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migration: fix deadlock 

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 free inside call_rcu.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reported-by: Igor Redko <redkoi@virtuozzo.com>
Tested-by: Igor Redko <redkoi@virtuozzo.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

CC: Anna Melekhova <annam@virtuozzo.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Amit Shah <amit.shah@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Wen Congyang <wency@cn.fujitsu.com>
This commit is contained in:
Denis V. Lunev 2015-09-28 14:41:58 +03:00 committed by Juan Quintela
parent 92e3762237
commit 60be634079
1 changed files with 27 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
migration/ram.c
View File

@ -219,7 +219,6 @@ 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;
@ -236,6 +235,11 @@ struct PageSearchStatus {
};
typedef struct PageSearchStatus PageSearchStatus;
static struct BitmapRcu {
struct rcu_head rcu;
unsigned long *bmap;
} *migration_bitmap_rcu;
struct CompressParam {
bool start;
bool done;
@ -540,7 +544,7 @@ ram_addr_t migration_bitmap_find_and_reset_dirty(RAMBlock *rb,
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 {
@ -558,7 +562,7 @@ ram_addr_t migration_bitmap_find_and_reset_dirty(RAMBlock *rb,
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);
}
@ -1090,17 +1094,22 @@ void free_xbzrle_decoded_buf(void)
xbzrle_decoded_buf = NULL;
}
static void migration_bitmap_free(struct BitmapRcu *bmap)
{
g_free(bmap->bmap);
g_free(bmap);
}
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);
call_rcu(bitmap, migration_bitmap_free, rcu);
}
XBZRLE_cache_lock();
@ -1136,9 +1145,10 @@ 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 = g_new(struct BitmapRcu, 1);
bitmap->bmap = bitmap_new(new);
/* prevent migration_bitmap content from being set bit
* by migration_bitmap_sync_range() at the same time.
@ -1146,13 +1156,12 @@ 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);
bitmap_set(bitmap, old, new - old);
atomic_rcu_set(&migration_bitmap, bitmap);
bitmap_copy(bitmap->bmap, old_bitmap->bmap, old);
bitmap_set(bitmap->bmap, old, new - 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);
call_rcu(old_bitmap, migration_bitmap_free, rcu);
}
}
@ -1210,8 +1219,9 @@ 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);
bitmap_set(migration_bitmap, 0, ram_bitmap_pages);
migration_bitmap_rcu = g_new(struct BitmapRcu, 1);
migration_bitmap_rcu->bmap = bitmap_new(ram_bitmap_pages);
bitmap_set(migration_bitmap_rcu->bmap, 0, ram_bitmap_pages);
/*
* Count the total number of pages used by ram blocks not including any