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Message-ID: <e0adcdedab52521111c2aa157eca276ae838fdb8.camel@fb.com>
Date: Fri, 31 May 2019 00:01:04 +0000
From: Matt Mullins <mmullins@...com>
To: Song Liu <songliubraving@...com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"daniel@...earbox.net" <daniel@...earbox.net>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
"ast@...nel.org" <ast@...nel.org>,
"rostedt@...dmis.org" <rostedt@...dmis.org>,
Andrew Hall <hall@...com>,
"mingo@...hat.com" <mingo@...hat.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf] bpf: preallocate a perf_sample_data per event fd
On Thu, 2019-05-30 at 23:28 +0000, Song Liu wrote:
> > On May 30, 2019, at 3:55 PM, Matt Mullins <mmullins@...com> wrote:
> >
> > It is possible that a BPF program can be called while another BPF
> > program is executing bpf_perf_event_output. This has been observed with
> > I/O completion occurring as a result of an interrupt:
> >
> > bpf_prog_247fd1341cddaea4_trace_req_end+0x8d7/0x1000
> > ? trace_call_bpf+0x82/0x100
> > ? sch_direct_xmit+0xe2/0x230
> > ? blk_mq_end_request+0x1/0x100
> > ? blk_mq_end_request+0x5/0x100
> > ? kprobe_perf_func+0x19b/0x240
> > ? __qdisc_run+0x86/0x520
> > ? blk_mq_end_request+0x1/0x100
> > ? blk_mq_end_request+0x5/0x100
> > ? kprobe_ftrace_handler+0x90/0xf0
> > ? ftrace_ops_assist_func+0x6e/0xe0
> > ? ip6_input_finish+0xbf/0x460
> > ? 0xffffffffa01e80bf
> > ? nbd_dbg_flags_show+0xc0/0xc0 [nbd]
> > ? blkdev_issue_zeroout+0x200/0x200
> > ? blk_mq_end_request+0x1/0x100
> > ? blk_mq_end_request+0x5/0x100
> > ? flush_smp_call_function_queue+0x6c/0xe0
> > ? smp_call_function_single_interrupt+0x32/0xc0
> > ? call_function_single_interrupt+0xf/0x20
> > ? call_function_single_interrupt+0xa/0x20
> > ? swiotlb_map_page+0x140/0x140
> > ? refcount_sub_and_test+0x1a/0x50
> > ? tcp_wfree+0x20/0xf0
> > ? skb_release_head_state+0x62/0xc0
> > ? skb_release_all+0xe/0x30
> > ? napi_consume_skb+0xb5/0x100
> > ? mlx5e_poll_tx_cq+0x1df/0x4e0
> > ? mlx5e_poll_tx_cq+0x38c/0x4e0
> > ? mlx5e_napi_poll+0x58/0xc30
> > ? mlx5e_napi_poll+0x232/0xc30
> > ? net_rx_action+0x128/0x340
> > ? __do_softirq+0xd4/0x2ad
> > ? irq_exit+0xa5/0xb0
> > ? do_IRQ+0x7d/0xc0
> > ? common_interrupt+0xf/0xf
> > </IRQ>
> > ? __rb_free_aux+0xf0/0xf0
> > ? perf_output_sample+0x28/0x7b0
> > ? perf_prepare_sample+0x54/0x4a0
> > ? perf_event_output+0x43/0x60
> > ? bpf_perf_event_output_raw_tp+0x15f/0x180
> > ? blk_mq_start_request+0x1/0x120
> > ? bpf_prog_411a64a706fc6044_should_trace+0xad4/0x1000
> > ? bpf_trace_run3+0x2c/0x80
> > ? nbd_send_cmd+0x4c2/0x690 [nbd]
> >
> > This also cannot be alleviated by further splitting the per-cpu
> > perf_sample_data structs (as in commit 283ca526a9bd ("bpf: fix
> > corruption on concurrent perf_event_output calls")), as a raw_tp could
> > be attached to the block:block_rq_complete tracepoint and execute during
> > another raw_tp. Instead, keep a pre-allocated perf_sample_data
> > structure per perf_event_array element and fail a bpf_perf_event_output
> > if that element is concurrently being used.
