Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

Steven Rostedt
2010-01-07T05:40:59Z

On Wed, 2010-01-06 at 23:40 -0500, Mathieu Desnoyers wrote:
> Here is an implementation of a new system call, sys_membarrier(), which
> executes a memory barrier on all threads of the current process.
>
> It aims at greatly simplifying and enhancing the current signal-based
> liburcu userspace RCU synchronize_rcu() implementation.
> (found at http://lttng.org/urcu)
>

Nice.

> Both the signal-based and the sys_membarrier userspace RCU schemes
> permit us to remove the memory barrier from the userspace RCU
> rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
> accelerating them. These memory barriers are replaced by compiler
> barriers on the read-side, and all matching memory barriers on the
> write-side are turned into an invokation of a memory barrier on all
> active threads in the process. By letting the kernel perform this
> synchronization rather than dumbly sending a signal to every process
> threads (as we currently do), we diminish the number of unnecessary wake
> ups and only issue the memory barriers on active threads. Non-running
> threads do not need to execute such barrier anyway, because these are
> implied by the scheduler context switches.
>
> To explain the benefit of this scheme, let's introduce two example threads:
>
> Thread A (non-frequent, e.g. executing liburcu synchronize_rcu())
> Thread B (frequent, e.g. executing liburcu rcu_read_lock()/rcu_read_unlock())
>
> In a scheme where all smp_mb() in thread A synchronize_rcu() are
> ordering memory accesses with respect to smp_mb() present in
> rcu_read_lock/unlock(), we can change all smp_mb() from
> synchronize_rcu() into calls to sys_membarrier() and all smp_mb() from
> rcu_read_lock/unlock() into compiler barriers "barrier()".
>
> Before the change, we had, for each smp_mb() pairs:
>
> Thread A Thread B
> prev mem accesses prev mem accesses
> smp_mb() smp_mb()
> follow mem accesses follow mem accesses
>
> After the change, these pairs become:
>
> Thread A Thread B
> prev mem accesses prev mem accesses
> sys_membarrier() barrier()
> follow mem accesses follow mem accesses
>
> As we can see, there are two possible scenarios: either Thread B memory
> accesses do not happen concurrently with Thread A accesses (1), or they
> do (2).
>
> 1) Non-concurrent Thread A vs Thread B accesses:
>
> Thread A Thread B
> prev mem accesses
> sys_membarrier()
> follow mem accesses
> prev mem accesses
> barrier()
> follow mem accesses
>
> In this case, thread B accesses will be weakly ordered. This is OK,
> because at that point, thread A is not particularly interested in
> ordering them with respect to its own accesses.
>
> 2) Concurrent Thread A vs Thread B accesses
>
> Thread A Thread B
> prev mem accesses prev mem accesses
> sys_membarrier() barrier()
> follow mem accesses follow mem accesses
>
> In this case, thread B accesses, which are ensured to be in program
> order thanks to the compiler barrier, will be "upgraded" to full
> smp_mb() thanks to the IPIs executing memory barriers on each active
> system threads. Each non-running process threads are intrinsically
> serialized by the scheduler.
>
> The current implementation simply executes a memory barrier in an IPI
> handler on each active cpu. Going through the hassle of taking run queue
> locks and checking if the thread running on each online CPU belongs to
> the current thread seems more heavyweight than the cost of the IPI
> itself (not measured though).
>

I don't think you need to grab any locks. Doing an rcu_read_lock()
should prevent tasks from disappearing (since destruction of tasks use
RCU). You may still need to grab the tasklist_lock under read_lock().

So what you could do, is find each task that is a thread of the calling
task, and then just check task_rq(task)->curr != task. Just send the
IPI's to those tasks that pass the test.

If the task->rq changes, or the task->rq->curr changes, and makes the
condition fail (or even pass), the events that cause those changes are
probably good enough than needing to call smp_mb();

-- Steve

> The system call number is only assigned for x86_64 in this RFC patch.