Re: [RFC PATCH 2/2] membarrier: provide register sync core cmd

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Aug 27, 2017 at 12:54:04PM -0700, Mathieu Desnoyers wrote:
> Add a new MEMBARRIER_CMD_REGISTER_SYNC_CORE command to the membarrier
> system call. It allows processes to register their intent to have their
> threads issue core serializing barriers in addition to memory barriers
> whenever a membarrier command is performed.
> 
> It is relevant for reclaim of JIT code, which requires to issue core
> serializing barriers on all threads running on behalf of a process
> after ensuring the old code is not visible anymore, before re-using
> memory for new code.
> 
> When a processes returns from a MEMBARRIER_CMD_REGISTER_SYNC_CORE
> command, it is guaranteed that all following MEMBARRIER_CMD_SHARED and
> MEMBARRIER_CMD_PRIVATE_EXPEDITED issue core serializing barriers, in
> addition to the memory barriers, on all of its running threads.

I have queued both of these for testing and review:

  git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
  f54cf5123eb9 ("membarrier: Speed up single-threaded private cmd")
  0d6eb99818da ("membarrier: Provide register sync core cmd")

Could the people who asked for this please test it?  The second commit
is non-obvious enough to need some careful review and intensive testing
before I can in good conscious pass it upstream.

							Thanx, Paul

> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Andrew Hunter <ahh@google.com>
> CC: Maged Michael <maged.michael@gmail.com>
> CC: gromer@google.com
> CC: Avi Kivity <avi@scylladb.com>
> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> CC: Paul Mackerras <paulus@samba.org>
> CC: Michael Ellerman <mpe@ellerman.id.au>
> CC: Dave Watson <davejwatson@fb.com>
> CC: Andy Lutomirski <luto@kernel.org>
> CC: Will Deacon <will.deacon@arm.com>
> CC: Hans Boehm <hboehm@google.com>
> ---
>  fs/exec.c                       |  1 +
>  include/linux/sched.h           | 52 +++++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/membarrier.h |  1 +
>  kernel/fork.c                   |  2 ++
>  kernel/sched/core.c             |  3 +++
>  kernel/sched/membarrier.c       | 37 ++++++++++++++++++++++++++---
>  6 files changed, 93 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 62175cbcc801..a4ab3253bac7 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1794,6 +1794,7 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	/* execve succeeded */
>  	current->fs->in_exec = 0;
>  	current->in_execve = 0;
> +	membarrier_execve(current);
>  	acct_update_integrals(current);
>  	task_numa_free(current);
>  	free_bprm(bprm);
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8337e2db0bb2..b1ecdc4e8b84 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1086,6 +1086,9 @@ struct task_struct {
>  	/* Used by LSM modules for access restriction: */
>  	void				*security;
>  #endif
> +#ifdef CONFIG_MEMBARRIER
> +	int membarrier_sync_core;
> +#endif
> 
>  	/*
>  	 * New fields for task_struct should be added above here, so that
> @@ -1623,4 +1626,53 @@ extern long sched_getaffinity(pid_t pid, struct cpumask *mask);
>  #define TASK_SIZE_OF(tsk)	TASK_SIZE
>  #endif
> 
> +#ifdef CONFIG_MEMBARRIER
> +static inline void membarrier_fork(struct task_struct *t,
> +		unsigned long clone_flags)
> +{
> +	/*
> +	 * Coherence of membarrier_sync_core against thread fork is
> +	 * protected by siglock. membarrier_fork is called with siglock
> +	 * held.
> +	 */
> +	t->membarrier_sync_core = current->membarrier_sync_core;
> +}
> +static inline void membarrier_execve(struct task_struct *t)
> +{
> +	t->membarrier_sync_core = 0;
> +}
> +static inline void membarrier_sched_out(struct task_struct *t)
> +{
> +	/*
> +	 * Core serialization is performed before the memory barrier
> +	 * preceding the store to rq->curr.
> +	 */
> +	if (unlikely(READ_ONCE(t->membarrier_sync_core)))
> +		sync_core();
> +}
> +static inline void membarrier_sched_in(struct task_struct *t)
> +{
> +	/*
> +	 * Core serialization is performed after the memory barrier
