Re: [PATCH v8 1/3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Sep 23, 2020, at 7:36 PM, Peter Oskolkov posk@google.com wrote:

> This patchset is based on Google-internal RSEQ
> work done by Paul Turner and Andrew Hunter.
> 
> When working with per-CPU RSEQ-based memory allocations,
> it is sometimes important to make sure that a global
> memory location is no longer accessed from RSEQ critical
> sections. For example, there can be two per-CPU lists,
> one is "active" and accessed per-CPU, while another one
> is inactive and worked on asynchronously "off CPU" (e.g.
> garbage collection is performed). Then at some point
> the two lists are swapped, and a fast RCU-like mechanism
> is required to make sure that the previously active
> list is no longer accessed.
> 
> This patch introduces such a mechanism: in short,
> membarrier() syscall issues an IPI to a CPU, restarting
> a potentially active RSEQ critical section on the CPU.
> 

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

For the next time, you should move the changelog below (vN->vN+1)
after a "---" line, which comes after all the Signed-off-by, acked-by
and others.

Thanks,

Mathieu

> v1->v2:
>  - removed the ability to IPI all CPUs in a single sycall;
>  - use task->mm rather than task->group_leader to identify
>    tasks belonging to the same process.
> v2->v3:
>  - re-added the ability to IPI all CPUs in a single syscall;
>  - integrated with membarrier_private_expedited() to
>    make sure only CPUs running tasks with the same mm as
>    the current task are interrupted;
>  - also added MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ;
>  - flags in membarrier_private_expedited are never actually
>    bit flags but always distinct values (i.e. never two flags
>    are combined), so I modified bit testing to full equation
>    comparison for simplicity (otherwise the code needs to
>    work when several bits are set, for example).
> v3->v4:
>  - added the third parameter to membarrier syscall: @cpu_id:
>    if @flags == MEMBARRIER_CMD_FLAG_CPU, then @cpu_id indicates
>    the cpu on which RSEQ CS should be restarted.
> v4->v5:
>  - added @cpu_id parameter to sys_membarrier in syscalls.h.
> v5->v6:
>  - made membarrier_private_expedited more efficient in a
>    single-cpu case;
>  - a couple of minor refactorings.
> v6->v7:
>  - made @flags an unsigned int in sys_membarrier;
>  - a couple of minor refactorings.
> v7->v8:
>  - replaced BUG_ON with WARN_ON_ONCE in membarrier.c.
> 
> The second patch in the patchset adds a selftest
> of this feature.
> 
> Signed-off-by: Peter Oskolkov <posk@google.com>
> ---
> include/linux/sched/mm.h        |   3 +
> include/linux/syscalls.h        |   2 +-
> include/uapi/linux/membarrier.h |  26 ++++++
> kernel/sched/membarrier.c       | 136 +++++++++++++++++++++++++-------
> 4 files changed, 136 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index f889e332912f..15bfb06f2884 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -348,10 +348,13 @@ enum {
> 	MEMBARRIER_STATE_GLOBAL_EXPEDITED			= (1U << 3),
> 	MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY	= (1U << 4),
> 	MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE		= (1U << 5),
> +	MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY		= (1U << 6),
> +	MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ			= (1U << 7),
> };
> 
> enum {
> 	MEMBARRIER_FLAG_SYNC_CORE	= (1U << 0),
> +	MEMBARRIER_FLAG_RSEQ		= (1U << 1),
> };
> 
> #ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 75ac7f8ae93c..466c993e52bf 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -974,7 +974,7 @@ asmlinkage long sys_execveat(int dfd, const char __user
> *filename,
> 			const char __user *const __user *argv,
> 			const char __user *const __user *envp, int flags);
> asmlinkage long sys_userfaultfd(int flags);
> -asmlinkage long sys_membarrier(int cmd, int flags);
> +asmlinkage long sys_membarrier(int cmd, int flags, int cpu_id);
> asmlinkage long sys_mlock2(unsigned long start, size_t len, int flags);
> asmlinkage long sys_copy_file_range(int fd_in, loff_t __user *off_in,
> 				    int fd_out, loff_t __user *off_out,
> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> index 5891d7614c8c..737605897f36 100644
> --- a/include/uapi/linux/membarrier.h
> +++ b/include/uapi/linux/membarrier.h
> @@ -114,6 +114,26 @@
>  *                          If this command is not implemented by an
>  *                          architecture, -EINVAL is returned.
>  *                          Returns 0 on success.
> + * @MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
> + *                          Ensure the caller thread, upon return from
> + *                          system call, that all its running thread
> + *                          siblings have any currently running rseq
> + *                          critical sections restarted if @flags
> + *                          parameter is 0; if @flags parameter is
> + *                          MEMBARRIER_CMD_FLAG_CPU,
