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SubjectRe: [PATCH 1/2 v6] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
Any comments here? Should I change anything?

Thanks,
Peter

On Mon, Aug 31, 2020 at 3:58 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.
>
> 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 other minor refactorings.
>
> 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 | 29 +++++++
> kernel/sched/membarrier.c | 140 +++++++++++++++++++++++++-------
> 4 files changed, 143 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..98c2b0e7c0d8 100644
> --- a/include/uapi/linux/membarrier.h
> +++ b/include/uapi/linux/membarrier.h
> @@ -114,6 +114,29 @@
> * If this command is not implemented by an
> * architecture, -EINVAL is returned.
> * Returns 0 on success.
> + * @MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
> + * In addition to provide memory ordering
> + * guarantees described in
> + * MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE,
> + * 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 +154,15 @@ 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..e32e9476ccf3 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 +38,13 @@ static void ipi_mb(void *info)
> smp_mb(); /* IPIs should be serializing but paranoid. */
> }
>
> +#ifdef CONFIG_RSEQ
> +static void ipi_rseq(void *info)
> +{
> + rseq_preempt(current);
> +}
> +#endif
> +
> static void ipi_sync_rq_state(void *info)
> {
> struct mm_struct *mm = (struct mm_struct *) info;
> @@ -129,19 +144,29 @@ 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) {
> +#ifdef CONFIG_RSEQ
> + if (!(atomic_read(&mm->membarrier_state) &
> + MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY))
> + return -EPERM;
> + ipi_func = ipi_rseq;
> +#else
> + return -EINVAL;
> +#endif
> } else {
> + BUG_ON(flags != 0);
> if (!(atomic_read(&mm->membarrier_state) &
> MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
> return -EPERM;
> @@ -156,35 +181,59 @@ 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 +332,18 @@ 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 {
> + BUG_ON(flags != 0);
> }
>
> /*
> @@ -299,6 +355,8 @@ 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 +368,15 @@ 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 +402,21 @@ 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, 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 +438,17 @@ 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;
> }
> --
> 2.28.0.402.g5ffc5be6b7-goog
>

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