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Revert "cgroup: simplify threadgroup locking"

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This reverts commit b5ba75b5fc.

d59cfc09c3 ("sched, cgroup: replace signal_struct->group_rwsem with
a global percpu_rwsem") and b5ba75b5fc ("cgroup: simplify
threadgroup locking") changed how cgroup synchronizes against task
fork and exits so that it uses global percpu_rwsem instead of
per-process rwsem; unfortunately, the write [un]lock paths of
percpu_rwsem always involve synchronize_rcu_expedited() which turned
out to be too expensive.

Improvements for percpu_rwsem are scheduled to be merged in the coming
v4.4-rc1 merge window which alleviates this issue.  For now, revert
the two commits to restore per-process rwsem.  They will be re-applied
for the v4.4-rc1 merge window.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org # v4.2+
This commit is contained in:
Tejun Heo 2015-09-16 11:51:12 -04:00
parent 6ff33f3902
commit f9f9e7b776
1 changed files with 33 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

45
kernel/cgroup.c
View File

@ -2460,13 +2460,14 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
if (!cgrp)
return -ENODEV;
percpu_down_write(&cgroup_threadgroup_rwsem);
retry_find_task:
rcu_read_lock();
if (pid) {
tsk = find_task_by_vpid(pid);
if (!tsk) {
rcu_read_unlock();
ret = -ESRCH;
goto out_unlock_rcu;
goto out_unlock_cgroup;
}
} else {
tsk = current;
@ -2482,23 +2483,37 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
*/
if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
ret = -EINVAL;
goto out_unlock_rcu;
rcu_read_unlock();
goto out_unlock_cgroup;
}
get_task_struct(tsk);
rcu_read_unlock();
percpu_down_write(&cgroup_threadgroup_rwsem);
if (threadgroup) {
if (!thread_group_leader(tsk)) {
/*
* a race with de_thread from another thread's exec()
* may strip us of our leadership, if this happens,
* there is no choice but to throw this task away and
* try again; this is
* "double-double-toil-and-trouble-check locking".
*/
percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(tsk);
goto retry_find_task;
}
}
ret = cgroup_procs_write_permission(tsk, cgrp, of);
if (!ret)
ret = cgroup_attach_task(cgrp, tsk, threadgroup);
put_task_struct(tsk);
goto out_unlock_threadgroup;
out_unlock_rcu:
rcu_read_unlock();
out_unlock_threadgroup:
percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(tsk);
out_unlock_cgroup:
cgroup_kn_unlock(of->kn);
return ret ?: nbytes;
}
@ -2643,8 +2658,6 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
lockdep_assert_held(&cgroup_mutex);
percpu_down_write(&cgroup_threadgroup_rwsem);
/* look up all csses currently attached to @cgrp's subtree */
down_read(&css_set_rwsem);
css_for_each_descendant_pre(css, cgroup_css(cgrp, NULL)) {
@ -2700,8 +2713,17 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
goto out_finish;
last_task = task;
percpu_down_write(&cgroup_threadgroup_rwsem);
/* raced against de_thread() from another thread? */
if (!thread_group_leader(task)) {
percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(task);
continue;
}
ret = cgroup_migrate(src_cset->dfl_cgrp, task, true);
percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(task);
if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret))
@ -2711,7 +2733,6 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
out_finish:
cgroup_migrate_finish(&preloaded_csets);
percpu_up_write(&cgroup_threadgroup_rwsem);
return ret;
}