[PATCH] drm/i915: Only prune fences after wait-for-all

[PATCH] drm/i915: Only prune fences after wait-for-all

[Chris Wilson]

Currently, we only allow ourselves to prune the fences so long as
all the waits completed (i.e. all the fences we checked were signaled),
and that the reservation snapshot did not change across the wait.
However, if we only waited for a subset of the reservation object, i.e.
just waiting for the last writer to complete as opposed to all readers
as well, then we would erroneously conclude we could prune the fences as
indeed although all of our waits were successful, they did not represent
the totality of the reservation object.

Fixes: e54ca9774777 ("drm/i915: Remove completed fences after a wait")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index a5bd07338b46..e3e52b9a74e9 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -407,7 +407,7 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
 {
 	unsigned int seq = __read_seqcount_begin(&resv->seq);
 	struct dma_fence *excl;
-	bool prune_fences = false;
+	bool prune_fences;
 
 	if (flags & I915_WAIT_ALL) {
 		struct dma_fence **shared;
@@ -432,17 +432,13 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
 		for (; i < count; i++)
 			dma_fence_put(shared[i]);
 		kfree(shared);
-
-		prune_fences = count && timeout >= 0;
 	} else {
 		excl = reservation_object_get_excl_rcu(resv);
 	}
 
-	if (excl && timeout >= 0) {
+	if (excl && timeout >= 0)
 		timeout = i915_gem_object_wait_fence(excl, flags, timeout,
 						     rps_client);
-		prune_fences = timeout >= 0;
-	}
 
 	dma_fence_put(excl);
 
@@ -450,6 +446,7 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
 	 * signaled and that the reservation object has not been changed (i.e.
 	 * no new fences have been added).
 	 */
+	prune_fences = flags & I915_WAIT_ALL && timeout >= 0;
 	if (prune_fences && !__read_seqcount_retry(&resv->seq, seq)) {
 		if (reservation_object_trylock(resv)) {
 			if (!__read_seqcount_retry(&resv->seq, seq))
-- 
2.16.2

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[PATCH v2] drm/i915: Only prune fences after wait-for-all

[Chris Wilson]

Currently, we only allow ourselves to prune the fences so long as
all the waits completed (i.e. all the fences we checked were signaled),
and that the reservation snapshot did not change across the wait.
However, if we only waited for a subset of the reservation object, i.e.
just waiting for the last writer to complete as opposed to all readers
as well, then we would erroneously conclude we could prune the fences as
indeed although all of our waits were successful, they did not represent
the totality of the reservation object.

v2: We only need to check the shared fences due to construction (i.e.
all of the shared fences will be later than the exclusive fence, if
any).

Fixes: e54ca9774777 ("drm/i915: Remove completed fences after a wait")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index a5bd07338b46..9b48b5101357 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -433,20 +433,28 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
 			dma_fence_put(shared[i]);
 		kfree(shared);
 
+		/*
+		 * If both shared fences and an exclusive fence exist,
+		 * then by construction the shared fences must be later
+		 * than the exclusive fence. If we successfully wait for
+		 * all the shared fences, we know that the exclusive fence
+		 * must all be signaled. If all the shared fences are
+		 * signaled, we can prune the array and recover the
+		 * floating references on the fences/requests.
+		 */
 		prune_fences = count && timeout >= 0;
 	} else {
 		excl = reservation_object_get_excl_rcu(resv);
 	}
 
-	if (excl && timeout >= 0) {
+	if (excl && timeout >= 0)
 		timeout = i915_gem_object_wait_fence(excl, flags, timeout,
 						     rps_client);
-		prune_fences = timeout >= 0;
-	}
 
 	dma_fence_put(excl);
 
-	/* Oportunistically prune the fences iff we know they have *all* been
+	/*
+	 * Oportunistically prune the fences iff we know they have *all* been
 	 * signaled and that the reservation object has not been changed (i.e.
 	 * no new fences have been added).
 	 */
-- 
2.16.2

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Re: [PATCH v2] drm/i915: Only prune fences after wait-for-all

[Chris Wilson]

