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linux/kernel/sched/membarrier.c

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// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
*
* membarrier system call
*/
#include "sched.h"
/*
* For documentation purposes, here are some membarrier ordering
* scenarios to keep in mind:
*
* A) Userspace thread execution after IPI vs membarrier's memory
* barrier before sending the IPI
*
* Userspace variables:
*
* int x = 0, y = 0;
*
* The memory barrier at the start of membarrier() on CPU0 is necessary in
* order to enforce the guarantee that any writes occurring on CPU0 before
* the membarrier() is executed will be visible to any code executing on
* CPU1 after the IPI-induced memory barrier:
*
* CPU0 CPU1
*
* x = 1
* membarrier():
* a: smp_mb()
* b: send IPI IPI-induced mb
* c: smp_mb()
* r2 = y
* y = 1
* barrier()
* r1 = x
*
* BUG_ON(r1 == 0 && r2 == 0)
*
* The write to y and load from x by CPU1 are unordered by the hardware,
* so it's possible to have "r1 = x" reordered before "y = 1" at any
* point after (b). If the memory barrier at (a) is omitted, then "x = 1"
* can be reordered after (a) (although not after (c)), so we get r1 == 0
* and r2 == 0. This violates the guarantee that membarrier() is
* supposed by provide.
*
* The timing of the memory barrier at (a) has to ensure that it executes
* before the IPI-induced memory barrier on CPU1.
*
* B) Userspace thread execution before IPI vs membarrier's memory
* barrier after completing the IPI
*
* Userspace variables:
*
* int x = 0, y = 0;
*
* The memory barrier at the end of membarrier() on CPU0 is necessary in
* order to enforce the guarantee that any writes occurring on CPU1 before
* the membarrier() is executed will be visible to any code executing on
* CPU0 after the membarrier():
*
* CPU0 CPU1
*
* x = 1
* barrier()
* y = 1
* r2 = y
* membarrier():
* a: smp_mb()
* b: send IPI IPI-induced mb
* c: smp_mb()
* r1 = x
* BUG_ON(r1 == 0 && r2 == 1)
*
* The writes to x and y are unordered by the hardware, so it's possible to
* have "r2 = 1" even though the write to x doesn't execute until (b). If
* the memory barrier at (c) is omitted then "r1 = x" can be reordered
* before (b) (although not before (a)), so we get "r1 = 0". This violates
* the guarantee that membarrier() is supposed to provide.
*
* The timing of the memory barrier at (c) has to ensure that it executes
* after the IPI-induced memory barrier on CPU1.
*
* C) Scheduling userspace thread -> kthread -> userspace thread vs membarrier
*
* CPU0 CPU1
*
* membarrier():
* a: smp_mb()
* d: switch to kthread (includes mb)
* b: read rq->curr->mm == NULL
* e: switch to user (includes mb)
* c: smp_mb()
*
* Using the scenario from (A), we can show that (a) needs to be paired
* with (e). Using the scenario from (B), we can show that (c) needs to
* be paired with (d).
*
* D) exit_mm vs membarrier
*
* Two thread groups are created, A and B. Thread group B is created by
* issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD.
* Let's assume we have a single thread within each thread group (Thread A
* and Thread B). Thread A runs on CPU0, Thread B runs on CPU1.
*
* CPU0 CPU1
*
* membarrier():
* a: smp_mb()
* exit_mm():
* d: smp_mb()
* e: current->mm = NULL
* b: read rq->curr->mm == NULL
* c: smp_mb()
*
* Using scenario (B), we can show that (c) needs to be paired with (d).
*
* E) kthread_{use,unuse}_mm vs membarrier
*
* CPU0 CPU1
*
* membarrier():
* a: smp_mb()
* kthread_unuse_mm()
* d: smp_mb()
* e: current->mm = NULL
* b: read rq->curr->mm == NULL
* kthread_use_mm()
* f: current->mm = mm
* g: smp_mb()
* c: smp_mb()
*
* Using the scenario from (A), we can show that (a) needs to be paired
* with (g). Using the scenario from (B), we can show that (c) needs to
* be paired with (d).
*/
/*
* Bitmask made from a "or" of all commands within enum membarrier_cmd,
* except MEMBARRIER_CMD_QUERY.
*/
#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK \
(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE)
#else
#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 \
| MEMBARRIER_CMD_PRIVATE_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \
| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK)
static void ipi_mb(void *info)
{
smp_mb(); /* IPIs should be serializing but paranoid. */
}
static void ipi_sync_core(void *info)
{
/*
* The smp_mb() in membarrier after all the IPIs is supposed to
* ensure that memory on remote CPUs that occur before the IPI
* become visible to membarrier()'s caller -- see scenario B in
* the big comment at the top of this file.
