker and eviction is special as they operate
> + * on the inactive lists which reference limbo objects.
> + */
This confused me a bit (imo makes only sense with an s/inactive/active/).
And I think this needs a some clarification to make it clear why we don't need
this same trickery for the unbound list. What about this instead?
/*
* Hold a reference whilst we unbind this object, as we may
* end up waiting for and retiring requests. This might
* release the final reference (held by the active list)
* and result in the object being freed from under us.
* in this object being freed.
*
* Note that only shrinking the bound list is special
* since only active (and hence bound objects) can contain
* such limbo objects.
*/
> + drm_gem_object_reference(&obj->base);
I also think we need a comment here explaining why it is safe to grab a
reference despite that the bound (or unbound list fwiw) doesn't hold a
reference itself. Ok if I add
/*
* Note: Even though the bound list doesn't hold a
* reference to the object we can savely grab one here:
* The final object unreferencing and the bound_list are
* both protected by the dev->struct_mutex and so we won't
* ever be able to observe an object on the bound_list
* with a reference count equals 0.
*/
when applying?
Cheers, Daniel
> +
> list_for_each_entry_safe(vma, v, &obj->vma_list, vma_link)
> if (i915_vma_unbind(vma))
> break;
>
> - if (!i915_gem_object_put_pages(obj)) {
> + if (i915_gem_object_put_pages(obj) == 0)
> count += obj->base.size >> PAGE_SHIFT;
> - if (count >= target)
> - return count;
> - }
> +
> + drm_gem_object_unreference(&obj->base);
> }
> + list_splice(&still_bound_list, &dev_priv->mm.bound_list);
>
> return count;
> }
> --
> 1.8.4.rc3
>
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--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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