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Date: Mon, 16 Mar 2015 15:19:39 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel@...r.kernel.org,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Steven Rostedt <rostedt@...dmis.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Nicholas Miell <nmiell@...cast.net>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...hat.com>,
Alan Cox <gnomes@...rguk.ukuu.org.uk>,
Lai Jiangshan <laijs@...fujitsu.com>,
Stephen Hemminger <stephen@...workplumber.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Josh Triplett <josh@...htriplett.org>,
Thomas Gleixner <tglx@...utronix.de>,
David Howells <dhowells@...hat.com>,
Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier
(x86) (v12)
On Sun, Mar 15, 2015 at 03:24:19PM -0400, Mathieu Desnoyers wrote:
TL;DR
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -45,6 +45,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> #endif
> cpumask_set_cpu(cpu, mm_cpumask(next));
>
> + /*
> + * smp_mb() between mm_cpumask set and following memory
> + * accesses to user-space addresses is required by
> + * sys_membarrier(). A smp_mb() is also needed between
> + * prior memory accesses and mm_cpumask clear. This
> + * ensures that all user-space address memory accesses
> + * performed by the current thread are in program order
> + * when the mm_cpumask is set. Implied by load_cr3.
> + */
> +
> /* Re-load page tables */
> load_cr3(next->pgd);
> trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
> @@ -82,6 +92,13 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> * We were in lazy tlb mode and leave_mm disabled
> * tlb flush IPI delivery. We must reload CR3
> * to make sure to use no freed page tables.
> + *
> + * smp_mb() between mm_cpumask set and memory accesses
> + * to user-space addresses is required by
> + * sys_membarrier(). This ensures that all user-space
> + * address memory accesses performed by the current
> + * thread are in program order when the mm_cpumask is
> + * set. Implied by load_cr3.
> */
> load_cr3(next->pgd);
> trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
In both cases the cpumask_set_cpu() will also imply a MB.
> +enum {
> + /*
> + * Private flag set: only synchronize across a single process. If this
> + * flag is not set, it means "shared": synchronize across multiple
> + * processes. The shared mode is useful for shared memory mappings
> + * across processes.
> + */
> + MEMBARRIER_PRIVATE_FLAG = (1 << 0),
> +
> + /*
> + * Expedited flag set: adds some overhead, fast execution (few
> + * microseconds). If this flag is not set, it means "delayed": low
> + * overhead, but slow execution (few milliseconds).
> + */
> + MEMBARRIER_EXPEDITED_FLAG = (1 << 1),
I suppose this is an unprivileged syscall; so what do we do about:
for (;;)
sys_membar(EXPEDITED);
Which would spray the entire system with IPIs at break neck speed.
> +static void membarrier_ipi(void *unused)
> +{
> + /* Order memory accesses with respects to sys_membarrier caller. */
> + smp_mb();
> +}
> +
> +/*
> + * Handle out-of-memory by sending per-cpu IPIs instead.
> + */
> +static void membarrier_fallback(void)
> +{
> + struct mm_struct *mm;
> + int cpu;
> +
> + for_each_cpu(cpu, mm_cpumask(current->mm)) {
> + raw_spin_lock_irq(&cpu_rq(cpu)->lock);
> + mm = cpu_curr(cpu)->mm;
> + raw_spin_unlock_irq(&cpu_rq(cpu)->lock);
> + if (current->mm == mm)
> + smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
> + }
> +}
> +static void membarrier_expedited(void)
> +{
> + struct mm_struct *mm;
> + cpumask_var_t tmpmask;
> + int cpu;
> +
> + /*
> + * Memory barrier on the caller thread between previous memory accesses
> + * to user-space addresses and sending memory-barrier IPIs. Orders all
> + * user-space address memory accesses prior to sys_membarrier() before
> + * mm_cpumask read and membarrier_ipi executions. This barrier is paired
> + * with memory barriers in:
> + * - membarrier_ipi() (for each running threads of the current process)
> + * - switch_mm() (ordering scheduler mm_cpumask update wrt memory
> + * accesses to user-space addresses)
> + * - Each CPU ->mm update performed with rq lock held by the scheduler.
> + * A memory barrier is issued each time ->mm is changed while the rq
> + * lock is held.
> + */
> + smp_mb();
> + if (!alloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
> + membarrier_fallback();
> + goto out;
> + }
> + cpumask_copy(tmpmask, mm_cpumask(current->mm));
> + preempt_disable();
> + cpumask_clear_cpu(smp_processor_id(), tmpmask);
> + for_each_cpu(cpu, tmpmask) {
> + raw_spin_lock_irq(&cpu_rq(cpu)->lock);
> + mm = cpu_curr(cpu)->mm;
> + raw_spin_unlock_irq(&cpu_rq(cpu)->lock);
> + if (current->mm != mm)
> + cpumask_clear_cpu(cpu, tmpmask);
> + }
> + smp_call_function_many(tmpmask, membarrier_ipi, NULL, 1);
> + preempt_enable();
> + free_cpumask_var(tmpmask);
> +out:
> + /*
> + * Memory barrier on the caller thread between sending & waiting for
> + * memory-barrier IPIs and following memory accesses to user-space
> + * addresses. Orders mm_cpumask read and membarrier_ipi executions
> + * before all user-space address memory accesses following
> + * sys_membarrier(). This barrier is paired with memory barriers in:
> + * - membarrier_ipi() (for each running threads of the current process)
> + * - switch_mm() (ordering scheduler mm_cpumask update wrt memory
> + * accesses to user-space addresses)
> + * - Each CPU ->mm update performed with rq lock held by the scheduler.
> + * A memory barrier is issued each time ->mm is changed while the rq
> + * lock is held.
> + */
> + smp_mb();
> +}
Did you just write:
bool membar_cpu_is_mm(int cpu, void *info)
{
struct mm_struct *mm = info;
struct rq *rq = cpu_rq(cpu);
bool ret;
raw_spin_lock_irq(&rq->lock);
ret = rq->curr->mm == mm;
raw_spin_unlock_irq(&rq->lock);
return ret;
}
on_each_cpu_cond(membar_cpu_is_mm, membar_ipi, mm, 1, GFP_NOWAIT);
On which; I absolutely hate that rq->lock thing in there. What is
'wrong' with doing a lockless compare there? Other than not actually
being able to deref rq->curr of course, but we need to fix that anyhow.
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