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Message-ID: <CAEf4BzZw=E_fP77HT_jSUXxCho4zBPGED0_YbvjxHHGZFvbcsw@mail.gmail.com>
Date: Wed, 28 Jan 2026 11:13:57 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
namhyung@...nel.org, mark.rutland@....com, alexander.shishkin@...ux.intel.com,
jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com,
kan.liang@...ux.intel.com, song@...nel.org, ast@...nel.org,
daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
eddyz87@...il.com, yonghong.song@...ux.dev, john.fastabend@...il.com,
kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v8 3/3] bpf: Hold ther perf callchain entry until
used completely
On Tue, Jan 27, 2026 at 8:21 PM Tao Chen <chen.dylane@...ux.dev> wrote:
>
> 在 2026/1/28 05:35, Andrii Nakryiko 写道:
> > On Sun, Jan 25, 2026 at 11:46 PM Tao Chen <chen.dylane@...ux.dev> wrote:
> >>
> >> As Alexei noted, get_perf_callchain() return values may be reused
> >> if a task is preempted after the BPF program enters migrate disable
> >> mode. The perf_callchain_entres has a small stack of entries, and
> >> we can reuse it as follows:
> >>
> >> 1. get the perf callchain entry
> >> 2. BPF use...
> >> 3. put the perf callchain entry
> >>
> >> And Peter suggested that get_recursion_context used with preemption
> >> disabled, so we should disable preemption at BPF side.
> >>
> >> Acked-by: Yonghong Song <yonghong.song@...ux.dev>
> >> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
> >> ---
> >> kernel/bpf/stackmap.c | 55 ++++++++++++++++++++++++++++++++++++-------
> >> 1 file changed, 47 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> >> index e77dcdc2164..6bdee6cc05f 100644
> >> --- a/kernel/bpf/stackmap.c
> >> +++ b/kernel/bpf/stackmap.c
> >> @@ -215,7 +215,9 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
> >> #ifdef CONFIG_STACKTRACE
> >> struct perf_callchain_entry *entry;
> >>
> >> + preempt_disable();
> >> entry = get_callchain_entry();
> >> + preempt_enable();
> >
> > pass perf_callchain_entry as input argument, to keep similar approach
> > to __get_perf_callchain, see below
> >
> >>
> >> if (!entry)
> >> return NULL;
> >> @@ -237,14 +239,40 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
> >> to[i] = (u64)(from[i]);
> >> }
> >>
> >> - put_callchain_entry(entry);
> >> -
> >> return entry;
> >> #else /* CONFIG_STACKTRACE */
> >> return NULL;
> >> #endif
> >> }
> >>
> >> +static struct perf_callchain_entry *
> >> +bpf_get_perf_callchain(struct pt_regs *regs, bool kernel, bool user, int max_stack,
> >> + bool crosstask)
> >> +{
> >
> > I don't really like having this wrapper, it feels like the flow will
> > be cleaner and easier to follow if we modify the code as suggested
> > below
> >
>
> Ok, will use it directly.
> >> + struct perf_callchain_entry *entry;
> >> + int ret;
> >> +
> >> + preempt_disable();
> >> + entry = get_callchain_entry();
> >> + preempt_enable();
> >
> > I'd actually consider having __get_callchain_entry() that does what
> > get_callchain_entry() does right now under assumption that
> > preemption/migration is disabled, and then make get_callchain_entry do
> > preempt_disable + fetch entry + preevent_enable + return entry dance.
> >
>
> in v4, YongHong suggested add preempt_disable in get_callchain_entry,
> but Peter suggested that do it from BPF side, so maybe keeping the
> existing method is a compromise.
