From 1e886639791762e89b51aa0507f523c6a1448831 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Thu, 29 Jun 2017 15:27:41 +0200 Subject: [PATCH] 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 Reviewed-by: Stefan Hajnoczi Reviewed-by: Fam Zheng Signed-off-by: Paolo Bonzini Message-Id: <20170629132749.997-4-pbonzini@redhat.com> Signed-off-by: Fam Zheng --- block/vdi.c | 48 ++++++++++++++++++++++++------------------------ 1 file changed, 24 insertions(+), 24 deletions(-) diff --git a/block/vdi.c b/block/vdi.c index 2b6e8fa1ed..8da5dfc897 100644 --- a/block/vdi.c +++ b/block/vdi.c @@ -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);