Re: [Qemu-devel] [RFC PATCH v5 2/4] Migration thread mutex

[Marcelo Tosatti]

On Tue, Aug 23, 2011 at 11:12:48PM -0400, Umesh Deshpande wrote:
> ramlist mutex is implemented to protect the RAMBlock list traversal in the
> migration thread from their addition/removal from the iothread.
> 
> Note: Combination of iothread mutex and migration thread mutex works as a
> rw-lock. Both mutexes are acquired while modifying the ram_list members or RAM
> block list.
> 
> Signed-off-by: Umesh Deshpande <address@hidden>
> ---
>  arch_init.c   |   21 +++++++++++++++++++++
>  cpu-all.h     |    3 +++
>  exec.c        |   23 +++++++++++++++++++++++
>  qemu-common.h |    2 ++
>  4 files changed, 49 insertions(+), 0 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 484b39d..9d02270 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -109,6 +109,7 @@ static int is_dup_page(uint8_t *page, uint8_t ch)
>  
>  static RAMBlock *last_block;
>  static ram_addr_t last_offset;
> +static uint64_t last_version;
>  
>  static int ram_save_block(QEMUFile *f)
>  {
> @@ -170,6 +171,7 @@ static int ram_save_block(QEMUFile *f)
>  
>      last_block = block;
>      last_offset = offset;
> +    last_version = ram_list.version;
>  
>      return bytes_sent;
>  }
> @@ -270,6 +272,7 @@ int ram_save_live(Monitor *mon, QEMUFile *f, int stage, 
> void *opaque)
>          bytes_transferred = 0;
>          last_block = NULL;
>          last_offset = 0;
> +        last_version = ram_list.version = 0;
>          sort_ram_list();
>  
>          /* Make sure all dirty bits are set */
> @@ -298,6 +301,17 @@ int ram_save_live(Monitor *mon, QEMUFile *f, int stage, 
> void *opaque)
>      bytes_transferred_last = bytes_transferred;
>      bwidth = qemu_get_clock_ns(rt_clock);
>  
> +    if (stage != 3) {
> +        qemu_mutex_lock_migthread();
> +        qemu_mutex_unlock_iothread();
> +    }
> +
> +    if (ram_list.version != last_version) {
> +        /* RAM block added or removed */
> +        last_block = NULL;
> +        last_offset = 0;
> +    }
> +
>      while (!qemu_file_rate_limit(f)) {
>          int bytes_sent;
>  
> @@ -308,6 +322,13 @@ int ram_save_live(Monitor *mon, QEMUFile *f, int stage, 
> void *opaque)
>          }
>      }
>  
> +    if (stage != 3) {
> +        qemu_mutex_unlock_migthread();
> +        qemu_mutex_lock_iothread();
> +        /* Lock ordering : iothread mutex is always acquired outside 
> migthread
> +         * mutex critical section to avoid deadlock */
> +    }
> +
>      bwidth = qemu_get_clock_ns(rt_clock) - bwidth;
>      bwidth = (bytes_transferred - bytes_transferred_last) / bwidth;
>  
> diff --git a/cpu-all.h b/cpu-all.h
> index 6b217a2..b85483f 100644
> --- a/cpu-all.h
> +++ b/cpu-all.h
> @@ -21,6 +21,7 @@
>  
>  #include "qemu-common.h"
>  #include "cpu-common.h"
> +#include "qemu-thread.h"
>  
>  /* some important defines:
>   *
> @@ -932,7 +933,9 @@ typedef struct RAMBlock {
>  } RAMBlock;
>  
>  typedef struct RAMList {
> +    QemuMutex mutex;    /* Protects RAM block list */
>      uint8_t *phys_dirty;
> +    uint32_t version;   /* To detect ram block addition/removal */
>      QLIST_HEAD(ram, RAMBlock) blocks;
>      QLIST_HEAD(, RAMBlock) blocks_mru;
>  } RAMList;
> diff --git a/exec.c b/exec.c
> index c5c247c..7627483 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -582,6 +582,7 @@ void cpu_exec_init_all(unsigned long tb_size)
>      code_gen_alloc(tb_size);
>      code_gen_ptr = code_gen_buffer;
>      page_init();
> +    qemu_mutex_init(&ram_list.mutex);
>  #if !defined(CONFIG_USER_ONLY)
>      io_mem_init();
>  #endif
> @@ -2802,6 +2803,16 @@ static long gethugepagesize(const char *path)
>      return fs.f_bsize;
>  }
>  
> +void qemu_mutex_lock_migthread(void)
> +{
> +    qemu_mutex_lock(&ram_list.mutex);
> +}

qemu_mutex_lock_ramlist is a better name, _migthread is confusing.