*
* A sync_core() would provide this guarantee, but
* sync_core_before_usermode() might end up being deferred until
* after membarrier()'s smp_mb().
*/
smp_mb(); /* IPIs should be serializing but paranoid. */
sync_core_before_usermode();
}
static void ipi_rseq(void *info)
{
/*
* Ensure that all stores done by the calling thread are visible
* to the current task before the current task resumes. We could
* probably optimize this away on most architectures, but by the
* time we've already sent an IPI, the cost of the extra smp_mb()
* is negligible.
*/
smp_mb();
rseq_preempt(current);
}
static void ipi_sync_rq_state(void *info)
{
struct mm_struct *mm = (struct mm_struct *) info;
if (current->mm != mm)
return;
this_cpu_write(runqueues.membarrier_state,
atomic_read(&mm->membarrier_state));
/*
* Issue a memory barrier after setting
* MEMBARRIER_STATE_GLOBAL_EXPEDITED in the current runqueue to
* guarantee that no memory access following registration is reordered
* before registration.
*/
smp_mb();
}
void membarrier_exec_mmap(struct mm_struct *mm)
{
/*
* Issue a memory barrier before clearing membarrier_state to
* guarantee that no memory access prior to exec is reordered after
* clearing this state.
*/
smp_mb();
atomic_set(&mm->membarrier_state, 0);
/*
* Keep the runqueue membarrier_state in sync with this mm
* membarrier_state.
*/
this_cpu_write(runqueues.membarrier_state, 0);
}
void membarrier_update_current_mm(struct mm_struct *next_mm)
{
struct rq *rq = this_rq();
int membarrier_state = 0;
if (next_mm)
membarrier_state = atomic_read(&next_mm->membarrier_state);
if (READ_ONCE(rq->membarrier_state) == membarrier_state)
return;
WRITE_ONCE(rq->membarrier_state, membarrier_state);
}
static int membarrier_global_expedited(void)
{
int cpu;
cpumask_var_t tmpmask;
if (num_online_cpus() == 1)
return 0;
/*
* Matches memory barriers around rq->curr modification in
* scheduler.
*/
smp_mb(); /* system call entry is not a mb. */
if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
return -ENOMEM;
cpus_read_lock();
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;
if (!(READ_ONCE(cpu_rq(cpu)->membarrier_state) &
MEMBARRIER_STATE_GLOBAL_EXPEDITED))
continue;
/*
* Skip the CPU if it runs a kernel thread which is not using
* a task mm.
*/
p = rcu_dereference(cpu_rq(cpu)->curr);
if (!p->mm)
continue;
__cpumask_set_cpu(cpu, tmpmask);
}
rcu_read_unlock();
preempt_disable();
smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
preempt_enable();
free_cpumask_var(tmpmask);
cpus_read_unlock();
/*
* Memory barrier on the caller thread _after_ we finished
* waiting for the last IPI. Matches memory barriers around
* rq->curr modification in scheduler.
*/
smp_mb(); /* exit from system call is not a mb */
return 0;
}
static int membarrier_private_expedited(int flags, int cpu_id)
{
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 (!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;
ipi_func = ipi_sync_core;
} 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;
}
if (flags != MEMBARRIER_FLAG_SYNC_CORE &&
(atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1))
return 0;
/*
* Matches memory barriers around rq->curr modification in
* scheduler.
*/
smp_mb(); /* system call entry is not a mb. */
if (cpu_id < 0 && !zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
return -ENOMEM;
cpus_read_lock();
if (cpu_id >= 0) {
struct task_struct *p;
if (cpu_id >= nr_cpu_ids || !cpu_online(cpu_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;
p = rcu_dereference(cpu_rq(cpu)->curr);
if (p && p->mm == mm)
__cpumask_set_cpu(cpu, tmpmask);
}
rcu_read_unlock();
}
if (cpu_id >= 0) {
/*
* smp_call_function_single() will call ipi_func() if cpu_id
* is the calling CPU.
*/
smp_call_function_single(cpu_id, ipi_func, NULL, 1);
} else {
/*
* For regular membarrier, we can save a few cycles by
* skipping the current cpu -- we're about to do smp_mb()
* below, and if we migrate to a different cpu, this cpu
* and the new cpu will execute a full barrier in the
* scheduler.
*
* For SYNC_CORE, we do need a barrier on the current cpu --
* otherwise, if we are migrated and replaced by a different
* task in the same mm just before, during, or after
* membarrier, we will end up with some thread in the mm
* running without a core sync.
*
* For RSEQ, don't rseq_preempt() the caller. User code
* is not supposed to issue syscalls at all from inside an
* rseq critical section.
