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));
+ 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
--
2.1.4
