> This almost stalls the system, this patch moves the threadgroup_change_begin > from before cgroup_fork() to just before cgroup_canfork(). Ideally we shouldn't > have to worry about threadgroup changes till the task is actually added to > the threadgroup. This avoids having to call reclaim with cgroup_threadgroup_rwsem > held. > > There are other theoretical issues with this semaphore > > systemd can do > > 1. cgroup_mutex (cgroup_kn_lock_live) > 2. cgroup_threadgroup_rwsem (W) (__cgroup_procs_write) > > and other threads can go > > 1. cgroup_threadgroup_rwsem (R) (copy_process) > 2. mem_cgroup_iter (as a part of reclaim) (cgroup_mutex -- rcu lock or cgroup_mutex) > > However, I've not examined them in too much detail or looked at lockdep > wait chains for those paths. > > I am sure there is a good reason for placing cgroup_threadgroup_rwsem > where it is today and I might be missing something. I am also surprised > no-one else has run into it so far. > > Comments? > We used to use cgroup_threadgroup_rwsem for syncronization between threads in the same threadgroup, but now it has evolved to ensure atomic operations across multi processes. For example, I'm trying to fix a race. See https://lkml.org/lkml/2016/8/8/900 And the fix kind of relies on the fact that cgroup_post_fork() is placed inside the read section of cgroup_threadgroup_rwsem, so that cpuset_fork() won't race with cgroup migration. > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Tejun Heo <tj@xxxxxxxxxx> > > Signed-off-by: Balbir Singh <bsingharora@xxxxxxxxx> > --- > kernel/fork.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index 5c2c355..0474fa8 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1406,7 +1406,6 @@ static struct task_struct *copy_process(unsigned long clone_flags, > p->real_start_time = ktime_get_boot_ns(); > p->io_context = NULL; > p->audit_context = NULL; > - threadgroup_change_begin(current); > cgroup_fork(p); > #ifdef CONFIG_NUMA > p->mempolicy = mpol_dup(p->mempolicy); > @@ -1558,6 +1557,7 @@ static struct task_struct *copy_process(unsigned long clone_flags, > INIT_LIST_HEAD(&p->thread_group); > p->task_works = NULL; > > + threadgroup_change_begin(current); > /* > * Ensure that the cgroup subsystem policies allow the new process to be > * forked. It should be noted the the new process's css_set can be changed > @@ -1658,6 +1658,7 @@ static struct task_struct *copy_process(unsigned long clone_flags, > bad_fork_cancel_cgroup: > cgroup_cancel_fork(p); > bad_fork_free_pid: > + threadgroup_change_end(current); > if (pid != &init_struct_pid) > free_pid(pid); > bad_fork_cleanup_thread: > @@ -1690,7 +1691,6 @@ bad_fork_cleanup_policy: > mpol_put(p->mempolicy); > bad_fork_cleanup_threadgroup_lock: > #endif > - threadgroup_change_end(current); > delayacct_tsk_free(p); > bad_fork_cleanup_count: > atomic_dec(&p->cred->user->processes); > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>