On 27/02/2015 20:54, Max Reitz wrote:
> 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 <qemu-sta...@nongnu.org>
> Signed-off-by: Max Reitz <mre...@redhat.com>
Usage of CoMutex is tricky, but well commented. So:
Reviewed-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> 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);
> }
>
>
