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[Qemu-devel] [PATCH v3] block/vdi: Add locking for parallel requests
From: |
Max Reitz |
Subject: |
[Qemu-devel] [PATCH v3] block/vdi: Add locking for parallel requests |
Date: |
Fri, 27 Feb 2015 14:54:39 -0500 |
When allocating a new cluster, the first write to it must be the one
doing the allocation, because that one pads its write request to the
cluster size; if another write to that cluster is executed before it,
that write will be overwritten due to the padding.
See https://bugs.launchpad.net/qemu/+bug/1422307 for what can go wrong
without this patch.
Cc: qemu-stable <address@hidden>
Signed-off-by: Max Reitz <address@hidden>
---
v3: Hopefully finally found the real issue which causes the problems
described in the bug report; at least it sounds very reasonable and
I can no longer reproduce any of the issues described there.
Thank you, Paolo and Stefan!
---
block/vdi.c | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/block/vdi.c b/block/vdi.c
index 74030c6..53bd02f 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -53,6 +53,7 @@
#include "block/block_int.h"
#include "qemu/module.h"
#include "migration/migration.h"
+#include "block/coroutine.h"
#if defined(CONFIG_UUID)
#include <uuid/uuid.h>
@@ -196,6 +197,8 @@ typedef struct {
/* VDI header (converted to host endianness). */
VdiHeader header;
+ CoMutex write_lock;
+
Error *migration_blocker;
} BDRVVdiState;
@@ -504,6 +507,8 @@ static int vdi_open(BlockDriverState *bs, QDict *options,
int flags,
"vdi", bdrv_get_device_name(bs), "live migration");
migrate_add_blocker(s->migration_blocker);
+ qemu_co_mutex_init(&s->write_lock);
+
return 0;
fail_free_bmap:
@@ -639,11 +644,31 @@ static int vdi_co_write(BlockDriverState *bs,
buf, n_sectors * SECTOR_SIZE);
memset(block + (sector_in_block + n_sectors) * SECTOR_SIZE, 0,
(s->block_sectors - n_sectors - sector_in_block) *
SECTOR_SIZE);
+
+ /* Note that this coroutine does not yield anywhere from reading
the
+ * bmap entry until here, so in regards to all the coroutines
trying
+ * to write to this cluster, the one doing the allocation will
+ * always be the first to try to acquire the lock.
+ * Therefore, it is also the first that will actually be able to
+ * acquire the lock and thus the padded cluster is written before
+ * the other coroutines can write to the affected area. */
+ qemu_co_mutex_lock(&s->write_lock);
ret = bdrv_write(bs->file, offset, block, s->block_sectors);
+ qemu_co_mutex_unlock(&s->write_lock);
} else {
uint64_t offset = s->header.offset_data / SECTOR_SIZE +
(uint64_t)bmap_entry * s->block_sectors +
sector_in_block;
+ qemu_co_mutex_lock(&s->write_lock);
+ /* This lock is only used to make sure the following write
operation
+ * is executed after the write issued by the coroutine allocating
+ * this cluster, therefore we do not need to keep it locked.
+ * As stated above, the allocating coroutine will always try to
lock
+ * the mutex before all the other concurrent accesses to that
+ * cluster, therefore at this point we can be absolutely certain
+ * that that write operation has returned (there may be other
writes
+ * in flight, but they do not concern this very operation). */
+ qemu_co_mutex_unlock(&s->write_lock);
ret = bdrv_write(bs->file, offset, buf, n_sectors);
}
--
2.1.0
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