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libbpf-tools/klockstat: Support nested mutex_trylock #5461

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8144799
libbpf-tools/klockstat: Don't needlessly duplicate BPF programs
jeromemarchand Mar 10, 2026
a53eaab
libbpf-tools/klockstat: Support nested mutex_trylock
jeromemarchand Feb 3, 2026
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195 changes: 0 additions & 195 deletions libbpf-tools/klockstat.bpf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -832,201 +832,6 @@ int BPF_KPROBE(kprobe_up_write, struct rw_semaphore *lock)
return 0;
}

/* CONFIG_DEBUG_LOCK_ALLOC is enabled */

SEC("kprobe/mutex_lock_nested")
int BPF_KPROBE(kprobe_mutex_lock_nested, struct mutex *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/mutex_lock_nested")
int BPF_KRETPROBE(kprobe_mutex_lock_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);
lock_acquired(*lock);
return 0;
}

SEC("kprobe/mutex_lock_interruptible_nested")
int BPF_KPROBE(kprobe_mutex_lock_interruptible_nested, struct mutex *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/mutex_lock_interruptible_nested")
int BPF_KRETPROBE(kprobe_mutex_lock_interruptible_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);

if (ret)
lock_aborted(*lock);
else
lock_acquired(*lock);
return 0;
}

SEC("kprobe/mutex_lock_killable_nested")
int BPF_KPROBE(kprobe_mutex_lock_killable_nested, struct mutex *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/mutex_lock_killable_nested")
int BPF_KRETPROBE(kprobe_mutex_lock_killable_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);

if (ret)
lock_aborted(*lock);
else
lock_acquired(*lock);
return 0;
}

SEC("kprobe/down_read_nested")
int BPF_KPROBE(kprobe_down_read_nested, struct rw_semaphore *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/down_read_nested")
int BPF_KRETPROBE(kprobe_down_read_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);

lock_acquired(*lock);
return 0;
}

SEC("kprobe/down_read_killable_nested")
int BPF_KPROBE(kprobe_down_read_killable_nested, struct rw_semaphore *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/down_read_killable_nested")
int BPF_KRETPROBE(kprobe_down_read_killable_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);

if (ret)
lock_aborted(*lock);
else
lock_acquired(*lock);
return 0;
}

SEC("kprobe/down_write_nested")
int BPF_KPROBE(kprobe_down_write_nested, struct rw_semaphore *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/down_write_nested")
int BPF_KRETPROBE(kprobe_down_write_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);

lock_acquired(*lock);
return 0;
}

SEC("kprobe/down_write_killable_nested")
int BPF_KPROBE(kprobe_down_write_killable_nested, struct rw_semaphore *lock)
{
u32 tid = (u32)bpf_get_current_pid_tgid();

bpf_map_update_elem(&locks, &tid, &lock, BPF_ANY);
lock_contended(ctx, lock);
return 0;
}

SEC("kretprobe/down_write_killable_nested")
int BPF_KRETPROBE(kprobe_down_write_killable_exit_nested, long ret)
{
u32 tid = (u32)bpf_get_current_pid_tgid();
void **lock;

lock = bpf_map_lookup_elem(&locks, &tid);
if (!lock)
return 0;

bpf_map_delete_elem(&locks, &tid);

if (ret)
lock_aborted(*lock);
else
lock_acquired(*lock);
return 0;
}

SEC("kprobe/rtnetlink_rcv_msg")
int BPF_KPROBE(kprobe_rtnetlink_rcv_msg, struct sk_buff *skb, struct nlmsghdr *nlh,
struct netlink_ext_ack *ext)
Expand Down
120 changes: 65 additions & 55 deletions libbpf-tools/klockstat.c
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Original file line number Diff line number Diff line change
Expand Up @@ -103,7 +103,9 @@ static const char *lock_ksym_names[] = {
"mutex_lock_interruptible_nested",
"mutex_lock_killable",
"mutex_lock_killable_nested",
"_mutex_lock_killable",
"mutex_trylock",
"_mutex_trylock_nest_lock",
"down_read",
"down_read_nested",
"down_read_interruptible",
Expand Down Expand Up @@ -775,8 +777,6 @@ static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va

static void enable_fentry(struct klockstat_bpf *obj)
{
bool debug_lock;

bpf_program__set_autoload(obj->progs.kprobe_mutex_lock, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_trylock, false);
Expand Down Expand Up @@ -804,22 +804,6 @@ static void enable_fentry(struct klockstat_bpf *obj)
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_up_write, false);

bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_interruptible_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_interruptible_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_killable_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_killable_exit_nested, false);

bpf_program__set_autoload(obj->progs.kprobe_down_read_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_killable_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_killable_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable_exit_nested, false);

bpf_program__set_autoload(obj->progs.kprobe_rtnetlink_rcv_msg, false);
bpf_program__set_autoload(obj->progs.kprobe_rtnetlink_rcv_msg_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_netlink_dump, false);
Expand All @@ -828,8 +812,7 @@ static void enable_fentry(struct klockstat_bpf *obj)
bpf_program__set_autoload(obj->progs.kprobe_sock_do_ioctl_exit, false);

