----- On Dec 4, 2020, at 12:07 AM, Andy Lutomirski luto@xxxxxxxxxx wrote:
> membarrier()'s MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is documented
> as syncing the core on all sibling threads but not necessarily the
> calling thread. This behavior is fundamentally buggy and cannot be used
> safely. Suppose a user program has two threads. Thread A is on CPU 0
> and thread B is on CPU 1. Thread A modifies some text and calls
> membarrier(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE). Then thread B
> executes the modified code. If, at any point after membarrier() decides
> which CPUs to target, thread A could be preempted and replaced by thread
> B on CPU 0. This could even happen on exit from the membarrier()
> syscall. If this happens, thread B will end up running on CPU 0 without
> having synced.
>
> In principle, this could be fixed by arranging for the scheduler to
> sync_core_before_usermode() whenever switching between two threads in
> the same mm if there is any possibility of a concurrent membarrier()
> call, but this would have considerable overhead. Instead, make
> membarrier() sync the calling CPU as well.
>
> As an optimization, this avoids an extra smp_mb() in the default
> barrier-only mode.
^ we could also add to the commit message that it avoids doing rseq preempt
on the caller as well.
Other than that:
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Thanks!
Mathieu
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
> kernel/sched/membarrier.c | 51 +++++++++++++++++++++++++--------------
> 1 file changed, 33 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 01538b31f27e..57266ab32ef9 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -333,7 +333,8 @@ static int membarrier_private_expedited(int flags, int
> cpu_id)
> return -EPERM;
> }
>
> - if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1)
> + if (flags != MEMBARRIER_FLAG_SYNC_CORE &&
> + (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1))
> return 0;
>
> /*
> @@ -352,8 +353,6 @@ static int membarrier_private_expedited(int flags, int
> cpu_id)
>
> 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) {
> @@ -368,16 +367,6 @@ static int membarrier_private_expedited(int flags, int
> cpu_id)
> 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);
> @@ -385,12 +374,38 @@ static int membarrier_private_expedited(int flags, int
> cpu_id)
> rcu_read_unlock();
> }
>
> - preempt_disable();
> - if (cpu_id >= 0)
> + 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
> - smp_call_function_many(tmpmask, ipi_func, NULL, 1);
> - preempt_enable();
> + } 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 CORE_SYNC, 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)
> --
> 2.28.0
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
