membarrier: riscv: Add full memory barrier in switch_mm()

commit d6cfd1770f20392d7009ae1fdb04733794514fa9 upstream.

The membarrier system call requires a full memory barrier after storing
to rq->cu

membarrier: riscv: Add full memory barrier in switch_mm()

commit d6cfd1770f20392d7009ae1fdb04733794514fa9 upstream.

The membarrier system call requires a full memory barrier after storing
to rq->curr, before going back to user-space. The barrier is only
needed when switching between processes: the barrier is implied by
mmdrop() when switching from kernel to userspace, and it's not needed
when switching from userspace to kernel.

Rely on the feature/mechanism ARCH_HAS_MEMBARRIER_CALLBACKS and on the
primitive membarrier_arch_switch_mm(), already adopted by the PowerPC
architecture, to insert the required barrier.

Fixes: fab957c11efe2f ("RISC-V: Atomic and Locking Code")
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20240131144936.29190-2-parri.andrea@gmail.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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