diff options
| -rw-r--r-- | cpu-exec-common.c | 20 |
1 files changed, 14 insertions, 6 deletions
diff --git a/cpu-exec-common.c b/cpu-exec-common.c index 16d305b911..b95b09a77d 100644 --- a/cpu-exec-common.c +++ b/cpu-exec-common.c @@ -42,13 +42,21 @@ void cpu_reload_memory_map(CPUState *cpu) AddressSpaceDispatch *d; if (qemu_in_vcpu_thread()) { - /* Do not let the guest prolong the critical section as much as it - * as it desires. + /* The guest can in theory prolong the RCU critical section as long + * as it feels like. The major problem with this is that because it + * can do multiple reconfigurations of the memory map within the + * critical section, we could potentially accumulate an unbounded + * collection of memory data structures awaiting reclamation. * - * Currently, this is prevented by the I/O thread's periodinc kicking - * of the VCPU thread (iothread_requesting_mutex, qemu_cpu_kick_thread) - * but this will go away once TCG's execution moves out of the global - * mutex. + * Because the only thing we're currently protecting with RCU is the + * memory data structures, it's sufficient to break the critical section + * in this callback, which we know will get called every time the + * memory map is rearranged. + * + * (If we add anything else in the system that uses RCU to protect + * its data structures, we will need to implement some other mechanism + * to force TCG CPUs to exit the critical section, at which point this + * part of this callback might become unnecessary.) * * This pair matches cpu_exec's rcu_read_lock()/rcu_read_unlock(), which * only protects cpu->as->dispatch. Since we reload it below, we can |