> + *                          then this operation is performed only
> + *                          on CPU indicated by @cpu_id. If this command is
> + *                          not implemented by an architecture, -EINVAL
> + *                          is returned. A process needs to register its
> + *                          intent to use the private expedited rseq
> + *                          command prior to using it, otherwise
> + *                          this command returns -EPERM.
> + * @MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
> + *                          Register the process intent to use
> + *                          MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ.
> + *                          If this command is not implemented by an
> + *                          architecture, -EINVAL is returned.
> + *                          Returns 0 on success.
>  * @MEMBARRIER_CMD_SHARED:
>  *                          Alias to MEMBARRIER_CMD_GLOBAL. Provided for
>  *                          header backward compatibility.
> @@ -131,9 +151,15 @@ enum membarrier_cmd {
> 	MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED		= (1 << 4),
> 	MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE		= (1 << 5),
> 	MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE	= (1 << 6),
> +	MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ			= (1 << 7),
> +	MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ		= (1 << 8),
> 
> 	/* Alias for header backward compatibility. */
> 	MEMBARRIER_CMD_SHARED			= MEMBARRIER_CMD_GLOBAL,
> };
> 
> +enum membarrier_cmd_flag {
> +	MEMBARRIER_CMD_FLAG_CPU		= (1 << 0),
> +};
> +
> #endif /* _UAPI_LINUX_MEMBARRIER_H */
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 168479a7d61b..e23e74d52db5 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -18,6 +18,14 @@
> #define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK	0
> #endif
> 
> +#ifdef CONFIG_RSEQ
> +#define MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ_BITMASK		\
> +	(MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ			\
> +	| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ_BITMASK)
> +#else
> +#define MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ_BITMASK	0
> +#endif
> +
> #define MEMBARRIER_CMD_BITMASK						\
> 	(MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED	\
> 	| MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED			\
> @@ -30,6 +38,11 @@ static void ipi_mb(void *info)
> 	smp_mb();	/* IPIs should be serializing but paranoid. */
> }
> 
> +static void ipi_rseq(void *info)
> +{
> +	rseq_preempt(current);
> +}
> +
> static void ipi_sync_rq_state(void *info)
> {
> 	struct mm_struct *mm = (struct mm_struct *) info;
> @@ -129,19 +142,27 @@ static int membarrier_global_expedited(void)
> 	return 0;
> }
> 
> -static int membarrier_private_expedited(int flags)
> +static int membarrier_private_expedited(int flags, int cpu_id)
> {
> -	int cpu;
> 	cpumask_var_t tmpmask;
> 	struct mm_struct *mm = current->mm;
> +	smp_call_func_t ipi_func = ipi_mb;
> 
> -	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
> +	if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
> 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> 			return -EINVAL;
> 		if (!(atomic_read(&mm->membarrier_state) &
> 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
> 			return -EPERM;
> +	} else if (flags == MEMBARRIER_FLAG_RSEQ) {
> +		if (!IS_ENABLED(CONFIG_RSEQ))
> +			return -EINVAL;
> +		if (!(atomic_read(&mm->membarrier_state) &
> +		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY))
> +			return -EPERM;
> +		ipi_func = ipi_rseq;
> 	} else {
> +		WARN_ON_ONCE(flags);
> 		if (!(atomic_read(&mm->membarrier_state) &
> 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
> 			return -EPERM;
> @@ -156,35 +177,59 @@ static int membarrier_private_expedited(int flags)
> 	 */
> 	smp_mb();	/* system call entry is not a mb. */
> 
> -	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
> +	if (cpu_id < 0 && !zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
> 		return -ENOMEM;
> 
> 	cpus_read_lock();
> -	rcu_read_lock();
> -	for_each_online_cpu(cpu) {
> +
> +	if (cpu_id >= 0) {
> 		struct task_struct *p;
> 
> -		/*
> -		 * Skipping the current CPU is OK even through we can be
> -		 * migrated at any point. The current CPU, at the point
> -		 * where we read raw_smp_processor_id(), is ensured to
> -		 * be in program order with respect to the caller
> -		 * thread. Therefore, we can skip this CPU from the
> -		 * iteration.
> -		 */
> -		if (cpu == raw_smp_processor_id())
> -			continue;
> -		p = rcu_dereference(cpu_rq(cpu)->curr);
> -		if (p && p->mm == mm)
> -			__cpumask_set_cpu(cpu, tmpmask);
> +		if (cpu_id >= nr_cpu_ids || !cpu_online(cpu_id))
> +			goto out;
> +		if (cpu_id == raw_smp_processor_id())
> +			goto out;
> +		rcu_read_lock();
> +		p = rcu_dereference(cpu_rq(cpu_id)->curr);
> +		if (!p || p->mm != mm) {
> +			rcu_read_unlock();
> +			goto out;
> +		}
> +		rcu_read_unlock();
> +	} else {
> +		int cpu;
> +
> +		rcu_read_lock();