/* CONFIG_DEBUG_LOCK_ALLOC is on */
debug_lock = fentry_can_attach("mutex_lock_nested", NULL);
if (!debug_lock)
if (!fentry_can_attach("mutex_lock_nested", NULL))
return;

bpf_program__set_attach_target(obj->progs.mutex_lock, 0,
Expand Down Expand Up @@ -857,10 +840,24 @@ static void enable_fentry(struct klockstat_bpf *obj)
"down_write_nested");
bpf_program__set_attach_target(obj->progs.down_write_exit, 0,
"down_write_nested");
bpf_program__set_attach_target(obj->progs.down_write_killable, 0,
"down_write_killable_nested");
bpf_program__set_attach_target(obj->progs.down_write_killable_exit, 0,
"down_write_killable_nested");

/* Since v6.16 mutex_lock_killable nested variant is implemented differently */
if (fentry_can_attach("_mutex_lock_killable", NULL)) {
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@ekyooo ekyooo Mar 1, 2026

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_mutex_lock_killable handling is inside DEBUG-only section.
This function exists in all kernels >= 6.16 (not debug-specific) ?
Line 849 may also need to be adjusted depending on line 877.

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@jeromemarchand jeromemarchand Mar 10, 2026

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I don't see it in /proc/kallsyms on a non-debug kernel and both definitions of _mutex_lock_killable(), in mutex.c and rtmutex_api.c are inside a #ifdef CONFIG_DEBUG_LOCK_ALLOC section.

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@ekyooo ekyooo Mar 15, 2026

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  1. You are right — _mutex_lock_killable handling belongs in the debug section. Thanks for the correction.
  2. The two assignments to mutex_lock_killable / mutex_lock_killable_exit are mutually exclusive alternatives depending on the kernel version, but the code expresses this as an unconditional default followed by a conditional update. An if/else would make the intent clearer.

Thank you.

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@jeromemarchand jeromemarchand Mar 17, 2026

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Alright. I'll update the pull request soon with your suggestion.

bpf_program__set_attach_target(obj->progs.mutex_lock_killable, 0,
"_mutex_lock_killable");
bpf_program__set_attach_target(obj->progs.mutex_lock_killable_exit, 0,
"_mutex_lock_killable");
} else {
bpf_program__set_attach_target(obj->progs.down_write_killable, 0,
"down_write_killable_nested");
bpf_program__set_attach_target(obj->progs.down_write_killable_exit, 0,
"down_write_killable_nested");
Comment on lines +850 to +854
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Copilot AI Mar 31, 2026

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In enable_fentry(), the fallback branch of the _mutex_lock_killable check is setting attach targets for down_write_killable(_exit), which is unrelated to mutex_lock_killable. This also means down_write_killable(_exit) won’t be retargeted to down_write_killable_nested when _mutex_lock_killable exists. Please retarget down_write_killable(_exit) to down_write_killable_nested unconditionally (for CONFIG_DEBUG_LOCK_ALLOC), and keep the mutex_lock_killable targets on mutex_lock_killable_nested when _mutex_lock_killable isn’t available.

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}

/* Since v6.16 mutex_trylock also have a nested variant */
if (fentry_can_attach("_mutex_trylock_nest_lock", NULL))
bpf_program__set_attach_target(obj->progs.mutex_trylock_exit, 0,
"_mutex_trylock_nest_lock");
}

static void enable_kprobes(struct klockstat_bpf *obj)
Expand Down Expand Up @@ -897,39 +894,52 @@ static void enable_kprobes(struct klockstat_bpf *obj)
bpf_program__set_autoload(obj->progs.sock_do_ioctl_exit, false);

/* CONFIG_DEBUG_LOCK_ALLOC is on */
if (kprobe_exists("mutex_lock_nested")) {
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_interruptible, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_interruptible_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_killable, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_killable_exit, false);

bpf_program__set_autoload(obj->progs.kprobe_down_read, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_killable, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_killable_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_exit, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable_exit, false);
if (!kprobe_exists("mutex_lock_nested"))
return;

bpf_program__set_attach_target(obj->progs.mutex_lock, 0,
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enable_kprobes() is retargeting obj->progs.mutex_lock (the fentry program that was just set to autoload=false) to mutex_lock_nested, instead of retargeting the kprobe entry program. This likely leaves kprobe_mutex_lock still attached to mutex_lock (non-nested), breaking DEBUG_LOCK_ALLOC nested tracing. Please set the attach target on obj->progs.kprobe_mutex_lock (and its kretprobe) instead of the fentry program.

Suggested change
bpf_program__set_attach_target(obj->progs.mutex_lock, 0,
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock, 0,

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@jeromemarchand jeromemarchand Apr 1, 2026

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Yes indeed, that's a typo. Will fix.

