[Intel-gfx] [CI] drm/i915/gem: Delay tracking the GEM context until it is registered

[Chris Wilson]

Avoid exposing a partially constructed context by deferring the
list_add() from the initial construction to the end of registration.
Otherwise, if we peek into the list of contexts from inside debugfs, we
may see the partially constructed context and chase down some dangling
incomplete pointers.

Reported-by: CQ Tang <cq.t...@intel.com>
Fixes: 3aa9945a528e ("drm/i915: Separate GEM context construction and 
registration to userspace")
References: f6e8aa387171 ("drm/i915: Report the number of closed vma held by 
each context in debugfs")
Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
Cc: CQ Tang <cq.t...@intel.com>
Cc: <sta...@vger.kernel.org> # v5.2+
Reviewed-by: Mika Kuoppala <mika.kuopp...@linux.intel.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_context.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c 
b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index d0bdb6d447ed..efc4ba34c06e 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -713,6 +713,7 @@ __create_context(struct drm_i915_private *i915)
        ctx->i915 = i915;
        ctx->sched.priority = I915_USER_PRIORITY(I915_PRIORITY_NORMAL);
        mutex_init(&ctx->mutex);
+       INIT_LIST_HEAD(&ctx->link);
 
        spin_lock_init(&ctx->stale.lock);
        INIT_LIST_HEAD(&ctx->stale.engines);
@@ -740,10 +741,6 @@ __create_context(struct drm_i915_private *i915)
        for (i = 0; i < ARRAY_SIZE(ctx->hang_timestamp); i++)
                ctx->hang_timestamp[i] = jiffies - CONTEXT_FAST_HANG_JIFFIES;
 
-       spin_lock(&i915->gem.contexts.lock);
-       list_add_tail(&ctx->link, &i915->gem.contexts.list);
-       spin_unlock(&i915->gem.contexts.lock);
-
        return ctx;
 
 err_free:
@@ -931,6 +928,7 @@ static int gem_context_register(struct i915_gem_context 
*ctx,
                                struct drm_i915_file_private *fpriv,
                                u32 *id)
 {
+       struct drm_i915_private *i915 = ctx->i915;
        struct i915_address_space *vm;
        int ret;
 
@@ -949,8 +947,16 @@ static int gem_context_register(struct i915_gem_context 
*ctx,
        /* And finally expose ourselves to userspace via the idr */
        ret = xa_alloc(&fpriv->context_xa, id, ctx, xa_limit_32b, GFP_KERNEL);
        if (ret)
-               put_pid(fetch_and_zero(&ctx->pid));
+               goto err_pid;
+
+       spin_lock(&i915->gem.contexts.lock);
+       list_add_tail(&ctx->link, &i915->gem.contexts.list);
+       spin_unlock(&i915->gem.contexts.lock);
+
+       return 0;
 
+err_pid:
+       put_pid(fetch_and_zero(&ctx->pid));
        return ret;
 }
 
-- 
2.20.1

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