> +		for_each_online_cpu(cpu) {
> +			struct task_struct *p;
> +
> +			/*
> +			 * Skipping the current CPU is OK even through we can be
> +			 * migrated at any point. The current CPU, at the point
> +			 * where we read raw_smp_processor_id(), is ensured to
> +			 * be in program order with respect to the caller
> +			 * thread. Therefore, we can skip this CPU from the
> +			 * iteration.
> +			 */
> +			if (cpu == raw_smp_processor_id())
> +				continue;
> +			p = rcu_dereference(cpu_rq(cpu)->curr);
> +			if (p && p->mm == mm)
> +				__cpumask_set_cpu(cpu, tmpmask);
> +		}
> +		rcu_read_unlock();
> 	}
> -	rcu_read_unlock();
> 
> 	preempt_disable();
> -	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> +	if (cpu_id >= 0)
> +		smp_call_function_single(cpu_id, ipi_func, NULL, 1);
> +	else
> +		smp_call_function_many(tmpmask, ipi_func, NULL, 1);
> 	preempt_enable();
> 
> -	free_cpumask_var(tmpmask);
> +out:
> +	if (cpu_id < 0)
> +		free_cpumask_var(tmpmask);
> 	cpus_read_unlock();
> 
> 	/*
> @@ -283,11 +328,18 @@ static int membarrier_register_private_expedited(int
> flags)
> 	    set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED,
> 	    ret;
> 
> -	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
> +	if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
> 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> 			return -EINVAL;
> 		ready_state =
> 			MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY;
> +	} else if (flags == MEMBARRIER_FLAG_RSEQ) {
> +		if (!IS_ENABLED(CONFIG_RSEQ))
> +			return -EINVAL;
> +		ready_state =
> +			MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY;
> +	} else {
> +		WARN_ON_ONCE(flags);
> 	}
> 
> 	/*
> @@ -299,6 +351,8 @@ static int membarrier_register_private_expedited(int flags)
> 		return 0;
> 	if (flags & MEMBARRIER_FLAG_SYNC_CORE)
> 		set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE;
> +	if (flags & MEMBARRIER_FLAG_RSEQ)
> +		set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ;
> 	atomic_or(set_state, &mm->membarrier_state);
> 	ret = sync_runqueues_membarrier_state(mm);
> 	if (ret)
> @@ -310,8 +364,15 @@ static int membarrier_register_private_expedited(int flags)
> 
> /**
>  * sys_membarrier - issue memory barriers on a set of threads
> - * @cmd:   Takes command values defined in enum membarrier_cmd.
> - * @flags: Currently needs to be 0. For future extensions.
> + * @cmd:    Takes command values defined in enum membarrier_cmd.
> + * @flags:  Currently needs to be 0 for all commands other than
> + *          MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ: in the latter
> + *          case it can be MEMBARRIER_CMD_FLAG_CPU, indicating that @cpu_id
> + *          contains the CPU on which to interrupt (= restart)
> + *          the RSEQ critical section.
> + * @cpu_id: if @flags == MEMBARRIER_CMD_FLAG_CPU, indicates the cpu on which
> + *          RSEQ CS should be interrupted (@cmd must be
> + *          MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ).
>  *
>  * If this system call is not implemented, -ENOSYS is returned. If the
>  * command specified does not exist, not available on the running
> @@ -337,10 +398,21 @@ static int membarrier_register_private_expedited(int
> flags)
>  *        smp_mb()           X           O            O
>  *        sys_membarrier()   O           O            O
>  */
> -SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> +SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
> {
> -	if (unlikely(flags))
> -		return -EINVAL;
> +	switch (cmd) {
> +	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
> +		if (unlikely(flags && flags != MEMBARRIER_CMD_FLAG_CPU))
> +			return -EINVAL;
> +		break;
> +	default:
> +		if (unlikely(flags))
> +			return -EINVAL;
> +	}
> +
> +	if (!(flags & MEMBARRIER_CMD_FLAG_CPU))
> +		cpu_id = -1;
> +
> 	switch (cmd) {
> 	case MEMBARRIER_CMD_QUERY:
> 	{
> @@ -362,13 +434,17 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> 	case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
> 		return membarrier_register_global_expedited();
> 	case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> -		return membarrier_private_expedited(0);
> +		return membarrier_private_expedited(0, cpu_id);
> 	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
> 		return membarrier_register_private_expedited(0);
> 	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE:
> -		return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
> +		return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE, cpu_id);
> 	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE:
> 		return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
> +	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
> +		return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, cpu_id);
> +	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
> +		return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
> 	default:
> 		return -EINVAL;
> 	}
> --
> 2.28.0.709.gb0816b6eb0-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com