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Subject[PATCH 04/30] sched: Allow sched_cgroup_fork() to fail and introduce sched_cancel_fork()
Date
A new BPF extensible sched_class will need more control over the forking
process. It wants to be able to fail from sched_cgroup_fork() after the new
task's sched_task_group is initialized so that the loaded BPF program can
prepare the task with its cgroup association is established and reject fork
if e.g. allocation fails.

Allow sched_cgroup_fork() to fail by making it return int instead of void
and adding sched_cancel_fork() to undo sched_fork() in the error path.

sched_cgroup_fork() doesn't fail yet and this patch shouldn't cause any
behavior changes.

v2: Patch description updated to detail the expected use.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
---
include/linux/sched/task.h | 3 ++-
kernel/fork.c | 15 ++++++++++-----
kernel/sched/core.c | 8 +++++++-
3 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 357e0068497c..dcff721170c3 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -58,7 +58,8 @@ extern asmlinkage void schedule_tail(struct task_struct *prev);
extern void init_idle(struct task_struct *idle, int cpu);

extern int sched_fork(unsigned long clone_flags, struct task_struct *p);
-extern void sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs);
+extern int sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs);
+extern void sched_cancel_fork(struct task_struct *p);
extern void sched_post_fork(struct task_struct *p);
extern void sched_dead(struct task_struct *p);

diff --git a/kernel/fork.c b/kernel/fork.c
index 9f7fe3541897..184c622b5513 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2239,7 +2239,7 @@ static __latent_entropy struct task_struct *copy_process(

retval = perf_event_init_task(p, clone_flags);
if (retval)
- goto bad_fork_cleanup_policy;
+ goto bad_fork_sched_cancel_fork;
retval = audit_alloc(p);
if (retval)
goto bad_fork_cleanup_perf;
@@ -2380,7 +2380,9 @@ static __latent_entropy struct task_struct *copy_process(
* cgroup specific, it unconditionally needs to place the task on a
* runqueue.
*/
- sched_cgroup_fork(p, args);
+ retval = sched_cgroup_fork(p, args);
+ if (retval)
+ goto bad_fork_cancel_cgroup;

/*
* From this point on we must avoid any synchronous user-space
@@ -2426,13 +2428,13 @@ static __latent_entropy struct task_struct *copy_process(
/* Don't start children in a dying pid namespace */
if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
retval = -ENOMEM;
- goto bad_fork_cancel_cgroup;
+ goto bad_fork_core_free;
}

/* Let kill terminate clone/fork in the middle */
if (fatal_signal_pending(current)) {
retval = -EINTR;
- goto bad_fork_cancel_cgroup;
+ goto bad_fork_core_free;
}

/* No more failure paths after this point. */
@@ -2507,10 +2509,11 @@ static __latent_entropy struct task_struct *copy_process(

return p;

-bad_fork_cancel_cgroup:
+bad_fork_core_free:
sched_core_free(p);
spin_unlock(&current->sighand->siglock);
write_unlock_irq(&tasklist_lock);
+bad_fork_cancel_cgroup:
cgroup_cancel_fork(p, args);
bad_fork_put_pidfd:
if (clone_flags & CLONE_PIDFD) {
@@ -2549,6 +2552,8 @@ static __latent_entropy struct task_struct *copy_process(
audit_free(p);
bad_fork_cleanup_perf:
perf_event_free_task(p);
+bad_fork_sched_cancel_fork:
+ sched_cancel_fork(p);
bad_fork_cleanup_policy:
lockdep_free_task(p);
#ifdef CONFIG_NUMA
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 65f21f8bf738..49b3d8ce84ca 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4709,7 +4709,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
return 0;
}

-void sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs)
+int sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs)
{
unsigned long flags;

@@ -4736,6 +4736,12 @@ void sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs)
if (p->sched_class->task_fork)
p->sched_class->task_fork(p);
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
+ return 0;
+}
+
+void sched_cancel_fork(struct task_struct *p)
+{
}

void sched_post_fork(struct task_struct *p)
--
2.39.1
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