[Devel] [PATCH RHEL7 COMMIT] ms/sched/core: Fix unserialized r-m-w scribbling stuff

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-229.7.2.vz7.9.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-229.7.2.vz7.9.23
------>
commit fc53d0571bb5ca7f98c023c52f57a98688abe9de
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Mon Jan 18 17:56:50 2016 +0400

    ms/sched/core: Fix unserialized r-m-w scribbling stuff
    
    Port commit be958bdc96f18bc1356177bbb79d46ea0c037b96 from mainstream
    ("sched/core: Fix unserialized r-m-w scribbling stuff").
    
    Some of the sched bitfieds (notably sched_reset_on_fork) can be set
    on other than current, this can cause the r-m-w to race with other
    updates.
    
    Since all the sched bits are serialized by scheduler locks, pull them
    in a separate word.
    
    Reported-by: Tejun Heo <tj at kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
    Cc: Dmitry Vyukov <dvyukov at google.com>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Peter Zijlstra <peterz at infradead.org>
    Cc: Sasha Levin <sasha.levin at oracle.com>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Cc: akpm at linux-foundation.org
    Cc: hannes at cmpxchg.org
    Cc: mhocko at kernel.org
    Cc: Dmitry Vyukov <dvyukov at google.com>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Peter Zijlstra <peterz at infradead.org>
    
    Really the below 4 bits are protected by different locks:
    
    unsigned sched_reset_on_fork:1; rq pi
    unsigned sched_contributes_to_load:1; pi
    unsigned sched_interruptible_sleep:1; pi
    unsigned woken_while_running:1; rq
    
    But we may say, all of them are protected by rq lock,
    because sched_contributes_to_load and sched_interruptible_sleep
    are modified in try_to_wake_up(), when task is not on rq,
    so nobody can modify sched_reset_on_fork and woken_while_running
    this moment.
    
    Thus we won't use one more bitfield and save a memory of one word.
    
    Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
    Reviewed-by: Vladimir Davydov <vdavydov at virtuozzo.com>
---
 include/linux/sched.h | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4bbd391..81dc3d7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1231,21 +1231,22 @@ struct task_struct {
 	/* Used for emulating ABI behavior of previous Linux versions */
 	unsigned int personality;
 
+	/* scheduler bits, serialized by rq lock: */
+	unsigned sched_reset_on_fork:1;
+	/* Two below are really protected by pi_lock, but they are modified in
+         * the place where nobody else can modify other fields using rq->lock */
+	unsigned sched_contributes_to_load:1;
+	unsigned sched_interruptible_sleep:1;
+	unsigned woken_while_running:1;
+	unsigned :0; /* force alignment to the next boundary */
+
+	/* unserialized, strictly 'current' */
 	unsigned did_exec:1;
 	unsigned in_execve:1;	/* Tell the LSMs that the process is doing an
 				 * execve */
 	unsigned in_iowait:1;
 	unsigned did_ve_enter:1;
-
-	/* task may not gain privileges */
-	unsigned no_new_privs:1;
-
-	/* Revert to default priority/policy when forking */
-	unsigned sched_reset_on_fork:1;
-	unsigned sched_contributes_to_load:1;
-	unsigned sched_interruptible_sleep:1;
-
-	unsigned woken_while_running:1;
+	unsigned no_new_privs:1; /* task may not gain privileges */
 	unsigned may_throttle:1;
 
 	pid_t pid;