Quoting Chris Wilson (2018-03-07 17:13:03)
> Currently, we only allow ourselves to prune the fences so long as
> all the waits completed (i.e. all the fences we checked were signaled),
> and that the reservation snapshot did not change across the wait.
> However, if we only waited for a subset of the reservation object, i.e.
> just waiting for the last writer to complete as opposed to all readers
> as well, then we would erroneously conclude we could prune the fences as
> indeed although all of our waits were successful, they did not represent
> the totality of the reservation object.
> 
> v2: We only need to check the shared fences due to construction (i.e.
> all of the shared fences will be later than the exclusive fence, if
> any).
> 

Testcase: igt/drv_hangman

As it turns out, that is pretty much the only way we can observe this
bug. (At least not without drinking more than one cup of tea.)

You could construct something to try and overwrite the reader on another
engine, tricky to observe though. However, it is the goal of
selftests/live_coherency and gem_exec_flush to try and catch this type
of error, so should try harder.
-Chris
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✓ Fi.CI.BAT: success for drm/i915: Only prune fences after wait-for-all (rev2)

[Patchwork]

== Series Details ==

Series: drm/i915: Only prune fences after wait-for-all (rev2)
URL   : https://patchwork.freedesktop.org/series/39547/
State : success

== Summary ==

Series 39547v2 drm/i915: Only prune fences after wait-for-all
https://patchwork.freedesktop.org/api/1.0/series/39547/revisions/2/mbox/

---- Known issues:

Test kms_pipe_crc_basic:
        Subgroup suspend-read-crc-pipe-b:
                pass       -> INCOMPLETE (fi-snb-2520m) fdo#103713

fdo#103713 https://bugs.freedesktop.org/show_bug.cgi?id=103713

fi-bdw-5557u     total:288  pass:267  dwarn:0   dfail:0   fail:0   skip:21  time:422s
fi-bdw-gvtdvm    total:288  pass:264  dwarn:0   dfail:0   fail:0   skip:24  time:421s
fi-blb-e6850     total:288  pass:223  dwarn:1   dfail:0   fail:0   skip:64  time:372s
fi-bsw-n3050     total:288  pass:242  dwarn:0   dfail:0   fail:0   skip:46  time:503s
fi-bwr-2160      total:288  pass:183  dwarn:0   dfail:0   fail:0   skip:105 time:279s
fi-bxt-dsi       total:288  pass:258  dwarn:0   dfail:0   fail:0   skip:30  time:488s
fi-bxt-j4205     total:288  pass:259  dwarn:0   dfail:0   fail:0   skip:29  time:492s
fi-byt-j1900     total:288  pass:253  dwarn:0   dfail:0   fail:0   skip:35  time:478s
fi-byt-n2820     total:288  pass:249  dwarn:0   dfail:0   fail:0   skip:39  time:469s
fi-cfl-8700k     total:288  pass:260  dwarn:0   dfail:0   fail:0   skip:28  time:407s
fi-cfl-s2        total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:579s
fi-cfl-u         total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:504s
fi-elk-e7500     total:288  pass:229  dwarn:0   dfail:0   fail:0   skip:59  time:414s
fi-gdg-551       total:288  pass:179  dwarn:0   dfail:0   fail:1   skip:108 time:287s
fi-glk-1         total:288  pass:260  dwarn:0   dfail:0   fail:0   skip:28  time:514s
fi-hsw-4770      total:288  pass:261  dwarn:0   dfail:0   fail:0   skip:27  time:395s
fi-ilk-650       total:288  pass:228  dwarn:0   dfail:0   fail:0   skip:60  time:411s
fi-ivb-3520m     total:288  pass:259  dwarn:0   dfail:0   fail:0   skip:29  time:454s
fi-ivb-3770      total:288  pass:255  dwarn:0   dfail:0   fail:0   skip:33  time:421s
fi-kbl-7500u     total:288  pass:263  dwarn:1   dfail:0   fail:0   skip:24  time:467s
fi-kbl-7567u     total:288  pass:268  dwarn:0   dfail:0   fail:0   skip:20  time:462s
fi-kbl-r         total:288  pass:261  dwarn:0   dfail:0   fail:0   skip:27  time:510s
fi-pnv-d510      total:288  pass:222  dwarn:1   dfail:0   fail:0   skip:65  time:585s
fi-skl-6260u     total:288  pass:268  dwarn:0   dfail:0   fail:0   skip:20  time:425s
fi-skl-6600u     total:288  pass:261  dwarn:0   dfail:0   fail:0   skip:27  time:521s
fi-skl-6700hq    total:288  pass:262  dwarn:0   dfail:0   fail:0   skip:26  time:534s
fi-skl-6700k2    total:288  pass:264  dwarn:0   dfail:0   fail:0   skip:24  time:498s
fi-skl-6770hq    total:288  pass:268  dwarn:0   dfail:0   fail:0   skip:20  time:484s
fi-skl-guc       total:288  pass:260  dwarn:0   dfail:0   fail:0   skip:28  time:425s
fi-skl-gvtdvm    total:288  pass:265  dwarn:0   dfail:0   fail:0   skip:23  time:425s
fi-snb-2520m     total:245  pass:211  dwarn:0   dfail:0   fail:0   skip:33 
fi-snb-2600      total:288  pass:248  dwarn:0   dfail:0   fail:0   skip:40  time:388s

