[Devel] [PATCH rh7] sched/core: Fix unserialized r-m-w scribbling stuff

[Kirill Tkhai]

Ignore this, I'll better split this patch into two.

On 15.01.2016 18:09, Kirill Tkhai wrote:
> Port commit be958bdc96f18bc1356177bbb79d46ea0c037b96 from mainstream.
> 
> 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 (patch makes this)
> 
> 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>
> ---
>  include/linux/sched.h |   21 +++++++++++----------
>  kernel/sched/core.c   |    5 ++---
>  2 files changed, 13 insertions(+), 13 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;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c8ac8bd..f61a4d1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1616,6 +1616,7 @@ static int ttwu_remote(struct task_struct *p, int wake_flags)
>  	rq = __task_rq_lock(p);
>  	if (p->on_rq) {
>  		ttwu_do_wakeup(rq, p, wake_flags);
> +		p->woken_while_running = 1;
>  		ret = 1;
>  	}
>  	__task_rq_unlock(rq);
> @@ -1739,10 +1740,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  	success = 1; /* we're going to change ->state */
>  	cpu = task_cpu(p);
>  
> -	if (p->on_rq && ttwu_remote(p, wake_flags)) {
> -		p->woken_while_running = 1;
> +	if (p->on_rq && ttwu_remote(p, wake_flags))
>  		goto stat;
> -	}
>  
>  #ifdef CONFIG_SMP
>  	/*
> 
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