*/
if (flags != MEMBARRIER_FLAG_SYNC_CORE) {
preempt_disable();
smp_call_function_many(tmpmask, ipi_func, NULL, true);
preempt_enable();
} else {
on_each_cpu_mask(tmpmask, ipi_func, NULL, true);
}
}
out:
if (cpu_id < 0)
free_cpumask_var(tmpmask);
cpus_read_unlock();
/*
* Memory barrier on the caller thread _after_ we finished
* waiting for the last IPI. Matches memory barriers around
* rq->curr modification in scheduler.
*/
smp_mb(); /* exit from system call is not a mb */
return 0;
}
static int sync_runqueues_membarrier_state(struct mm_struct *mm)
{
int membarrier_state = atomic_read(&mm->membarrier_state);
cpumask_var_t tmpmask;
int cpu;
if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) {
this_cpu_write(runqueues.membarrier_state, membarrier_state);
/*
* For single mm user, we can simply issue a memory barrier
* after setting MEMBARRIER_STATE_GLOBAL_EXPEDITED in the
* mm and in the current runqueue to guarantee that no memory
* access following registration is reordered before
* registration.
*/
smp_mb();
return 0;
}
if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
return -ENOMEM;
/*
* For mm with multiple users, we need to ensure all future
* scheduler executions will observe @mm's new membarrier
* state.
*/
synchronize_rcu();
/*
* For each cpu runqueue, if the task's mm match @mm, ensure that all
* @mm's membarrier state set bits are also set in in the runqueue's
* membarrier state. This ensures that a runqueue scheduling
* between threads which are users of @mm has its membarrier state
* updated.
*/
cpus_read_lock();
rcu_read_lock();
for_each_online_cpu(cpu) {
struct rq *rq = cpu_rq(cpu);
struct task_struct *p;
p = rcu_dereference(rq->curr);
if (p && p->mm == mm)
__cpumask_set_cpu(cpu, tmpmask);
}
rcu_read_unlock();
preempt_disable();
smp_call_function_many(tmpmask, ipi_sync_rq_state, mm, 1);
preempt_enable();
free_cpumask_var(tmpmask);
cpus_read_unlock();
return 0;
}
static int membarrier_register_global_expedited(void)
{
struct task_struct *p = current;
struct mm_struct *mm = p->mm;
int ret;
if (atomic_read(&mm->membarrier_state) &
MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY)
return 0;
atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED, &mm->membarrier_state);
ret = sync_runqueues_membarrier_state(mm);
if (ret)
return ret;
atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
&mm->membarrier_state);
return 0;
}
static int membarrier_register_private_expedited(int flags)
{
struct task_struct *p = current;
struct mm_struct *mm = p->mm;
int ready_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED,
ret;
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);
}
/*
* We need to consider threads belonging to different thread
* groups, which use the same mm. (CLONE_VM but not
* CLONE_THREAD).
*/
if ((atomic_read(&mm->membarrier_state) & ready_state) == ready_state)
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)
return ret;
atomic_or(ready_state, &mm->membarrier_state);
return 0;
}
/**
* 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 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
* kernel, or if the command argument is invalid, this system call
* returns -EINVAL. For a given command, with flags argument set to 0,
* if this system call returns -ENOSYS or -EINVAL, it is guaranteed to
* always return the same value until reboot. In addition, it can return
* -ENOMEM if there is not enough memory available to perform the system
* call.
*
* All memory accesses performed in program order from each targeted thread
* is guaranteed to be ordered with respect to sys_membarrier(). If we use
* the semantic "barrier()" to represent a compiler barrier forcing memory
* accesses to be performed in program order across the barrier, and
* smp_mb() to represent explicit memory barriers forcing full memory
* ordering across the barrier, we have the following ordering table for
* each pair of barrier(), sys_membarrier() and smp_mb():
*
* The pair ordering is detailed as (O: ordered, X: not ordered):
*
* barrier() smp_mb() sys_membarrier()
* barrier() X X O
* smp_mb() X O O
* sys_membarrier() O O O
*/
SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
{
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:
{
int cmd_mask = MEMBARRIER_CMD_BITMASK;
if (tick_nohz_full_enabled())
cmd_mask &= ~MEMBARRIER_CMD_GLOBAL;
return cmd_mask;
}
case MEMBARRIER_CMD_GLOBAL:
/* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */
if (tick_nohz_full_enabled())
return -EINVAL;
if (num_online_cpus() > 1)
synchronize_rcu();
return 0;
case MEMBARRIER_CMD_GLOBAL_EXPEDITED:
return membarrier_global_expedited();
case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
return membarrier_register_global_expedited();
case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
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, 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;
}
}
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