Re: [PATCH v3 4/4] membarrier: Execute SYNC_CORE on the calling thread

[Andy Lutomirski <luto@kernel.org>]

On Thu, Dec 3, 2020 at 9:07 PM Andy Lutomirski <luto@kernel.org> 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.

Fixes: 70216e18e519 ("membarrier: Provide core serializing command,
*_SYNC_CORE")

also:

> +               /*
> +                * 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 --

s/CORE_SYNC/SYNC_CORE/

--Andy