Re: [PATCH v2 3/4] membarrier: Explicitly sync remote cores when SYNC_CORE is requested

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

----- On Dec 2, 2020, at 10:35 AM, Andy Lutomirski luto@kernel.org wrote:

> membarrier() does not explicitly sync_core() remote CPUs; instead, it
> relies on the assumption that an IPI will result in a core sync.  On
> x86, I think this may be true in practice, but it's not architecturally
> reliable.  In particular, the SDM and APM do not appear to guarantee
> that interrupt delivery is serializing.  While IRET does serialize, IPI
> return can schedule, thereby switching to another task in the same mm
> that was sleeping in a syscall.  The new task could then SYSRET back to
> usermode without ever executing IRET.
> 
> Make this more robust by explicitly calling sync_core_before_usermode()
> on remote cores.  (This also helps people who search the kernel tree for
> instances of sync_core() and sync_core_before_usermode() -- one might be
> surprised that the core membarrier code doesn't currently show up in a
> such a search.)
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Andy Lutomirski <luto@kernel.org>

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

> ---
> kernel/sched/membarrier.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
> 
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 6251d3d12abe..01538b31f27e 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -166,6 +166,23 @@ 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)
> {
> 	/*
> @@ -301,6 +318,7 @@ static int membarrier_private_expedited(int flags, int
> cpu_id)
> 		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;
> --
> 2.28.0

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