Linux kernel: tracing: Unregister field variable histograms when the command fails

echo 'hist:keys=pid:ts0=commontimestamp.usecs' >> \

events/sched/sched_waking/trigger

echo 'mysynth u64 lat; int a; int b' >> syntheticevents

echo 'hist:keys=nextpid:wakeuplat=commontimestamp.usecs-$ts0:\

onmatch(sched.sched_waking).my_synth($wakeup_lat,prio,nosuchfld)'\

> events/sched/sched_switch/trigger

cat events/sched/schedwaking/trigger

hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active] hist:keys=pid:vals=hitcount:synthetic_prio=prio:sort=hitcount:size=2048 [active]

The second histogram is the one created for 'prio'. It is still active and still counting.

out_free reaches destroy_field_var_hists(), which frees the field_var_hists[] entries, but never unregister_field_var_hists(), which is what removes the triggers and runs only from event_hist_trigger_free(). The entry is freed and the trigger is left with nothing tracking it.

Unregister the field variable histograms in out_free as well, and record each one as soon as its trigger is registered rather than after the lookup that follows it, so no exit can leave a registered trigger untracked. The entry then belongs to field_var_hists[] and is released by destroy_field_var_hists(), so freeing it on that exit would be a double free.