linux-kernel - [PATCH] drm/msm: Fix pagetables setup/teardown serialization

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Message-ID: <20250717151931.10228-1-robin.clark@oss.qualcomm.com>
Date: Thu, 17 Jul 2025 08:19:30 -0700
From: Rob Clark <robin.clark@....qualcomm.com>
To: dri-devel@...ts.freedesktop.org
Cc: linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org,
        Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/msm: Fix pagetables setup/teardown serialization

An atomic counter is not sufficient, as one task could still be in the
process of tearing things down while another task increments the counter
back up to one and begins setup again.  The race condition existed since
commit b145c6e65eb0 ("drm/msm: Add support to create a local pagetable")
but got bigger in commit dbbde63c9e9d ("drm/msm: Add PRR support").

Fixes: dbbde63c9e9d ("drm/msm: Add PRR support")
Fixes: b145c6e65eb0 ("drm/msm: Add support to create a local pagetable")
Signed-off-by: Rob Clark <robin.clark@....qualcomm.com>
---
 drivers/gpu/drm/msm/msm_iommu.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
index cefa50192391..89cdda73711a 100644
--- a/drivers/gpu/drm/msm/msm_iommu.c
+++ b/drivers/gpu/drm/msm/msm_iommu.c
@@ -14,7 +14,9 @@
 struct msm_iommu {
 	struct msm_mmu base;
 	struct iommu_domain *domain;
-	atomic_t pagetables;
+
+	struct mutex init_lock;  /* protects pagetables counter and prr_page */
+	int pagetables;
 	struct page *prr_page;
 
 	struct kmem_cache *pt_cache;
@@ -227,7 +229,8 @@ static void msm_iommu_pagetable_destroy(struct msm_mmu *mmu)
 	 * If this is the last attached pagetable for the parent,
 	 * disable TTBR0 in the arm-smmu driver
 	 */
-	if (atomic_dec_return(&iommu->pagetables) == 0) {
+	mutex_lock(&iommu->init_lock);
+	if (--iommu->pagetables == 0) {
 		adreno_smmu->set_ttbr0_cfg(adreno_smmu->cookie, NULL);
 
 		if (adreno_smmu->set_prr_bit) {
@@ -236,6 +239,7 @@ static void msm_iommu_pagetable_destroy(struct msm_mmu *mmu)
 			iommu->prr_page = NULL;
 		}
 	}
+	mutex_unlock(&iommu->init_lock);
 
 	free_io_pgtable_ops(pagetable->pgtbl_ops);
 	kfree(pagetable);
@@ -568,9 +572,12 @@ struct msm_mmu *msm_iommu_pagetable_create(struct msm_mmu *parent, bool kernel_m
 	 * If this is the first pagetable that we've allocated, send it back to
 	 * the arm-smmu driver as a trigger to set up TTBR0
 	 */
-	if (atomic_inc_return(&iommu->pagetables) == 1) {
+	mutex_lock(&iommu->init_lock);
+	if (iommu->pagetables++ == 0) {
 		ret = adreno_smmu->set_ttbr0_cfg(adreno_smmu->cookie, &ttbr0_cfg);
 		if (ret) {
+			iommu->pagetables--;
+			mutex_unlock(&iommu->init_lock);
 			free_io_pgtable_ops(pagetable->pgtbl_ops);
 			kfree(pagetable);
 			return ERR_PTR(ret);
@@ -595,6 +602,7 @@ struct msm_mmu *msm_iommu_pagetable_create(struct msm_mmu *parent, bool kernel_m
 			adreno_smmu->set_prr_bit(adreno_smmu->cookie, true);
 		}
 	}
+	mutex_unlock(&iommu->init_lock);
 
 	/* Needed later for TLB flush */
 	pagetable->parent = parent;
@@ -730,7 +738,7 @@ struct msm_mmu *msm_iommu_new(struct device *dev, unsigned long quirks)
 	iommu->domain = domain;
 	msm_mmu_init(&iommu->base, dev, &funcs, MSM_MMU_IOMMU);
 
-	atomic_set(&iommu->pagetables, 0);
+	mutex_init(&iommu->init_lock);
 
 	ret = iommu_attach_device(iommu->domain, dev);
 	if (ret) {
-- 
2.50.1