From d8628f35bae0d0b1f06ca32fa57de76a7055e731 Mon Sep 17 00:00:00 2001 From: "Uladzislau Rezki (Sony)" Date: Wed, 28 Apr 2021 15:44:22 +0200 Subject: [PATCH] kvfree_rcu: Fix comments according to current code The kvfree_rcu() function now defers allocations in the common case due to the fact that there is no lockless access to the memory-allocator caches/pools. In addition, in CONFIG_PREEMPT_NONE=y and in CONFIG_PREEMPT_VOLUNTARY=y kernels, there is no reliable way to determine if spinlocks are held. As a result, allocation is deferred in the common case, and the two-argument form of kvfree_rcu() thus uses the "channel 3" queue through all the rcu_head structures. This channel is called referred to as the emergency case in comments, and these comments are now obsolete. This commit therefore updates these comments to reflect the new common-case nature of such emergencies. Signed-off-by: Uladzislau Rezki (Sony) Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index d643fd8327b6..b043af7b0212 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3355,9 +3355,11 @@ static void kfree_rcu_work(struct work_struct *work) } /* - * Emergency case only. It can happen under low memory - * condition when an allocation gets failed, so the "bulk" - * path can not be temporary maintained. + * This is used when the "bulk" path can not be used for the + * double-argument of kvfree_rcu(). This happens when the + * page-cache is empty, which means that objects are instead + * queued on a linked list through their rcu_head structures. + * This list is named "Channel 3". */ for (; head; head = next) { unsigned long offset = (unsigned long)head->func; @@ -3403,8 +3405,8 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp) if ((krcp->bkvhead[0] && !krwp->bkvhead_free[0]) || (krcp->bkvhead[1] && !krwp->bkvhead_free[1]) || (krcp->head && !krwp->head_free)) { - // Channel 1 corresponds to SLAB ptrs. - // Channel 2 corresponds to vmalloc ptrs. + // Channel 1 corresponds to the SLAB-pointer bulk path. + // Channel 2 corresponds to vmalloc-pointer bulk path. for (j = 0; j < FREE_N_CHANNELS; j++) { if (!krwp->bkvhead_free[j]) { krwp->bkvhead_free[j] = krcp->bkvhead[j]; @@ -3412,7 +3414,8 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp) } } - // Channel 3 corresponds to emergency path. + // Channel 3 corresponds to both SLAB and vmalloc + // objects queued on the linked list. if (!krwp->head_free) { krwp->head_free = krcp->head; krcp->head = NULL;