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block-backend: Queue requests while drained

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This fixes devices like IDE that can still start new requests from I/O
handlers in the CPU thread while the block backend is drained.

The basic assumption is that in a drain section, no new requests should
be allowed through a BlockBackend (blk_drained_begin/end don't exist,
we get drain sections only on the node level). However, there are two
special cases where requests should not be queued:

1. Block jobs: We already make sure that block jobs are paused in a
   drain section, so they won't start new requests. However, if the
   drain_begin is called on the job's BlockBackend first, it can happen
   that we deadlock because the job stays busy until it reaches a pause
   point - which it can't if its requests aren't processed any more.

   The proper solution here would be to make all requests through the
   job's filter node instead of using a BlockBackend. For now, just
   disabling request queuing on the job BlockBackend is simpler.

2. In test cases where making requests through bdrv_* would be
   cumbersome because we'd need a BdrvChild. As we already got the
   functionality to disable request queuing from 1., use it in tests,
   too, for convenience.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Kevin Wolf 2019-07-22 17:46:23 +02:00
parent d2da5e288a
commit cf3129323f
7 changed files with 59 additions and 3 deletions
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2
block/commit.c
View file

@ -350,6 +350,7 @@ void commit_start(const char *job_id, BlockDriverState *bs,
if (ret < 0) {
goto fail;
}
blk_set_disable_request_queuing(s->base, true);
s->base_bs = base;
/* Required permissions are already taken with block_job_add_bdrv() */
@ -358,6 +359,7 @@ void commit_start(const char *job_id, BlockDriverState *bs,
if (ret < 0) {
goto fail;
}
blk_set_disable_request_queuing(s->top, true);
s->backing_file_str = g_strdup(backing_file_str);
s->on_error = on_error;