On 09/25/2015 05:09 PM, Denis V. Lunev wrote:
> Release qemu global mutex before call synchronize_rcu().
> synchronize_rcu() waiting for all readers to finish their critical
> sections. There is at least one critical section in which we try
> to get QGM (critical section is in address_space_rw() and
> prepare_mmio_access() is trying to aquire QGM).
>
> Both functions (migration_end() and migration_bitmap_extend())
> are called from main thread which is holding QGM.
>
> Thus there is a race condition that ends up with deadlock:
> main thread working thread
> Lock QGA |
> | Call KVM_EXIT_IO handler
> | |
> | Open rcu reader's critical section
> Migration cleanup bh |
> | |
> synchronize_rcu() is |
> waiting for readers |
> | prepare_mmio_access() is waiting for QGM
> \ /
> deadlock
>
> The patch changes bitmap freeing from direct g_free after synchronize_rcu
> to g_free_rcu.
>
> Signed-off-by: Denis V. Lunev <[email protected]>
> Reported-by: Igor Redko <[email protected]>
> CC: Igor Redko <[email protected]>
> CC: Anna Melekhova <[email protected]>
> CC: Juan Quintela <[email protected]>
> CC: Amit Shah <[email protected]>
> CC: Paolo Bonzini <[email protected]>
> CC: Wen Congyang <[email protected]>
> ---
> migration/ram.c | 43 ++++++++++++++++++++++++++++---------------
> 1 file changed, 28 insertions(+), 15 deletions(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index a712c68..56b6fce 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -221,12 +221,27 @@ static RAMBlock *last_seen_block;
> /* This is the last block from where we have sent data */
> static RAMBlock *last_sent_block;
> static ram_addr_t last_offset;
> -static unsigned long *migration_bitmap;
> static QemuMutex migration_bitmap_mutex;
> static uint64_t migration_dirty_pages;
> static uint32_t last_version;
> static bool ram_bulk_stage;
>
> +static struct BitmapRcu {
> + struct rcu_head rcu;
> + unsigned long bmap[0];
> +} *migration_bitmap_rcu;
> +
> +static inline struct BitmapRcu *bitmap_new_rcu(long nbits)
> +{
> + long len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> + struct BitmapRcu *ptr = g_try_malloc0(len + sizeof(struct BitmapRcu));
It is better to allocate memory twice, one is for BitmapRcu, another is calling
bitmap_new(). The user doesn't need to know how the bitmap is implemented.
> + if (ptr == NULL) {
> + abort();
> + }
> + return ptr;
> +}
> +
> +
> struct CompressParam {
> bool start;
> bool done;
> @@ -508,7 +523,7 @@ ram_addr_t
> migration_bitmap_find_and_reset_dirty(MemoryRegion *mr,
>
> unsigned long next;
>
> - bitmap = atomic_rcu_read(&migration_bitmap);
> + bitmap = atomic_rcu_read(&migration_bitmap_rcu)->bmap;
> if (ram_bulk_stage && nr > base) {
> next = nr + 1;
> } else {
> @@ -526,7 +541,7 @@ ram_addr_t
> migration_bitmap_find_and_reset_dirty(MemoryRegion *mr,
> static void migration_bitmap_sync_range(ram_addr_t start, ram_addr_t length)
> {
> unsigned long *bitmap;
> - bitmap = atomic_rcu_read(&migration_bitmap);
> + bitmap = atomic_rcu_read(&migration_bitmap_rcu)->bmap;
> migration_dirty_pages +=
> cpu_physical_memory_sync_dirty_bitmap(bitmap, start, length);
> }
> @@ -1029,12 +1044,11 @@ static void migration_end(void)
> /* caller have hold iothread lock or is in a bh, so there is
> * no writing race against this migration_bitmap
> */
> - unsigned long *bitmap = migration_bitmap;
> - atomic_rcu_set(&migration_bitmap, NULL);
> + struct BitmapRcu *bitmap = migration_bitmap_rcu;
> + atomic_rcu_set(&migration_bitmap_rcu, NULL);
> if (bitmap) {
> memory_global_dirty_log_stop();
> - synchronize_rcu();
> - g_free(bitmap);
> + g_free_rcu(bitmap, rcu);
> }
>
> XBZRLE_cache_lock();
> @@ -1070,9 +1084,9 @@ void migration_bitmap_extend(ram_addr_t old, ram_addr_t
> new)
> /* called in qemu main thread, so there is
> * no writing race against this migration_bitmap
> */
> - if (migration_bitmap) {
> - unsigned long *old_bitmap = migration_bitmap, *bitmap;
> - bitmap = bitmap_new(new);
> + if (migration_bitmap_rcu) {
> + struct BitmapRcu *old_bitmap = migration_bitmap_rcu, *bitmap;
> + bitmap = bitmap_new_rcu(new);
>
> /* prevent migration_bitmap content from being set bit
> * by migration_bitmap_sync_range() at the same time.
> @@ -1080,12 +1094,11 @@ void migration_bitmap_extend(ram_addr_t old,
> ram_addr_t new)
> * at the same time.
> */
> qemu_mutex_lock(&migration_bitmap_mutex);
> - bitmap_copy(bitmap, old_bitmap, old);
> - atomic_rcu_set(&migration_bitmap, bitmap);
> + bitmap_copy(bitmap->bmap, old_bitmap->bmap, old);
> + atomic_rcu_set(&migration_bitmap_rcu, bitmap);
> qemu_mutex_unlock(&migration_bitmap_mutex);
> migration_dirty_pages += new - old;
> - synchronize_rcu();
> - g_free(old_bitmap);
> + g_free_rcu(old_bitmap, rcu);
> }
> }
>
> @@ -1144,7 +1157,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
> reset_ram_globals();
>
> ram_bitmap_pages = last_ram_offset() >> TARGET_PAGE_BITS;
> - migration_bitmap = bitmap_new(ram_bitmap_pages);
> + migration_bitmap_rcu = bitmap_new_rcu(ram_bitmap_pages);
>
> /*
> * Count the total number of pages used by ram blocks not including any
>
