smart_ptr spinlocks leads to livelock on ARM Cortex-A9 CPUs

[alex]

Spinlock used in smart_ptr for ARM architecture (smart_ptr/details/spinlock_gcc_arm.hpp) relies on swp/mcr instruction sequence or ldrex/strex/dmb depending on ARM version. Both these implementations are affected by an ARM issue described in errata 754327 for ARM Cortex-A9 CPU.

Suggested fix is to add "dmb" instruction to change spinlock::unlock() method:

void unlock() {

asm volatile( BOOST_SP_ARM_BARRIER ::: "memory" ); *const_cast< int volatile* >( &v_ ) = 0; asm volatile( BOOST_SP_ARM_BARRIER ::: "memory" );

}

Second barrier was added as recommended workaround from ARM errata document.

[Peter Dimov]

In ​https://github.com/boostorg/smart_ptr/commit/a42dda0af44727466c07e2d7fb3cfbb649b51bf2.