From: Alexei Starovoitov <ast@xxxxxxxxxx>
The next patch will introduce cross-cpu llist access and existing
irq_work_sync() + drain_mem_cache() + rcu_barrier_tasks_trace() mechanism will
not be enough, since irq_work_sync() + drain_mem_cache() on cpu A won't
guarantee that llist on cpu A are empty. The free_bulk() on cpu B might add
objects back to llist of cpu A. Add 'bool draining' flag.
The modified sequence looks like:
for_each_cpu:
WRITE_ONCE(c->draining, true); // do_call_rcu_ttrace() won't be doing call_rcu() any more
irq_work_sync(); // wait for irq_work callback (free_bulk) to finish
drain_mem_cache(); // free all objects
rcu_barrier_tasks_trace(); // wait for RCU callbacks to execute
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
kernel/bpf/memalloc.c | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index 0ee566a7719a..2615f296f052 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -98,6 +98,7 @@ struct bpf_mem_cache {
int free_cnt;
int low_watermark, high_watermark, batch;
int percpu_size;
+ bool draining;
/* list of objects to be freed after RCU tasks trace GP */
struct llist_head free_by_rcu_ttrace;
@@ -301,6 +302,12 @@ static void do_call_rcu_ttrace(struct bpf_mem_cache *c)
* from __free_rcu() and from drain_mem_cache().
*/
__llist_add(llnode, &c->waiting_for_gp_ttrace);
+
+ if (unlikely(READ_ONCE(c->draining))) {
+ __free_rcu(&c->rcu_ttrace);
+ return;
+ }
+
/* Use call_rcu_tasks_trace() to wait for sleepable progs to finish.
* If RCU Tasks Trace grace period implies RCU grace period, free
* these elements directly, else use call_rcu() to wait for normal
@@ -544,15 +551,7 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
rcu_in_progress = 0;
for_each_possible_cpu(cpu) {
c = per_cpu_ptr(ma->cache, cpu);
- /*
- * refill_work may be unfinished for PREEMPT_RT kernel
- * in which irq work is invoked in a per-CPU RT thread.
- * It is also possible for kernel with
- * arch_irq_work_has_interrupt() being false and irq
- * work is invoked in timer interrupt. So waiting for
- * the completion of irq work to ease the handling of
- * concurrency.
- */
+ WRITE_ONCE(c->draining, true);
irq_work_sync(&c->refill_work);
drain_mem_cache(c);
rcu_in_progress += atomic_read(&c->call_rcu_ttrace_in_progress);
@@ -568,6 +567,7 @@ void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)
cc = per_cpu_ptr(ma->caches, cpu);
for (i = 0; i < NUM_CACHES; i++) {
c = &cc->cache[i];
+ WRITE_ONCE(c->draining, true);
irq_work_sync(&c->refill_work);
drain_mem_cache(c);
rcu_in_progress += atomic_read(&c->call_rcu_ttrace_in_progress);
--
2.34.1
