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block: Convert .bdrv_truncate callback to coroutine_fn

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bdrv_truncate() is an operation that can block (even for a quite long
time, depending on the PreallocMode) in I/O paths that shouldn't block.
Convert it to a coroutine_fn so that we have the infrastructure for
drivers to make their .bdrv_co_truncate implementation asynchronous.

This change could potentially introduce new race conditions because
bdrv_truncate() isn't necessarily executed atomically any more. Whether
this is a problem needs to be evaluated for each block driver that
supports truncate:

* file-posix/win32, gluster, iscsi, nfs, rbd, ssh, sheepdog: The
  protocol drivers are trivially safe because they don't actually yield
  yet, so there is no change in behaviour.

* copy-on-read, crypto, raw-format: Essentially just filter drivers that
  pass the request to a child node, no problem.

* qcow2: The implementation modifies metadata, so it needs to hold
  s->lock to be safe with concurrent I/O requests. In order to avoid
  double locking, this requires pulling the locking out into
  preallocate_co() and using qcow2_write_caches() instead of
  bdrv_flush().

* qed: Does a single header update, this is fine without locking.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Kevin Wolf 2018-06-21 17:54:35 +02:00
parent ae5475e82f
commit 061ca8a368
16 changed files with 162 additions and 89 deletions
Download patch file Download diff file Expand all files Collapse all files

8
block/iscsi.c
View file

@ -2085,8 +2085,8 @@ static void iscsi_reopen_commit(BDRVReopenState *reopen_state)
}
}
static int iscsi_truncate(BlockDriverState *bs, int64_t offset,
PreallocMode prealloc, Error **errp)
static int coroutine_fn iscsi_co_truncate(BlockDriverState *bs, int64_t offset,
PreallocMode prealloc, Error **errp)
{
IscsiLun *iscsilun = bs->opaque;
Error *local_err = NULL;
@ -2431,7 +2431,7 @@ static BlockDriver bdrv_iscsi = {
.bdrv_getlength = iscsi_getlength,
.bdrv_get_info = iscsi_get_info,
.bdrv_truncate = iscsi_truncate,
.bdrv_co_truncate = iscsi_co_truncate,
.bdrv_refresh_limits = iscsi_refresh_limits,
.bdrv_co_block_status = iscsi_co_block_status,
@ -2468,7 +2468,7 @@ static BlockDriver bdrv_iser = {
.bdrv_getlength = iscsi_getlength,
.bdrv_get_info = iscsi_get_info,
.bdrv_truncate = iscsi_truncate,
.bdrv_co_truncate = iscsi_co_truncate,
.bdrv_refresh_limits = iscsi_refresh_limits,
.bdrv_co_block_status = iscsi_co_block_status,