[PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

[Benjamin Segall]

When we clone a new thread, we do not inherit its posix_cputimers, and
clear them with posix_cputimers_init. However, this does not clear the
tick dependency it creates in tsk->tick_dep_mask, and the handler does
not reach the code to clear the dependency if there were no timers to
begin with.

Thus if a thread has a cputimer running before cloneing/forking, that
hierarchy will prevent nohz_full unless they create a cputimer of their
own.

Process-wide timers do not have this problem because fork does not copy
signal_struct as a baseline, it creates one from scratch.

Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
Signed-off-by: Ben Segall <bsegall@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index df8e4575ff01..b57cd63cfcd1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2290,10 +2290,11 @@ __latent_entropy struct task_struct *copy_process(
 
 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);
 
 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_clear_task(p, TICK_DEP_BIT_POSIX_TIMER);
 
 	p->io_context = NULL;
 	audit_set_context(p, NULL);
 	cgroup_fork(p);
 	if (args->kthread) {
-- 
2.47.0.rc1.288.g06298d1525-goog