Ever since 0eafec6d3244 ("drm/i915: Enable lockless lookup of request tracking via RCU"), the i915 driver has used SLAB_TYPESAFE_BY_RCU (it was called SLAB_DESTROY_BY_RCU at the time) in order to allow RCU on i915_request. As nifty as SLAB_TYPESAFE_BY_RCU may be, it comes with some serious disclaimers. In particular, objects can get recycled while RCU readers are still in-flight. This can be ok if everyone who touches these objects knows about the disclaimers and is careful. However, because we've chosen to use SLAB_TYPESAFE_BY_RCU for i915_request and because i915_request contains a dma_fence, we've leaked SLAB_TYPESAFE_BY_RCU and its whole pile of disclaimers to every driver in the kernel which may consume a dma_fence.
We've tried to keep it somewhat contained by doing most of the hard work to prevent access of recycled objects via dma_fence_get_rcu_safe(). However, a quick grep of kernel sources says that, of the 30 instances of dma_fence_get_rcu*, only 11 of them use dma_fence_get_rcu_safe(). It's likely there bear traps in DRM and related subsystems just waiting for someone to accidentally step in them.
This commit gets stops us using SLAB_TYPESAFE_BY_RCU for i915_request and, instead, does an RCU-safe slab free via rcu_call(). This should let us keep most of the perf benefits of slab allocation while avoiding the bear traps inherent in SLAB_TYPESAFE_BY_RCU.
Signed-off-by: Jason Ekstrand jason@jlekstrand.net Cc: Jon Bloomfield jon.bloomfield@intel.com Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: Christian König christian.koenig@amd.com Cc: Dave Airlie airlied@redhat.com Cc: Matthew Auld matthew.auld@intel.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com --- drivers/gpu/drm/i915/i915_request.c | 76 ++++++++++++++++------------- 1 file changed, 43 insertions(+), 33 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c index e531c74f0b0e2..55fa938126100 100644 --- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -111,9 +111,44 @@ void intel_engine_free_request_pool(struct intel_engine_cs *engine) if (!engine->request_pool) return;
+ /* + * It's safe to free this right away because we always put a fresh + * i915_request in the cache that's never been touched by an RCU + * reader. + */ kmem_cache_free(global.slab_requests, engine->request_pool); }
+static void __i915_request_free(struct rcu_head *head) +{ + struct i915_request *rq = container_of(head, typeof(*rq), fence.rcu); + + kmem_cache_free(global.slab_requests, rq); +} + +static void i915_request_free_rcu(struct i915_request *rq) +{ + /* + * Because we're on a slab allocator, memory may be re-used the + * moment we free it. There is no kfree_rcu() equivalent for + * slabs. Instead, we hand-roll it here with call_rcu(). This + * gives us all the perf benefits to slab allocation while ensuring + * that we never release a request back to the slab until there are + * no more readers. + * + * We do have to be careful, though, when calling kmem_cache_destroy() + * as there may be outstanding free requests. This is solved by + * inserting an rcu_barrier() before kmem_cache_destroy(). An RCU + * barrier is sufficient and we don't need synchronize_rcu() + * because the call_rcu() here will wait on any outstanding RCU + * readers and the rcu_barrier() will wait on any outstanding + * call_rcu() callbacks. So, if there are any readers who once had + * valid references to a request, rcu_barrier() will end up waiting + * on them by transitivity. + */ + call_rcu(&rq->fence.rcu, __i915_request_free); +} + static void i915_fence_release(struct dma_fence *fence) { struct i915_request *rq = to_request(fence); @@ -127,8 +162,7 @@ static void i915_fence_release(struct dma_fence *fence) */ i915_sw_fence_fini(&rq->submit); i915_sw_fence_fini(&rq->semaphore); - - kmem_cache_free(global.slab_requests, rq); + i915_request_free_rcu(rq); }
const struct dma_fence_ops i915_fence_ops = { @@ -933,35 +967,6 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp) */ ensure_cached_request(&ce->engine->request_pool, gfp);
- /* - * Beware: Dragons be flying overhead. - * - * We use RCU to look up requests in flight. The lookups may - * race with the request being allocated from the slab freelist. - * That is the request we are writing to here, may be in the process - * of being read by __i915_active_request_get_rcu(). As such, - * we have to be very careful when overwriting the contents. During - * the RCU lookup, we change chase the request->engine pointer, - * read the request->global_seqno and increment the reference count. - * - * The reference count is incremented atomically. If it is zero, - * the lookup knows the request is unallocated and complete. Otherwise, - * it is either still in use, or has been reallocated and reset - * with dma_fence_init(). This increment is safe for release as we - * check that the request we have a reference to and matches the active - * request. - * - * Before we increment the refcount, we chase the request->engine - * pointer. We must not call kmem_cache_zalloc() or else we set - * that pointer to NULL and cause a crash during the lookup. If - * we see the request is completed (based on the value of the - * old engine and seqno), the lookup is complete and reports NULL. - * If we decide the request is not completed (new engine or seqno), - * then we grab a reference and double check that it is still the - * active request - which it won't be and restart the lookup. - * - * Do not use kmem_cache_zalloc() here! - */ rq = kmem_cache_alloc(global.slab_requests, gfp | __GFP_RETRY_MAYFAIL | __GFP_NOWARN); if (unlikely(!rq)) { @@ -2116,6 +2121,12 @@ static void i915_global_request_shrink(void)
static void i915_global_request_exit(void) { + /* + * We need to rcu_barrier() before destroying slab_requests. See + * i915_request_free_rcu() for more details. + */ + rcu_barrier(); + kmem_cache_destroy(global.slab_execute_cbs); kmem_cache_destroy(global.slab_requests); } @@ -2132,8 +2143,7 @@ int __init i915_global_request_init(void) sizeof(struct i915_request), __alignof__(struct i915_request), SLAB_HWCACHE_ALIGN | - SLAB_RECLAIM_ACCOUNT | - SLAB_TYPESAFE_BY_RCU, + SLAB_RECLAIM_ACCOUNT, __i915_request_ctor); if (!global.slab_requests) return -ENOMEM;