> >
> > Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")
> > Signed-off-by: Matt Mullins <mmullins@...com>
> > ---
> > It felt a bit overkill, but I had to split bpf_event_entry into its own
> > header file to break an include cycle from perf_event.h -> cgroup.h ->
> > cgroup-defs.h -> bpf-cgroup.h -> bpf.h -> (potentially) perf_event.h.
> >
> > include/linux/bpf.h | 7 -------
> > include/linux/bpf_event.h | 20 ++++++++++++++++++++
> > kernel/bpf/arraymap.c | 2 ++
> > kernel/trace/bpf_trace.c | 30 +++++++++++++++++-------------
> > 4 files changed, 39 insertions(+), 20 deletions(-)
> > create mode 100644 include/linux/bpf_event.h
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 4fb3aa2dc975..13b253a36402 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -467,13 +467,6 @@ static inline bool bpf_map_flags_access_ok(u32 access_flags)
> > (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG);
> > }
> >
>
> I think we can avoid the include cycle as:
>
> +struct perf_sample_data *sd;
> struct bpf_event_entry {
> struct perf_event *event;
> struct file *perf_file;
> struct file *map_file;
> struct rcu_head rcu;
> + struct perf_sample_data *sd;
> };
Yeah, that totally works. I was mostly doing this so we had only one
kmalloc allocation, but I'm not too worried about having an extra
object in kmalloc-64 if it simplifies the code a lot.
>
> > -struct bpf_event_entry {
> > - struct perf_event *event;
> > - struct file *perf_file;
> > - struct file *map_file;
> > - struct rcu_head rcu;
> > -};
> > -
> > bool bpf_prog_array_compatible(struct bpf_array *array, const struct bpf_prog *fp);
> > int bpf_prog_calc_tag(struct bpf_prog *fp);
> >
> > diff --git a/include/linux/bpf_event.h b/include/linux/bpf_event.h
> > new file mode 100644
> > index 000000000000..9f415990f921
> > --- /dev/null
> > +++ b/include/linux/bpf_event.h
> > @@ -0,0 +1,20 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _LINUX_BPF_EVENT_H
> > +#define _LINUX_BPF_EVENT_H
> > +
> > +#include <linux/perf_event.h>
> > +#include <linux/types.h>
> > +
> > +struct file;
> > +
> > +struct bpf_event_entry {
> > + struct perf_event *event;
> > + struct file *perf_file;
> > + struct file *map_file;
> > + struct rcu_head rcu;
> > + struct perf_sample_data sd;
> > + atomic_t in_use;
> > +};
> > +
> > +#endif /* _LINUX_BPF_EVENT_H */
> > diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> > index 584636c9e2eb..08e5e486d563 100644
> > --- a/kernel/bpf/arraymap.c
> > +++ b/kernel/bpf/arraymap.c
> > @@ -11,6 +11,7 @@
> > * General Public License for more details.
> > */
> > #include <linux/bpf.h>
> > +#include <linux/bpf_event.h>
> > #include <linux/btf.h>
> > #include <linux/err.h>
> > #include <linux/slab.h>
> > @@ -659,6 +660,7 @@ static struct bpf_event_entry *bpf_event_entry_gen(struct file *perf_file,
> > ee->event = perf_file->private_data;
> > ee->perf_file = perf_file;
> > ee->map_file = map_file;
>
> And do the kzalloc() or some other trick here.
