Re: [PATCH] drm/shmem-helper: Fix locking for drm_gem_shmem_get_pages_sgt()

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Hi

Am 05.02.23 um 13:51 schrieb Asahi Lina:
Other functions touching shmem->sgt take the pages lock, so do that here

Really? I was just locking at the Lima driver and it apparently access sgt without locking in [1]. Not that I disagree with the patch in general.

Best regards
Thomas

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/lima/lima_gem.c#L21

too. drm_gem_shmem_get_pages() & co take the same lock, so move to the
_locked() variants to avoid recursive locking.

Discovered while auditing locking to write the Rust abstractions.

Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
Fixes: 4fa3d66f132b ("drm/shmem: Do dma_unmap_sg before purging pages")
Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
  drivers/gpu/drm/drm_gem_shmem_helper.c | 54 ++++++++++++++++----------
  1 file changed, 34 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index b602cd72a120..2c559b310cad 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -681,23 +681,7 @@ struct sg_table *drm_gem_shmem_get_sg_table(struct drm_gem_shmem_object *shmem)
  }
  EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);
-/**
- * drm_gem_shmem_get_pages_sgt - Pin pages, dma map them, and return a
- *				 scatter/gather table for a shmem GEM object.
- * @shmem: shmem GEM object
- *
- * This function returns a scatter/gather table suitable for driver usage. If
- * the sg table doesn't exist, the pages are pinned, dma-mapped, and a sg
- * table created.
- *
- * This is the main function for drivers to get at backing storage, and it hides
- * and difference between dma-buf imported and natively allocated objects.
- * drm_gem_shmem_get_sg_table() should not be directly called by drivers.
- *
- * Returns:
- * A pointer to the scatter/gather table of pinned pages or errno on failure.
- */
-struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
+static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_object *shmem)
  {
  	struct drm_gem_object *obj = &shmem->base;
  	int ret;
@@ -708,7 +692,7 @@ struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
WARN_ON(obj->import_attach); - ret = drm_gem_shmem_get_pages(shmem);
+	ret = drm_gem_shmem_get_pages_locked(shmem);
  	if (ret)
  		return ERR_PTR(ret);
@@ -730,10 +714,40 @@ struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
  	sg_free_table(sgt);
  	kfree(sgt);
  err_put_pages:
-	drm_gem_shmem_put_pages(shmem);
+	drm_gem_shmem_put_pages_locked(shmem);
  	return ERR_PTR(ret);
  }
-EXPORT_SYMBOL_GPL(drm_gem_shmem_get_pages_sgt);
+
+/**
+ * drm_gem_shmem_get_pages_sgt - Pin pages, dma map them, and return a
+ *				 scatter/gather table for a shmem GEM object.
+ * @shmem: shmem GEM object
+ *
+ * This function returns a scatter/gather table suitable for driver usage. If
+ * the sg table doesn't exist, the pages are pinned, dma-mapped, and a sg
+ * table created.
+ *
+ * This is the main function for drivers to get at backing storage, and it hides
+ * and difference between dma-buf imported and natively allocated objects.
+ * drm_gem_shmem_get_sg_table() should not be directly called by drivers.
+ *
+ * Returns:
+ * A pointer to the scatter/gather table of pinned pages or errno on failure.
+ */
+struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
+{
+	int ret;
+	struct sg_table *sgt;
+
+	ret = mutex_lock_interruptible(&shmem->pages_lock);
+	if (ret)
+		return ERR_PTR(ret);
+	sgt = drm_gem_shmem_get_pages_sgt_locked(shmem);
+	mutex_unlock(&shmem->pages_lock);
+
+	return sgt;
+}
+EXPORT_SYMBOL(drm_gem_shmem_get_pages_sgt);
/**
   * drm_gem_shmem_prime_import_sg_table - Produce a shmem GEM object from

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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