Hello Steven, Thank you for taking a look and really sorry. I did use ai assistance for commit message, but I reviewed, modified and tested(with syzbot locally) the code myself. I should have disclosed really sorry.
One thing I want to know (or I am still missing something): sched_cmdline_ref is incremented before tracing_sched_register() and register fails, but sched_cmdline_ref stays at 1 and on disable tracepoint_remove_func() sees NULL and return error (as syzbot reported and reproduce also locally). your suggestion WARN_ONCE correctly flags the upstream failure, but the secondary WARN at tracepoint.c:358 will still fire on the next disable, since the refcount desync isn't addressed. Was that intentional ? in my earlier approach, I tried to propagate the error and handle this desync under fault injection. I understand this may not align with the preferred direction, but that was the motivation behind the change. If you think it makes sense, I can prepare a v2 with your suggestion while also addressing the refcount consistency. Thank you On Fri, Apr 17, 2026 at 9:23 PM Steven Rostedt <[email protected]> wrote: > > On Fri, 17 Apr 2026 12:08:27 +0530 > Yash Suthar <[email protected]> wrote: > > > syzbot reported a WARN in tracepoint_probe_unregister(): > > > > tracing_start_sched_switch() increments sched_cmdline_ref / > > sched_tgid_ref before calling tracing_sched_register(), and its > > return value is discarded because the API is void. When the first > > register_trace_sched_*() fails (e.g. kmalloc under memory pressure > > or failslab), the function's fail_deprobe* labels roll back any > > partial probe registration, but the caller's refcount has already > > been bumped. The state is now desynced: refs > 0 but no probes in > > tp->funcs. > > > > Later, when the caller pairs the start with a stop, the refcount > > walks back to 0 and tracing_sched_unregister() calls > > unregister_trace_sched_*() against an empty tp->funcs. > > func_remove() returns -ENOENT and the > > WARN_ON_ONCE(IS_ERR(old)) in tracepoint_remove_func() fires. > > > > Fix: make tracing_start_sched_switch() and the two exported > > wrappers, tracing_start_cmdline_record() and > > tracing_start_tgid_record(), return int; register the probes > > before bumping the refcount; and propagate the error to callers > > so refs are only held on behalf of a caller whose registration > > actually succeeded. > > > > Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks") > > Reported-by: [email protected] > > Closes: > > https://syzkaller.appspot.com/bug?id=f93e97cd824071a2577a40cde9ecd957f59f87eb > > Did you use AI to create any of this? If so you must disclose it. This > reads very much like an AI patch. > > > > > Signed-off-by: Yash Suthar <[email protected]> > > --- > > kernel/trace/trace.c | 6 +++--- > > kernel/trace/trace.h | 4 ++-- > > kernel/trace/trace_events.c | 28 +++++++++++++++++++-------- > > kernel/trace/trace_functions.c | 8 +++++++- > > kernel/trace/trace_functions_graph.c | 6 +++++- > > kernel/trace/trace_sched_switch.c | 29 ++++++++++++++++++---------- > > kernel/trace/trace_selftest.c | 7 ++++++- > > 7 files changed, 62 insertions(+), 26 deletions(-) > > NAK on all this. If you are under severe memory constraints that causes > this to fail, then you'll be hitting a bunch more errors. > > > > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > > index 8bd4ec08fb36..e936eed99b27 100644 > > --- a/kernel/trace/trace.c > > +++ b/kernel/trace/trace.c > > @@ -3320,7 +3320,7 @@ void trace_printk_init_buffers(void) > > * allocated here, then this was called by module code. > > */ > > if (global_trace.array_buffer.buffer) > > - tracing_start_cmdline_record(); > > + (void)tracing_start_cmdline_record(); > > WTF??? Why are you adding the typecast of (void) here? Don't do that! > > > > } > > EXPORT_SYMBOL_GPL(trace_printk_init_buffers); > > > > @@ -3329,7 +3329,7 @@ void