[Devel] [PATCH RHEL8 COMMIT] ve: add a comment about possible pseudosuper race

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-240.1.1.vz8.5.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-240.1.1.vz8.5.35
------>
commit b7204691875013e109d250519edbe7ba8701156c
Author: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Date:   Mon Jun 7 15:54:02 2021 +0300

    ve: add a comment about possible pseudosuper race
    
    Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>
    
    (cherry-picked from vz7 commit fdac6d1dd408 ("ve: add a comment about possible
    pseudosuper race"))
    
    To_merge: 96c59ccc5780 ("ve/cgroup: Add pseudosuper state for restore sake")
    
    https://jira.sw.ru/browse/PSBM-127848
    Signed-off-by: Valeriy Vdovin <valeriy.vdovin at virtuozzo.com>
---
 kernel/ve/ve.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
index 350ff2990af5..12e91e6ee1a1 100644
--- a/kernel/ve/ve.c
+++ b/kernel/ve/ve.c
@@ -1228,6 +1228,18 @@ static int ve_pseudosuper_write(struct cgroup_subsys_state *css, struct cftype *
 		return -EBUSY;
 	}
 	ve->is_pseudosuper = val;
+	/*
+	 * In CRIU we do unset pseudosuper on ve cgroup just before doing
+	 * ptrace(PTRACE_DETACH) to release restored process, what if one of
+	 * them will see pseudosuper flag still set to 1?
+	 *
+	 * To be 100% sure that these will never happen we need to call
+	 * synchronize_sched_expedited(); here to make cross cpu memory
+	 * barrier.
+	 *
+	 * For now we rely on userspace that ptrace from CRIU will do wake-up
+	 * on CT tasks which should imply memory barrier.
+	 */
 	up_write(&ve->op_sem);
 
 	return 0;