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From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: intel-gfx@lists.freedesktop.org
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>, Ben Widawsky <ben@bwidawsk.net>
Subject: [PATCH 2/3] drm/i915: close PM interrupt masking races in the rps work func
Date: Sun,  4 Sep 2011 17:35:01 +0200
Message-ID: <1315150502-12537-3-git-send-email-daniel.vetter@ffwll.ch> (raw)
In-Reply-To: <1315150502-12537-1-git-send-email-daniel.vetter@ffwll.ch>

This patch closes the following race:

We get a PM interrupt A, mask it, set dev_priv->iir = PM_A and kick of the
work item. Scheduler isn't grumpy, so the work queue takes rps_lock,
grabs pm_iir = dev_priv->pm_iir and pm_imr = READ(PMIMR). Note that
pm_imr == pm_iir because we've just masked the interrupt we've got.

Now hw sends out PM interrupt B (not masked), we process it and mask
it.  Later on the irq handler also clears PMIIR.

Then the work item proceeds and at the end clears PMIMR. Because
(local) pm_imr == pm_iir we have
        pm_imr & ~pm_iir == 0
so all interrupts are enabled.

Hardware is still interrupt-happy, and sends out a new PM interrupt B.
PMIMR doesn't mask B (it does not mask anything), PMIIR is cleared, so
we get it and hit the WARN in the interrupt handler (because
dev_priv->pm_iir == PM_B).

That's why I've moved the
        WRITE(PMIMR, 0)
up under the protection of the rps_lock. And write an uncoditional 0
to PMIMR, because that's what we'll do anyway.

This races looks much more likely because we can arbitrarily extend
the window by grabing dev->struct mutex right after the irq handler
has processed the first PM_B interrupt.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/i915/i915_irq.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 2fdd9f9..21ebcbd 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -383,6 +383,7 @@ static void gen6_pm_rps_work(struct work_struct *work)
 	pm_iir = dev_priv->pm_iir;
 	dev_priv->pm_iir = 0;
 	pm_imr = I915_READ(GEN6_PMIMR);
+	I915_WRITE(GEN6_PMIMR, 0);
 	spin_unlock_irq(&dev_priv->rps_lock);
 
 	if (!pm_iir)
@@ -420,7 +421,6 @@ static void gen6_pm_rps_work(struct work_struct *work)
 	 * an *extremely* unlikely race with gen6_rps_enable() that is prevented
 	 * by holding struct_mutex for the duration of the write.
 	 */
-	I915_WRITE(GEN6_PMIMR, pm_imr & ~pm_iir);
 	mutex_unlock(&dev_priv->dev->struct_mutex);
 }
 
-- 
1.7.6

  parent reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-09-04  3:24 [PATCH] drm/i915: Fix rps irq warning Ben Widawsky
2011-09-04  9:03 ` Chris Wilson
2011-09-04 15:49   ` Ben Widawsky
2011-09-04 15:34     ` [PATCH 0/3] slaughter rps races some more Daniel Vetter
2011-09-04 15:35       ` [PATCH 1/3] drm/i915: close PM interrupt masking races in the irq handler Daniel Vetter
2011-09-04 17:09         ` Ben Widawsky
2011-09-04 15:35       ` Daniel Vetter [this message]
2011-09-04 17:08         ` [PATCH 2/3] drm/i915: close PM interrupt masking races in the rps work func Ben Widawsky
2011-09-04 19:26           ` Daniel Vetter
2011-09-04 19:56             ` Ben Widawsky
2011-09-04 20:10               ` Daniel Vetter
2011-09-04 21:38                 ` Ben Widawsky
2011-09-05  6:38                   ` Daniel Vetter
2011-09-05  6:51                     ` Ben Widawsky
2011-09-05 12:15                       ` Chris Wilson
2011-09-04 15:35       ` [PATCH 3/3] drm/i915: close rps work vs. rps disable races Daniel Vetter
2011-09-04 17:23         ` Ben Widawsky
2011-09-04 19:17           ` Daniel Vetter
2011-09-04 19:50             ` Ben Widawsky
2011-09-04 19:57               ` Daniel Vetter
2011-09-05  8:15                 ` [PATCH] drm/i915: properly cancel rps_work on module unload Daniel Vetter
2011-09-05 17:27                   ` Ben Widawsky
2011-09-08 12:00 [PATCH 1/3] drm/i915: close PM interrupt masking races in the irq handler Daniel Vetter
2011-09-08 12:00 ` [PATCH 2/3] drm/i915: close PM interrupt masking races in the rps work func Daniel Vetter
2011-09-08 15:19   ` Ben Widawsky

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