"mutex_lock_nested");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_exit, 0,
"mutex_lock_nested");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_interruptible, 0,
"mutex_lock_interruptible_nested");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_interruptible_exit, 0,
"mutex_lock_interruptible_nested");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_killable, 0,
"mutex_lock_killable_nested");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_killable_exit, 0,
"mutex_lock_killable_nested");

bpf_program__set_attach_target(obj->progs.kprobe_down_read, 0,
"down_read_nested");
bpf_program__set_attach_target(obj->progs.kprobe_down_read_exit, 0,
"down_read_nested");
bpf_program__set_attach_target(obj->progs.kprobe_down_read_killable, 0,
"down_read_killable_nested");
bpf_program__set_attach_target(obj->progs.kprobe_down_read_killable_exit, 0,
"down_read_killable_nested");
bpf_program__set_attach_target(obj->progs.kprobe_down_write, 0,
"down_write_nested");
bpf_program__set_attach_target(obj->progs.kprobe_down_write_exit, 0,
"down_write_nested");

/* Since v6.16 mutex_lock_killable nested variant is implemented differently */
if (kprobe_exists("_mutex_lock_killable")) {
bpf_program__set_attach_target(obj->progs.mutex_lock_killable, 0,
"_mutex_lock_killable");
bpf_program__set_attach_target(obj->progs.mutex_lock_killable_exit, 0,
Comment on lines +928 to +930
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In enable_kprobes(), when _mutex_lock_killable exists you’re calling bpf_program__set_attach_target() on the fentry mutex_lock_killable programs, but those were set to autoload=false at the top of enable_kprobes(). This means kprobe_mutex_lock_killable(_exit) will still attach to the old symbol and miss nested killable locks on newer kernels. Please retarget the kprobe mutex_lock_killable entry/exit programs to _mutex_lock_killable instead.

Suggested change
bpf_program__set_attach_target(obj->progs.mutex_lock_killable, 0,
"_mutex_lock_killable");
bpf_program__set_attach_target(obj->progs.mutex_lock_killable_exit, 0,
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_killable, 0,
"_mutex_lock_killable");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_lock_killable_exit, 0,

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@jeromemarchand jeromemarchand Apr 1, 2026

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Obviously some sloppy copy and paste business on my part. My bad! Will fix.

"_mutex_lock_killable");
} else {
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_interruptible_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_interruptible_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_killable_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_mutex_lock_killable_exit_nested, false);

bpf_program__set_autoload(obj->progs.kprobe_down_read_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_killable_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_read_killable_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_exit_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable_nested, false);
bpf_program__set_autoload(obj->progs.kprobe_down_write_killable_exit_nested, false);
bpf_program__set_attach_target(obj->progs.kprobe_down_write_killable, 0,
"down_write_killable_nested");
bpf_program__set_attach_target(obj->progs.kprobe_down_write_killable_exit, 0,
"down_write_killable_nested");
Comment on lines 932 to +936
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In enable_kprobes(), retargeting down_write_killable(_exit) to down_write_killable_nested is currently inside the else branch of the _mutex_lock_killable check. down_write_killable_nested selection shouldn’t depend on whether _mutex_lock_killable exists; on kernels where _mutex_lock_killable exists, down_write_killable(_exit) will remain attached to non-nested symbols. Please move the down_write_killable(_exit) retargeting out of this conditional (or duplicate it in both branches).

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@jeromemarchand jeromemarchand Apr 1, 2026

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Same issue as above. Same fix.

}

/* Since v6.16 mutex_trylock also have a nested variant */
if (kprobe_exists("_mutex_trylock_nest_lock"))
bpf_program__set_attach_target(obj->progs.mutex_trylock_exit, 0,
"_mutex_trylock_nest_lock");
Comment on lines +940 to +942
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In enable_kprobes(), the nested mutex_trylock handling is retargeting obj->progs.mutex_trylock_exit (fexit program) even though enable_kprobes() disables autoload for that program. It also retargets only the exit side; for kprobe/kretprobe pairs you need both entry and exit attached to the same target to keep the per-thread lock map consistent. Please retarget kprobe_mutex_trylock and kprobe_mutex_trylock_exit to _mutex_trylock_nest_lock when present.

Suggested change
if (kprobe_exists("_mutex_trylock_nest_lock"))
bpf_program__set_attach_target(obj->progs.mutex_trylock_exit, 0,
"_mutex_trylock_nest_lock");
if (kprobe_exists("_mutex_trylock_nest_lock")) {
bpf_program__set_attach_target(obj->progs.kprobe_mutex_trylock, 0,
"_mutex_trylock_nest_lock");
bpf_program__set_attach_target(obj->progs.kprobe_mutex_trylock_exit, 0,
"_mutex_trylock_nest_lock");
}

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@jeromemarchand jeromemarchand Apr 1, 2026

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Right. the *_trylock mutexes only have an exit version in the fentry case, not the kprobe one. Will fix.

}

static void disable_nldump_ioctl_probes(struct klockstat_bpf *obj)
Expand Down
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