Messages in this thread | | | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3a) | From | Peter Zijlstra <> | Date | Mon, 11 Jan 2010 18:50:40 +0100 |
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On Sun, 2010-01-10 at 23:29 -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.
Please start a new thread for new versions, I really didn't find this until I started reading in date order instead of thread order.
> Index: linux-2.6-lttng/arch/x86/include/asm/unistd_64.h > =================================================================== > --- linux-2.6-lttng.orig/arch/x86/include/asm/unistd_64.h 2010-01-10 19:21:31.000000000 -0500 > +++ linux-2.6-lttng/arch/x86/include/asm/unistd_64.h 2010-01-10 19:21:37.000000000 -0500 > @@ -661,6 +661,8 @@ __SYSCALL(__NR_pwritev, sys_pwritev) > __SYSCALL(__NR_rt_tgsigqueueinfo, sys_rt_tgsigqueueinfo) > #define __NR_perf_event_open 298 > __SYSCALL(__NR_perf_event_open, sys_perf_event_open) > +#define __NR_membarrier 299 > +__SYSCALL(__NR_membarrier, sys_membarrier) > > #ifndef __NO_STUBS > #define __ARCH_WANT_OLD_READDIR > Index: linux-2.6-lttng/kernel/sched.c > =================================================================== > --- linux-2.6-lttng.orig/kernel/sched.c 2010-01-10 19:21:31.000000000 -0500 > +++ linux-2.6-lttng/kernel/sched.c 2010-01-10 22:22:40.000000000 -0500 > @@ -2861,12 +2861,26 @@ context_switch(struct rq *rq, struct tas > */ > arch_start_context_switch(prev); > > + /* > + * sys_membarrier IPI-mb scheme requires a memory barrier between > + * user-space thread execution and update to mm_cpumask. > + */ > + if (likely(oldmm) && likely(oldmm != mm)) > + smp_mb__before_clear_bit(); > + > if (unlikely(!mm)) { > next->active_mm = oldmm; > atomic_inc(&oldmm->mm_count); > enter_lazy_tlb(oldmm, next); > - } else > + } else { > switch_mm(oldmm, mm, next); > + /* > + * sys_membarrier IPI-mb scheme requires a memory barrier > + * between update to mm_cpumask and user-space thread execution. > + */ > + if (likely(oldmm != mm)) > + smp_mb__after_clear_bit(); > + } > > if (unlikely(!prev->mm)) { > prev->active_mm = NULL; > @@ -10822,6 +10836,49 @@ struct cgroup_subsys cpuacct_subsys = { > }; > #endif /* CONFIG_CGROUP_CPUACCT */ > > +/* > + * Execute a memory barrier on all active threads from the current process > + * on SMP systems. Do not rely on implicit barriers in > + * smp_call_function_many(), just in case they are ever relaxed in the future. > + */ > +static void membarrier_ipi(void *unused) > +{ > + smp_mb(); > +} > + > +/* > + * sys_membarrier - issue memory barrier on current process running threads > + * > + * Execute a memory barrier on all running threads of the current process. > + * Upon completion, the caller thread is ensured that all process threads > + * have passed through a state where memory accesses match program order. > + * (non-running threads are de facto in such a state) > + */ > +SYSCALL_DEFINE0(membarrier) > +{ > +#ifdef CONFIG_SMP > + if (unlikely(thread_group_empty(current))) > + return 0; > + /* > + * Memory barrier on the caller thread _before_ sending first > + * IPI. Matches memory barriers around mm_cpumask modification in > + * context_switch(). > + */ > + smp_mb(); > + preempt_disable(); > + smp_call_function_many(mm_cpumask(current->mm), membarrier_ipi, > + NULL, 1); > + preempt_enable(); > + /* > + * Memory barrier on the caller thread _after_ we finished > + * waiting for the last IPI. Matches memory barriers around mm_cpumask > + * modification in context_switch(). > + */ > + smp_mb(); > +#endif /* #ifdef CONFIG_SMP */ > + return 0; > +} > + > #ifndef CONFIG_SMP > > int rcu_expedited_torture_stats(char *page)
Right, so here you rely on the arch switch_mm() implementation to keep mm_cpumask() current, but then stick a memory barrier in the generic code... seems odd.
x86 switch_mm() does indeed keep it current, but writing cr3 is also a rather serializing instruction.
Furthermore, do we really need that smp_mb() in the membarrier_ipi() function? Shouldn't we investigate if either: - receiving an IPI implies an mb, or - enter/leave kernelspace implies an mb ?
So while I much like the simplified version, that previous one was heavily over engineer, I 1) don't like that memory barrier in the generic code, 2) don't think that arch's necessarily keep that mask as tight.
[ even if for x86 smp_mb__{before,after}_clear_bit are a nop, tying that to switch_mm() semantics just reeks ]
See for example the sparc64 implementation of switch_mm() that only sets cpus in mm_cpumask(), but only tlb flushes clear them. Also, I wouldn't know if switch_mm() implies an mb on sparc64.
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