c362488c07f3f59581a479b2f3b82219fb5bfee3 drm-tip: 2018y-03m-07d-15h-23m-59s UTC integration manifest
3f4f3a155ac2 drm/i915: Only prune fences after wait-for-all

== Logs ==

For more details see: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_8261/issues.html
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✓ Fi.CI.IGT: success for drm/i915: Only prune fences after wait-for-all (rev2)

[Patchwork]

== Series Details ==

Series: drm/i915: Only prune fences after wait-for-all (rev2)
URL   : https://patchwork.freedesktop.org/series/39547/
State : success

== Summary ==

---- Known issues:

Test gem_eio:
        Subgroup in-flight-external:
                incomplete -> PASS       (shard-apl) fdo#105341
Test kms_chv_cursor_fail:
        Subgroup pipe-b-256x256-left-edge:
                dmesg-warn -> INCOMPLETE (shard-snb) fdo#105185 +3
Test kms_cursor_crc:
        Subgroup cursor-256x256-suspend:
                fail       -> PASS       (shard-apl) fdo#103375
Test kms_frontbuffer_tracking:
        Subgroup fbc-rgb565-draw-mmap-wc:
                pass       -> FAIL       (shard-apl) fdo#101623 +1
Test kms_rotation_crc:
        Subgroup primary-rotation-180:
                pass       -> FAIL       (shard-snb) fdo#103925
Test kms_sysfs_edid_timing:
                pass       -> WARN       (shard-apl) fdo#100047
Test pm_lpsp:
        Subgroup screens-disabled:
                fail       -> PASS       (shard-hsw) fdo#104941

fdo#105341 https://bugs.freedesktop.org/show_bug.cgi?id=105341
fdo#105185 https://bugs.freedesktop.org/show_bug.cgi?id=105185
fdo#103375 https://bugs.freedesktop.org/show_bug.cgi?id=103375
fdo#101623 https://bugs.freedesktop.org/show_bug.cgi?id=101623
fdo#103925 https://bugs.freedesktop.org/show_bug.cgi?id=103925
fdo#100047 https://bugs.freedesktop.org/show_bug.cgi?id=100047
fdo#104941 https://bugs.freedesktop.org/show_bug.cgi?id=104941

shard-apl        total:3467 pass:1823 dwarn:1   dfail:0   fail:10  skip:1632 time:12136s
shard-hsw        total:3467 pass:1773 dwarn:1   dfail:0   fail:1   skip:1691 time:11607s
shard-snb        total:3432 pass:1350 dwarn:1   dfail:0   fail:2   skip:2078 time:6772s
Blacklisted hosts:
shard-kbl        total:3308 pass:1859 dwarn:2   dfail:0   fail:7   skip:1438 time:8602s

== Logs ==

For more details see: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_8261/shards.html
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Re: [PATCH v2] drm/i915: Only prune fences after wait-for-all

[Matthew Auld]

