On Wed, Oct 19, 2022 at 6:08 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> On 10/20/2022 2:38 AM, sdf@xxxxxxxxxx wrote:
> > On 10/19, Hou Tao wrote:
> >> From: Hou Tao <houtao1@xxxxxxxxxx>
> >
> >> 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)
> >> 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 has not been invoked by irq work. 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:
> >> <TASK>
> >> 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
> >> </TASK>
> >
> >> Considering the ease of concurrency handling 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 waiting for busy refill_work to
> >> complete before memory draining and memory freeing.
> >
> >> Fixes: 7c8199e24fa0 ("bpf: Introduce any context BPF specific memory
> >> allocator.")
> >> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> >> ---
> >> kernel/bpf/memalloc.c | 11 +++++++++++
> >> 1 file changed, 11 insertions(+)
> >
> >> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> >> index 94f0f63443a6..48e606aaacf0 100644
> >> --- a/kernel/bpf/memalloc.c
> >> +++ b/kernel/bpf/memalloc.c
> >> @@ -497,6 +497,16 @@ 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 inovked in timer interrupt. So wait for the
> >> + * completion of irq work to ease the handling of
> >> + * concurrency.
> >> + */
> >> + irq_work_sync(&c->refill_work);
> >
> > Does it make sense to guard these with "IS_ENABLED(CONFIG_PREEMPT_RT)" ?
> > We do have a bunch of them sprinkled already to run alloc/free with
> > irqs disabled.
> No. As said in the commit message and the comments, irq_work_sync() is needed
> for both PREEMPT_RT kernel and kernel with arch_irq_work_has_interrupt() being
> false. And for other kernels, irq_work_sync() doesn't incur any overhead,
> because it is just a simple memory read through irq_work_is_busy() and nothing
> else. The reason is the irq work must have been completed when invoking
> bpf_mem_alloc_destroy() for these kernels.
>
> void irq_work_sync(struct irq_work *work)
> {
> /* Remove code snippet for PREEMPT_RT and arch_irq_work_has_interrupt() */
> /* irq wor*/
> while (irq_work_is_busy(work))
> cpu_relax();
> }
I see, thanks for clarifying! I was so carried away with that
PREEMPT_RT that I missed the fact that arch_irq_work_has_interrupt is
a separate thing. Agreed that doing irq_work_sync won't hurt in a
non-preempt/non-has_interrupt case.
In this case, can you still do a respin and fix the spelling issue in
the comment? You can slap my acked-by for the v2:
Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
s/work is inovked in timer interrupt. So wait for the/... invoked .../
> >
> > I was also trying to see if adding local_irq_save inside drain_mem_cache
> > to pair with the ones from refill might work, but waiting for irq to
> > finish seems easier...
> Disabling hard irq works, but irq_work_sync() is still needed to ensure it is
> completed before freeing its memory.
> >
> > Maybe also move both of these in some new "static void irq_work_wait"
> > to make it clear that the PREEMT_RT comment applies to both of them?
> >
> > Or maybe that helper should do 'for_each_possible_cpu(cpu)
> > irq_work_sync(&c->refill_work);'
> > in the PREEMPT_RT case so we don't have to call it twice?
> drain_mem_cache() is also time consuming somethings, so I think it is better to
> interleave irq_work_sync() and drain_mem_cache() to reduce waiting time.
>
> >
> >> drain_mem_cache(c);
> >> rcu_in_progress += atomic_read(&c->call_rcu_in_progress);
> >> }
> >> @@ -511,6 +521,7 @@ 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);
> >> }
> >> --
> >> 2.29.2
> >
> > .
>
