Re: [PATCH 1/2] memcg: flatten task_struct->memcg_oom

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 25, 2015 at 03:43:54PM +0100, Peter Zijlstra wrote:
> On Mon, Sep 21, 2015 at 04:01:41PM -0400, Tejun Heo wrote:
> > So, the only way the patch could have caused the above is if someone
> > who isn't the task itself is writing to the bitfields while the task
> > is running.  Looking through the fields, ->sched_reset_on_fork seems a
> > bit suspicious.  __sched_setscheduler() looks like it can modify the
> > bit while the target task is running.  Peter, am I misreading the
> > code?
> 
> Nope, that's quite possible. Looks like we need to break up those
> bitfields a bit. All the scheduler ones should be serialized by
> scheduler locks, but the others are fair game.

Maybe something like so; but my brain is a complete mess today.

---
 include/linux/sched.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f425aac63317..b474e0f05327 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1455,14 +1455,15 @@ struct task_struct {
 	/* Used for emulating ABI behavior of previous Linux versions */
 	unsigned int personality;
 
-	unsigned in_execve:1;	/* Tell the LSMs that the process is doing an
-				 * execve */
-	unsigned in_iowait:1;
-
-	/* Revert to default priority/policy when forking */
+	/* scheduler bits, serialized by scheduler locks */
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
 	unsigned sched_migrated:1;
+	unsigned __padding_sched:29;
+
+	/* unserialized, strictly 'current' */
+	unsigned in_execve:1; /* bit to tell LSMs we're in execve */
+	unsigned in_iowait:1;
 #ifdef CONFIG_MEMCG
 	unsigned memcg_may_oom:1;
 #endif

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