>
> > + atomic_set(&ee->in_use, 0);
> > }
> >
> > return ee;
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index f92d6ad5e080..a03e29957698 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -6,6 +6,7 @@
> > #include <linux/types.h>
> > #include <linux/slab.h>
> > #include <linux/bpf.h>
> > +#include <linux/bpf_event.h>
> > #include <linux/bpf_perf_event.h>
> > #include <linux/filter.h>
> > #include <linux/uaccess.h>
> > @@ -410,17 +411,17 @@ static const struct bpf_func_proto bpf_perf_event_read_value_proto = {
> > .arg4_type = ARG_CONST_SIZE,
> > };
> >
> > -static DEFINE_PER_CPU(struct perf_sample_data, bpf_trace_sd);
> > -
> > static __always_inline u64
> > __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> > - u64 flags, struct perf_sample_data *sd)
> > + u64 flags, struct perf_raw_record *raw)
> > {
> > struct bpf_array *array = container_of(map, struct bpf_array, map);
> > unsigned int cpu = smp_processor_id();
> > u64 index = flags & BPF_F_INDEX_MASK;
> > struct bpf_event_entry *ee;
> > struct perf_event *event;
> > + struct perf_sample_data *sd;
> > + u64 ret;
> >
> > if (index == BPF_F_CURRENT_CPU)
> > index = cpu;
> > @@ -439,13 +440,22 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> > if (unlikely(event->oncpu != cpu))
> > return -EOPNOTSUPP;
> >
> > - return perf_event_output(event, sd, regs);
> > + if (atomic_cmpxchg(&ee->in_use, 0, 1) != 0)
> > + return -EBUSY;
>
> And we only need xchg() here, so we can eliminate in_use.
>
> Does this make sense?
You mean xchg a null-pointer or something in there while it's in-use,
then xchg the slab back? Makes sense to me. I'll try that and see
where it gets me.
>
> Thanks,
> Song
>
> > +
> > + sd = &ee->sd;
> > + perf_sample_data_init(sd, 0, 0);
> > + sd->raw = raw;
> > +
> > + ret = perf_event_output(event, sd, regs);
> > +
> > + atomic_set(&ee->in_use, 0);
> > + return ret;
> > }
> >
> > BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
> > u64, flags, void *, data, u64, size)
> > {
> > - struct perf_sample_data *sd = this_cpu_ptr(&bpf_trace_sd);
> > struct perf_raw_record raw = {
> > .frag = {
> > .size = size,
> > @@ -456,10 +466,8 @@ BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
> > if (unlikely(flags & ~(BPF_F_INDEX_MASK)))
> > return -EINVAL;
> >
> > - perf_sample_data_init(sd, 0, 0);
> > - sd->raw = &raw;
> >
> > - return __bpf_perf_event_output(regs, map, flags, sd);
> > + return __bpf_perf_event_output(regs, map, flags, &raw);
> > }
> >
> > static const struct bpf_func_proto bpf_perf_event_output_proto = {
> > @@ -474,12 +482,10 @@ static const struct bpf_func_proto bpf_perf_event_output_proto = {
> > };
> >
> > static DEFINE_PER_CPU(struct pt_regs, bpf_pt_regs);
> > -static DEFINE_PER_CPU(struct perf_sample_data, bpf_misc_sd);
> >
> > u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
> > void *ctx, u64 ctx_size, bpf_ctx_copy_t ctx_copy)
> > {
> > - struct perf_sample_data *sd = this_cpu_ptr(&bpf_misc_sd);
> > struct pt_regs *regs = this_cpu_ptr(&bpf_pt_regs);
> > struct perf_raw_frag frag = {
> > .copy = ctx_copy,
> > @@ -497,10 +503,8 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
> > };
> >
> > perf_fetch_caller_regs(regs);
> > - perf_sample_data_init(sd, 0, 0);
> > - sd->raw = &raw;
> >
> > - return __bpf_perf_event_output(regs, map, flags, sd);
> > + return __bpf_perf_event_output(regs, map, flags, &raw);
> > }
> >
> > BPF_CALL_0(bpf_get_current_task)
> > --
> > 2.17.1
> >
>
>
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