trace_printk_start_comm(void) > > /* Start tracing comms if trace printk is set */ > > if (!buffers_allocated) > > return; > > - tracing_start_cmdline_record(); > > + (void)tracing_start_cmdline_record(); > > } > > > > static void trace_printk_start_stop_comm(int enabled) > > @@ -3338,7 +3338,7 @@ static void trace_printk_start_stop_comm(int enabled) > > return; > > > > if (enabled) > > - tracing_start_cmdline_record(); > > + (void)tracing_start_cmdline_record(); > > else > > tracing_stop_cmdline_record(); > > } > > diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h > > index b6d42fe06115..6fe2c8429560 100644 > > --- a/kernel/trace/trace.h > > +++ b/kernel/trace/trace.h > > @@ -751,9 +751,9 @@ void trace_graph_return(struct ftrace_graph_ret *trace, > > struct fgraph_ops *gops, > > int trace_graph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops > > *gops, > > struct ftrace_regs *fregs); > > > > -void tracing_start_cmdline_record(void); > > +int tracing_start_cmdline_record(void); > > void tracing_stop_cmdline_record(void); > > -void tracing_start_tgid_record(void); > > +int tracing_start_tgid_record(void); > > void tracing_stop_tgid_record(void); > > > > int register_tracer(struct tracer *type); > > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c > > index 137b4d9bb116..e6713aa80a03 100644 > > --- a/kernel/trace/trace_events.c > > +++ b/kernel/trace/trace_events.c > > @@ -734,9 +734,9 @@ void trace_event_enable_cmd_record(bool enable) > > continue; > > > > if (enable) { > > - tracing_start_cmdline_record(); > > - set_bit(EVENT_FILE_FL_RECORDED_CMD_BIT, &file->flags); > > - } else { > > + if (!tracing_start_cmdline_record()) > > + set_bit(EVENT_FILE_FL_RECORDED_CMD_BIT, > > &file->flags); > > + } else if (file->flags & EVENT_FILE_FL_RECORDED_CMD) { > > tracing_stop_cmdline_record(); > > clear_bit(EVENT_FILE_FL_RECORDED_CMD_BIT, > > &file->flags); > > } > > @@ -755,9 +755,9 @@ void trace_event_enable_tgid_record(bool enable) > > continue; > > > > if (enable) { > > - tracing_start_tgid_record(); > > - set_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, > > &file->flags); > > - } else { > > + if (!tracing_start_tgid_record()) > > + set_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, > > &file->flags); > > + } else if (file->flags & EVENT_FILE_FL_RECORDED_TGID) { > > tracing_stop_tgid_record(); > > clear_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, > > &file->flags); > > @@ -847,14 +847,26 @@ static int __ftrace_event_enable_disable(struct > > trace_event_file *file, > > set_bit(EVENT_FILE_FL_SOFT_DISABLED_BIT, > > &file->flags); > > > > if (tr->trace_flags & TRACE_ITER(RECORD_CMD)) { > > + ret = tracing_start_cmdline_record(); > > + if (ret) { > > + pr_info("event trace: Could not > > enable event %s\n", > > + trace_event_name(call)); > > + break; > > + } > > cmd = true; > > - tracing_start_cmdline_record(); > > set_bit(EVENT_FILE_FL_RECORDED_CMD_BIT, > > &file->flags); > > } > > > > if (tr->trace_flags & TRACE_ITER(RECORD_TGID)) { > > + ret = tracing_start_tgid_record(); > > + if (ret) { > > + if (cmd) > > + tracing_stop_cmdline_record(); > > + pr_info("event trace: Could not > > enable event %s\n", > > + trace_event_name(call)); > > + break; > > + } > > tgid = true; > > - tracing_start_tgid_record(); > > set_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, > > &file->flags); > > } > > > > diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c > > index c12795c2fb39..14d099734345 100644 > > --- a/kernel/trace/trace_functions.c > > +++ b/kernel/trace/trace_functions.c > > @@ -146,6 +146,8 @@ static bool handle_func_repeats(struct trace_array *tr, > > u32 flags_val) > > static int function_trace_init(struct trace_array *tr) > > { > > ftrace_func_t func; > > + int ret; > > + > > /* > > * Instance trace_arrays get their ops allocated > > * at instance creation. Unless it failed > > @@ -165,7 +167,11 @@ static int function_trace_init(struct trace_array *tr) > > > > tr->array_buffer.cpu = raw_smp_processor_id(); > > > > - tracing_start_cmdline_record(); > > + ret = tracing_start_cmdline_record(); > > + if (ret) { > > + ftrace_reset_array_ops(tr); > > + return ret; > > + } > > tracing_start_function_trace(tr); > > return 0; > > } > > diff --git a/kernel/trace/trace_functions_graph.c > > b/kernel/trace/trace_functions_graph.c > > index 1de6f1573621..6b27ed62fee8 100644 > > --- a/kernel/trace/trace_functions_graph.c > > +++ b/kernel/trace/trace_functions_graph.c > > @@ -487,7 +487,11 @@ static int graph_trace_init(struct trace_array *tr) > > ret = register_ftrace_graph(tr->gops); > > if (ret) > > return ret; > > - tracing_start_cmdline_record(); > > + ret = tracing_start_cmdline_record(); > > + if (ret) { > > + unregister_ftrace_graph(tr->gops); > > + return ret; > > + } > > > > return 0; > > } > > diff --git a/kernel/trace/trace_sched_switch.c > > b/kernel/trace/trace_sched_switch.c > > index c46d584ded3b..683ea4ca1498 100644 > > --- a/kernel/trace/trace_sched_switch.c > > +++ b/kernel/trace/trace_sched_switch.c > > @@ -89,12 +89,22 @@ static void tracing_sched_unregister(void) > > unregister_trace_sched_wakeup(probe_sched_wakeup, NULL); > > } > > > > -static void tracing_start_sched_switch(int ops) > > +static int tracing_start_sched_switch(int ops) > > { > > - bool sched_register; > > + int ret = 0; > > > > mutex_lock(&sched_register_mutex); > > - sched_register = (!sched_cmdline_ref && !sched_tgid_ref); > > + > > + /* > > + * If the registration fails, do not bump the reference count : the > > + * caller must observe the failure so it can avoid a later matching > > + * stop that would otherwise unregister probes that were never added. > > + */ > > + if (!sched_cmdline_ref && !sched_tgid_ref) { > > + ret = tracing_sched_register(); > > + if (ret) > > + goto out; > > + } > > > > switch (ops) { > > case RECORD_CMDLINE: > > @@ -105,10 +115,9 @@ static void tracing_start_sched_switch(int ops) > > sched_tgid_ref++; > > break; > > } > > - > > - if (sched_register && (sched_cmdline_ref || sched_tgid_ref)) > > - tracing_sched_register(); > > The only change that should deal with this would be: > > if (sched_register && (sched_cmdline_ref || sched_tgid_ref)) { > WARN_ONCE(tracing_sched_register() < 0, > "Failed to register trace command line caching. > Requires reboot to fix"); > } > > -- Steve > > > > > +out: > > mutex_unlock(&sched_register_mutex); > > + return ret; > > } > > > > static void tracing_stop_sched_switch(int ops) > > @@ -130,9 +139,9 @@ static void tracing_stop_sched_switch(int ops) > > mutex_unlock(&sched_register_mutex); > > } > > > > -void tracing_start_cmdline_record(void) > > +int tracing_start_cmdline_record(void) > > { > > - tracing_start_sched_switch(RECORD_CMDLINE); > > + return tracing_start_sched_switch(RECORD_CMDLINE); > > } > > > > void tracing_stop_cmdline_record(void) > > @@ -140,9 +149,9 @@ void tracing_stop_cmdline_record(void) > > tracing_stop_sched_switch(RECORD_CMDLINE); > > } > > > > -void tracing_start_tgid_record(void) > > +int tracing_start_tgid_record(void) > > { > > - tracing_start_sched_switch(RECORD_TGID); > > + return tracing_start_sched_switch(RECORD_TGID); > > } > > > > void tracing_stop_tgid_record(void) > > diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c > > index d88c44f1dfa5..238e7451f8e4 100644 > > --- a/kernel/trace/trace_selftest.c > > +++ b/kernel/trace/trace_selftest.c > > @@ -1084,7 +1084,12 @@ trace_selftest_startup_function_graph(struct tracer > > *trace, > > warn_failed_init_tracer(trace, ret); > > goto out; > > } > > - tracing_start_cmdline_record(); > > + ret = tracing_start_cmdline_record(); > > + if (ret) { > > + unregister_ftrace_graph(&fgraph_ops); > > + warn_failed_init_tracer(trace, ret); > > + goto out; > > + } > > > > /* Sleep for a 1/10 of a second */ > > msleep(100); >