> +	 * following the store to rq->curr.
> +	 */
> +	if (unlikely(READ_ONCE(t->membarrier_sync_core)))
> +		sync_core();
> +}
> +#else
> +static inline void membarrier_fork(struct task_struct *t,
> +		unsigned long clone_flags)
> +{
> +}
> +static inline void membarrier_execve(struct task_struct *t)
> +{
> +}
> +static inline void membarrier_sched_out(struct task_struct *t)
> +{
> +}
> +static inline void membarrier_sched_in(struct task_struct *t)
> +{
> +}
> +#endif
> +
>  #endif
> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> index 6d47b3249d8a..bc26fcf8fb7b 100644
> --- a/include/uapi/linux/membarrier.h
> +++ b/include/uapi/linux/membarrier.h
> @@ -67,6 +67,7 @@ enum membarrier_cmd {
>  	/* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
>  	/* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
>  	MEMBARRIER_CMD_PRIVATE_EXPEDITED	= (1 << 3),
> +	MEMBARRIER_CMD_REGISTER_SYNC_CORE	= (1 << 4),
>  };
> 
>  #endif /* _UAPI_LINUX_MEMBARRIER_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 17921b0390b4..713b3c932671 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1840,6 +1840,8 @@ static __latent_entropy struct task_struct *copy_process(
>  	 */
>  	copy_seccomp(p);
> 
> +	membarrier_fork(p, clone_flags);
> +
>  	/*
>  	 * Process group and session signals need to be delivered to just the
>  	 * parent before the fork or both the parent and the child after the
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0e36d9960d91..3ca27d066950 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3292,6 +3292,8 @@ static void __sched notrace __schedule(bool preempt)
>  	local_irq_disable();
>  	rcu_note_context_switch(preempt);
> 
> +	membarrier_sched_out(prev);
> +
>  	/*
>  	 * Make sure that signal_pending_state()->signal_pending() below
>  	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> @@ -3364,6 +3366,7 @@ static void __sched notrace __schedule(bool preempt)
> 
>  		/* Also unlocks the rq: */
>  		rq = context_switch(rq, prev, next, &rf);
> +		membarrier_sched_in(next);
>  	} else {
>  		rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
>  		rq_unlock_irq(rq, &rf);
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 7eec6914d2d2..f128caf64b49 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -25,12 +25,16 @@
>   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
>   * except MEMBARRIER_CMD_QUERY.
>   */
> -#define MEMBARRIER_CMD_BITMASK	\
> -	(MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
> +#define MEMBARRIER_CMD_BITMASK			\
> +	(MEMBARRIER_CMD_SHARED			\
> +	| MEMBARRIER_CMD_PRIVATE_EXPEDITED	\
> +	| MEMBARRIER_CMD_REGISTER_SYNC_CORE)
> 
>  static void ipi_mb(void *info)
>  {
> -	smp_mb();	/* IPIs should be serializing but paranoid. */
> +	/* IPIs should be serializing but paranoid. */
> +	smp_mb();
> +	sync_core();
>  }
> 
>  static void membarrier_private_expedited(void)
> @@ -96,6 +100,30 @@ static void membarrier_private_expedited(void)
>  	smp_mb();	/* exit from system call is not a mb */
>  }
> 
> +static void membarrier_register_sync_core(void)
> +{
> +	struct task_struct *p = current, *t;
> +
> +	if (get_nr_threads(p) == 1) {
> +		p->membarrier_sync_core = 1;
> +		return;
> +	}
> +
> +	/*
> +	 * Coherence of membarrier_sync_core against thread fork is
> +	 * protected by siglock.
> +	 */
> +	spin_lock(&p->sighand->siglock);
> +	for_each_thread(p, t)
> +		WRITE_ONCE(t->membarrier_sync_core, 1);
> +	spin_unlock(&p->sighand->siglock);
> +	/*
> +	 * Ensure all future scheduler execution will observe the new
> +	 * membarrier_sync_core state for this process.
> +	 */
> +	synchronize_sched();
> +}
> +
>  /**
>   * sys_membarrier - issue memory barriers on a set of threads
>   * @cmd:   Takes command values defined in enum membarrier_cmd.
> @@ -146,6 +174,9 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
>  	case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
>  		membarrier_private_expedited();
>  		return 0;
> +	case MEMBARRIER_CMD_REGISTER_SYNC_CORE:
> +		membarrier_register_sync_core();
> +		return 0;
>  	default:
>  		return -EINVAL;
>  	}
> -- 
> 2.11.0
>