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

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