On 7 March 2018 at 17:13, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> Currently, we only allow ourselves to prune the fences so long as
> all the waits completed (i.e. all the fences we checked were signaled),
> and that the reservation snapshot did not change across the wait.
> However, if we only waited for a subset of the reservation object, i.e.
> just waiting for the last writer to complete as opposed to all readers
> as well, then we would erroneously conclude we could prune the fences as
> indeed although all of our waits were successful, they did not represent
> the totality of the reservation object.
>
> v2: We only need to check the shared fences due to construction (i.e.
> all of the shared fences will be later than the exclusive fence, if
> any).
>
> Fixes: e54ca9774777 ("drm/i915: Remove completed fences after a wait")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index a5bd07338b46..9b48b5101357 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -433,20 +433,28 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
>                         dma_fence_put(shared[i]);
>                 kfree(shared);
>
> +               /*
> +                * If both shared fences and an exclusive fence exist,
> +                * then by construction the shared fences must be later
> +                * than the exclusive fence. If we successfully wait for
> +                * all the shared fences, we know that the exclusive fence
> +                * must all be signaled. If all the shared fences are
> +                * signaled, we can prune the array and recover the
> +                * floating references on the fences/requests.
> +                */
>                 prune_fences = count && timeout >= 0;
>         } else {
>                 excl = reservation_object_get_excl_rcu(resv);
>         }
>
> -       if (excl && timeout >= 0) {
> +       if (excl && timeout >= 0)
>                 timeout = i915_gem_object_wait_fence(excl, flags, timeout,
>                                                      rps_client);
> -               prune_fences = timeout >= 0;
> -       }
>
>         dma_fence_put(excl);
>
> -       /* Oportunistically prune the fences iff we know they have *all* been
> +       /*
> +        * Oportunistically prune the fences iff we know they have *all* been
Opportunistically

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
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Re: [PATCH v2] drm/i915: Only prune fences after wait-for-all

[Chris Wilson]

Quoting Matthew Auld (2018-03-08 17:48:48)
> On 7 March 2018 at 17:13, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > Currently, we only allow ourselves to prune the fences so long as
> > all the waits completed (i.e. all the fences we checked were signaled),
> > and that the reservation snapshot did not change across the wait.
> > However, if we only waited for a subset of the reservation object, i.e.
> > just waiting for the last writer to complete as opposed to all readers
> > as well, then we would erroneously conclude we could prune the fences as
> > indeed although all of our waits were successful, they did not represent
> > the totality of the reservation object.
> >
> > v2: We only need to check the shared fences due to construction (i.e.
> > all of the shared fences will be later than the exclusive fence, if
> > any).
> >
> > Fixes: e54ca9774777 ("drm/i915: Remove completed fences after a wait")
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > Cc: Matthew Auld <matthew.auld@intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index a5bd07338b46..9b48b5101357 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -433,20 +433,28 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
> >                         dma_fence_put(shared[i]);
> >                 kfree(shared);
> >
> > +               /*
> > +                * If both shared fences and an exclusive fence exist,
> > +                * then by construction the shared fences must be later
> > +                * than the exclusive fence. If we successfully wait for
> > +                * all the shared fences, we know that the exclusive fence
> > +                * must all be signaled. If all the shared fences are
> > +                * signaled, we can prune the array and recover the
> > +                * floating references on the fences/requests.
> > +                */
> >                 prune_fences = count && timeout >= 0;
> >         } else {
> >                 excl = reservation_object_get_excl_rcu(resv);
> >         }
> >
> > -       if (excl && timeout >= 0) {
> > +       if (excl && timeout >= 0)
> >                 timeout = i915_gem_object_wait_fence(excl, flags, timeout,
> >                                                      rps_client);
> > -               prune_fences = timeout >= 0;
> > -       }
> >
> >         dma_fence_put(excl);
> >
> > -       /* Oportunistically prune the fences iff we know they have *all* been
> > +       /*
> > +        * Oportunistically prune the fences iff we know they have *all* been
> Opportunistically
> 
> Reviewed-by: Matthew Auld <matthew.auld@intel.com>

And a bonus spelling fix, thank you!
Pushed,
-Chris
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