yeah, I guess perf's own usage of this is happening under constant
preempt_disable(), so this would be unnecessary for them. That's fine,
let's keep it outside
>
> > This will simplify the flow here to just with no explicit
> > preempt_{disable,enable} visible. Either way all of this has
> > assumption that we are staying on the same CPU throughout (so at the
> > very least we need to have migration disabled)
> >
> > entry = get_callchain_entry();
> > __get_perf_callchain(entry, ...);
> > put_callchain_entry();
> >
> >
> > BTW, is there a way to assert that either preemption or migration is
> > currently disabled? I think both get_callchain_entry and
> > put_callchain_entry would benefit from that
> >
> > pw-bot: cr
> >
> >
> >> +
> >> + if (unlikely(!entry))
> >> + return NULL;
> >> +
> >> + ret = __get_perf_callchain(entry, regs, kernel, user, max_stack, crosstask, false, 0);
> >> + if (ret) {
> >> + put_callchain_entry(entry);
> >> + return NULL;
> >> + }
> >> +
> >> + return entry;
> >> +}
> >> +
> >> +static void bpf_put_perf_callchain(struct perf_callchain_entry *entry)
> >> +{
> >> + put_callchain_entry(entry);
> >> +}
> >> +
> >> static long __bpf_get_stackid(struct bpf_map *map,
> >> struct perf_callchain_entry *trace, u64 flags)
> >> {
> >> @@ -327,20 +355,23 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
> >> struct perf_callchain_entry *trace;
> >> bool kernel = !user;
> >> u32 max_depth;
> >> + int ret;
> >>
> >> if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
> >> BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
> >> return -EINVAL;
> >>
> >> max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> >> - trace = get_perf_callchain(regs, kernel, user, max_depth,
> >> - false, false, 0);
> >> + trace = bpf_get_perf_callchain(regs, kernel, user, max_depth, false);
> >>
> >> if (unlikely(!trace))
> >> /* couldn't fetch the stack trace */
> >> return -EFAULT;
> >>
> >> - return __bpf_get_stackid(map, trace, flags);
> >> + ret = __bpf_get_stackid(map, trace, flags);
> >> + bpf_put_perf_callchain(trace);
> >
> > Just as above, I think get_callchain_entry + __get_perf_callchain +
> > put_callchain_entry is better, IMO
> >
> >> +
> >> + return ret;
> >> }
> >>
> >> const struct bpf_func_proto bpf_get_stackid_proto = {
> >> @@ -468,13 +499,19 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
> >> } else if (kernel && task) {
> >> trace = get_callchain_entry_for_task(task, max_depth);
> >> } else {
> >> - trace = get_perf_callchain(regs, kernel, user, max_depth,
> >> - crosstask, false, 0);
> >> + trace = bpf_get_perf_callchain(regs, kernel, user, max_depth, crosstask);
> >> }
> >
> > with the above suggestions this will be a pretty streamlined:
> >
> > trace = trace_in ?: get_callchain_entry();
> > if (!trace)
> > goto err_fault;
> >
> > if (trace_in) {
> > trace->nr = ...
> > err = 0
> > } else if (kernel && task) {
> > err = get_callchain_entry_for_task(trace, ...);
> > } else {
> > err = __get_perf_callchain(trace, ...);
> > }
> > if (err)
> > goto clear;
> >
>
> This code looks much cleaner, i will change it, thanks.
>
> > ... proceed as before, we have our stack trace inside trace ...
> >
> > for successful and failed paths (you'll have to duplicate this logic):
> >
> > if (trace != trace_in)
> > put_callchain_entry(trace);
> >
> >>
> >> - if (unlikely(!trace) || trace->nr < skip) {
> >> + if (unlikely(!trace)) {
> >
> > this condition cannot happen: we either get trace_in != NULL or we get
> > it using __get_callchain_entry and then validate it's not NULL
> > earlier, so drop this condition
> >
>
> will remove it.
>
> >> + if (may_fault)
> >> + rcu_read_unlock();
> >> + goto err_fault;
> >> + }
> >> + if (trace->nr < skip) {
> >> if (may_fault)
> >> rcu_read_unlock();
> >> + if (!trace_in)
> >> + bpf_put_perf_callchain(trace);
> >
> > do this clean up in one place, behind the new goto label? it's a bit
> > too easy to miss this, IMO
> >
>
> ok, will do.
>
> >> goto err_fault;
> >> }
> >>
> >> @@ -495,6 +532,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
> >> /* trace/ips should not be dereferenced after this point */
> >> if (may_fault)
> >> rcu_read_unlock();
> >
> > now that I looked at this code, I feel like we don't really need this
> > rcu_read_{lock,unlock}() dance (even though I added it in the first
> > place). I this RCU was supposed to be need to keep
> > perf_callchain_entry alive long enough, but for BPF this is guaranteed
> > because either BPF stack map will keep them alive by delaying
> > put_callchain_buffer() until freeing time (after RCU Tasks Trace + RCU
> > grace periods), or for bpf_get_stack/bpf_get_task_stack, BPF program
> > itself will hold these buffers alive again, until freeing time which
> > is delayed until after RCU Tasks Trace + RCU grace period.
>
> It seems so, for both, put_callchain_buffer is always called at the end,
> which ensures it won't be released during use, i will remove it as a new
> patch.
>
> >
> > Please send this clean up as the first patch in the series so we can
> > review and ack this separately. Thanks!
> >
> >> + if (!trace_in)
> >> + bpf_put_perf_callchain(trace);
> >>
> >> if (user_build_id)
> >> stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
> >> --
> >> 2.48.1
> >>
>
>
> --
> Best Regards
> Tao Chen
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