From 3d05818707bb2cf133ccdcd29f2d5585b5bc1298 Mon Sep 17 00:00:00 2001 From: Hou Tao Date: Fri, 21 Oct 2022 19:49:12 +0800 Subject: [PATCH] bpf: Wait for busy refill_work when destroying bpf memory allocator A busy irq work is an unfinished irq work and it can be either in the pending state or in the running state. When destroying bpf memory allocator, refill_work may be busy for PREEMPT_RT kernel in which irq work is invoked in a per-CPU RT-kthread. It is also possible for kernel with arch_irq_work_has_interrupt() being false (e.g. 1-cpu arm32 host or mips) and irq work is inovked in timer interrupt. The busy refill_work leads to various issues. The obvious one is that there will be concurrent operations on free_by_rcu and free_list between irq work and memory draining. Another one is call_rcu_in_progress will not be reliable for the checking of pending RCU callback because do_call_rcu() may have not been invoked by irq work yet. The other is there will be use-after-free if irq work is freed before the callback of irq work is invoked as shown below: BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor instruction fetch in kernel mode #PF: error_code(0x0010) - not-present page PGD 12ab94067 P4D 12ab94067 PUD 1796b4067 PMD 0 Oops: 0010 [#1] PREEMPT_RT SMP CPU: 5 PID: 64 Comm: irq_work/5 Not tainted 6.0.0-rt11+ #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) RIP: 0010:0x0 Code: Unable to access opcode bytes at 0xffffffffffffffd6. RSP: 0018:ffffadc080293e78 EFLAGS: 00010286 RAX: 0000000000000000 RBX: ffffcdc07fb6a388 RCX: ffffa05000a2e000 RDX: ffffa05000a2e000 RSI: ffffffff96cc9827 RDI: ffffcdc07fb6a388 ...... Call Trace: irq_work_single+0x24/0x60 irq_work_run_list+0x24/0x30 run_irq_workd+0x23/0x30 smpboot_thread_fn+0x203/0x300 kthread+0x126/0x150 ret_from_fork+0x1f/0x30 Considering the ease of concurrency handling, no overhead for irq_work_sync() under non-PREEMPT_RT kernel and has-irq-work-interrupt kernel and the short wait time used for irq_work_sync() under PREEMPT_RT (When running two test_maps on PREEMPT_RT kernel and 72-cpus host, the max wait time is about 8ms and the 99th percentile is 10us), just using irq_work_sync() to wait for busy refill_work to complete before memory draining and memory freeing. Fixes: 7c8199e24fa0 ("bpf: Introduce any context BPF specific memory allocator.") Acked-by: Stanislav Fomichev Signed-off-by: Hou Tao Link: https://lore.kernel.org/r/20221021114913.60508-2-houtao@huaweicloud.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/memalloc.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c index 5f83be1d2018..1cb24323826f 100644 --- a/kernel/bpf/memalloc.c +++ b/kernel/bpf/memalloc.c @@ -493,6 +493,16 @@ 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. + */ + irq_work_sync(&c->refill_work); drain_mem_cache(c); rcu_in_progress += atomic_read(&c->call_rcu_in_progress); } @@ -507,6 +517,7 @@ 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]; + irq_work_sync(&c->refill_work); drain_mem_cache(c); rcu_in_progress += atomic_read(&c->call_rcu_in_progress); } -- 2.34.1