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[Qemu-devel] [PULL 3/3] migration: fix deadlock
From: |
Juan Quintela |
Subject: |
[Qemu-devel] [PULL 3/3] migration: fix deadlock |
Date: |
Thu, 15 Oct 2015 08:28:00 +0200 |
From: "Denis V. Lunev" <address@hidden>
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 <address@hidden>
Reported-by: Igor Redko <address@hidden>
Tested-by: Igor Redko <address@hidden>
Reviewed-by: Paolo Bonzini <address@hidden>
Reviewed-by: Juan Quintela <address@hidden>
Signed-off-by: Juan Quintela <address@hidden>
CC: Anna Melekhova <address@hidden>
CC: Juan Quintela <address@hidden>
CC: Amit Shah <address@hidden>
CC: Paolo Bonzini <address@hidden>
CC: Wen Congyang <address@hidden>
---
migration/ram.c | 44 +++++++++++++++++++++++++++-----------------
1 file changed, 27 insertions(+), 17 deletions(-)
diff --git a/migration/ram.c b/migration/ram.c
index 2d1d0b9..a25bcc7 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -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
--
2.4.3