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vdi: make it thread-safe 

The VirtualBox driver is using a mutex to order all allocating writes,
but it is not protecting accesses to the bitmap because they implicitly
happen under the AioContext mutex.  Change this to use a CoRwlock
explicitly.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170629132749.997-4-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
This commit is contained in:
Paolo Bonzini 2017-06-29 15:27:41 +02:00 committed by Fam Zheng
parent 667221c10d
commit 1e88663979
1 changed files with 24 additions and 24 deletions
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48
block/vdi.c
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@ -172,7 +172,7 @@ typedef struct {
/* VDI header (converted to host endianness). */
VdiHeader header;
CoMutex write_lock;
CoRwlock bmap_lock;
Error *migration_blocker;
} BDRVVdiState;
@ -485,7 +485,7 @@ static int vdi_open(BlockDriverState *bs, QDict *options, int flags,
goto fail_free_bmap;
}
qemu_co_mutex_init(&s->write_lock);
qemu_co_rwlock_init(&s->bmap_lock);
return 0;
@ -557,7 +557,9 @@ vdi_co_preadv(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
n_bytes, offset);
/* prepare next AIO request */
qemu_co_rwlock_rdlock(&s->bmap_lock);
bmap_entry = le32_to_cpu(s->bmap[block_index]);
qemu_co_rwlock_unlock(&s->bmap_lock);
if (!VDI_IS_ALLOCATED(bmap_entry)) {
/* Block not allocated, return zeros, no need to wait. */
qemu_iovec_memset(qiov, bytes_done, 0, n_bytes);
@ -595,6 +597,7 @@ vdi_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
uint32_t block_index;
uint32_t offset_in_block;
uint32_t n_bytes;
uint64_t data_offset;
uint32_t bmap_first = VDI_UNALLOCATED;
uint32_t bmap_last = VDI_UNALLOCATED;
uint8_t *block = NULL;
@ -614,10 +617,19 @@ vdi_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
n_bytes, offset);
/* prepare next AIO request */
qemu_co_rwlock_rdlock(&s->bmap_lock);
bmap_entry = le32_to_cpu(s->bmap[block_index]);
if (!VDI_IS_ALLOCATED(bmap_entry)) {
/* Allocate new block and write to it. */
uint64_t data_offset;
qemu_co_rwlock_upgrade(&s->bmap_lock);
bmap_entry = le32_to_cpu(s->bmap[block_index]);
if (VDI_IS_ALLOCATED(bmap_entry)) {
/* A concurrent allocation did the work for us. */
qemu_co_rwlock_downgrade(&s->bmap_lock);
goto nonallocating_write;
}
bmap_entry = s->header.blocks_allocated;
s->bmap[block_index] = cpu_to_le32(bmap_entry);
s->header.blocks_allocated++;
@ -635,30 +647,18 @@ vdi_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
memset(block + offset_in_block + n_bytes, 0,
s->block_size - n_bytes - offset_in_block);
/* 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);
/* Write the new block under CoRwLock write-side protection,
* so this full-cluster write does not overlap a partial write
* of the same cluster, issued from the "else" branch.
*/
ret = bdrv_pwrite(bs->file, data_offset, block, s->block_size);
qemu_co_mutex_unlock(&s->write_lock);
qemu_co_rwlock_unlock(&s->bmap_lock);
} else {
uint64_t data_offset = s->header.offset_data +
(uint64_t)bmap_entry * s->block_size +
offset_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);
nonallocating_write:
data_offset = s->header.offset_data +
(uint64_t)bmap_entry * s->block_size +
offset_in_block;
qemu_co_rwlock_unlock(&s->bmap_lock);
qemu_iovec_reset(&local_qiov);
qemu_iovec_concat(&local_qiov, qiov, bytes_done, n_bytes);