Re: [Qemu-devel] [PATCH v2 03/15] migration: No need to take rcu during sync_dirty_bitmap

[Paolo Bonzini]

On 20/05/19 05:08, Peter Xu wrote:
> cpu_physical_memory_sync_dirty_bitmap() has one RAMBlock* as
> parameter, which means that it must be with RCU read lock held
> already.  Taking it again inside seems redundant.  Removing it.
> Instead comment on the functions about the RCU read lock.
> 
> Reviewed-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  include/exec/ram_addr.h | 5 +----
>  migration/ram.c         | 1 +
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index 139ad79390..993fb760f3 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -408,6 +408,7 @@ static inline void 
> cpu_physical_memory_clear_dirty_range(ram_addr_t start,
>  }
>  
>  
> +/* Must be with rcu read lock held */

The usual way to spell this is "Called within RCU critical section.",
otherwise the patch looks good.

Paolo

>  static inline
>  uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>                                                 ram_addr_t start,
> @@ -431,8 +432,6 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock 
> *rb,
>                                          DIRTY_MEMORY_BLOCK_SIZE);
>          unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
>  
> -        rcu_read_lock();
> -
>          src = atomic_rcu_read(
>                  &ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION])->blocks;
>  
> @@ -452,8 +451,6 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock 
> *rb,
>                  idx++;
>              }
>          }
> -
> -        rcu_read_unlock();
>      } else {
>          ram_addr_t offset = rb->offset;
>  
> diff --git a/migration/ram.c b/migration/ram.c
> index 4c60869226..05f9f36c7c 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1678,6 +1678,7 @@ static inline bool 
> migration_bitmap_clear_dirty(RAMState *rs,
>      return ret;
>  }
>  
> +/* Must be with rcu read lock held */
>  static void migration_bitmap_sync_range(RAMState *rs, RAMBlock *rb,
>                                          ram_addr_t length)
>  {
>