Re: [PATCH v2 7/8] block/replication: do not acquire AioContext

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/04/21 10:55, Emanuele Giuseppe Esposito wrote:
> Replication functions are mostly called when the BDS is quiescent and
> does not have any pending I/O.  They do not need to synchronize on
> anything since BDS and BB are now thread-safe.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

This patch has the same issue that Stefan mentioned in patch 2.  In 
particular, the following chain leads to the bdrv*drain family of 
functions and thus AIO_WAIT_WHILE:

replication_start -> reopen_backing_file -> bdrv_subtree_drained_begin

and also

replication_stop -> commit_active_start -> bdrv_reopen_set_read_only

The same is true of patch 8, where you have call sequences like

bdrv_commit -> blk_flush -> bdrv_flush (in the generated file 
block/block-gen.c) -> bdrv_poll_co

So patches 7 and 8 need to be shelved for now, as they can only go in 
with the overall removal of AioContext lock.

Paolo

> ---
>   block/replication.c | 54 ++++++++++-----------------------------------
>   1 file changed, 12 insertions(+), 42 deletions(-)
> 
> diff --git a/block/replication.c b/block/replication.c
> index 97be7ef4de..25ee37b21b 100644
> --- a/block/replication.c
> +++ b/block/replication.c
> @@ -45,6 +45,8 @@ typedef struct BDRVReplicationState {
>       Error *blocker;
>       bool orig_hidden_read_only;
>       bool orig_secondary_read_only;
> +
> +    /* This field is accessed asynchronously.  */
>       int error;
>   } BDRVReplicationState;
>   
> @@ -210,7 +212,7 @@ static int replication_return_value(BDRVReplicationState *s, int ret)
>       }
>   
>       if (ret < 0) {
> -        s->error = ret;
> +        qatomic_set(&s->error, ret);
>           ret = 0;
>       }
>   
> @@ -307,6 +309,7 @@ out:
>       return ret;
>   }
>   
> +/* Called with no I/O pending.  */
>   static void secondary_do_checkpoint(BDRVReplicationState *s, Error **errp)
>   {
>       Error *local_err = NULL;
> @@ -420,7 +423,7 @@ static void backup_job_completed(void *opaque, int ret)
>   
>       if (s->stage != BLOCK_REPLICATION_FAILOVER) {
>           /* The backup job is cancelled unexpectedly */
> -        s->error = -EIO;
> +        qatomic_set(&s->error, -EIO);
>       }
>   
>       backup_job_cleanup(bs);
> @@ -445,6 +448,7 @@ static bool check_top_bs(BlockDriverState *top_bs, BlockDriverState *bs)
>       return false;
>   }
>   
> +/* Called with no I/O pending.  */
>   static void replication_start(ReplicationState *rs, ReplicationMode mode,
>                                 Error **errp)
>   {
> @@ -452,12 +456,9 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>       BDRVReplicationState *s;
>       BlockDriverState *top_bs;
>       int64_t active_length, hidden_length, disk_length;
> -    AioContext *aio_context;
>       Error *local_err = NULL;
>       BackupPerf perf = { .use_copy_range = true, .max_workers = 1 };
>   
> -    aio_context = bdrv_get_aio_context(bs);
> -    aio_context_acquire(aio_context);
>       s = bs->opaque;
>   
>       if (s->stage == BLOCK_REPLICATION_DONE ||
> @@ -467,20 +468,17 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>            * Ignore the request because the secondary side of replication
>            * doesn't have to do anything anymore.
>            */
> -        aio_context_release(aio_context);
>           return;
>       }
>   
>       if (s->stage != BLOCK_REPLICATION_NONE) {
>           error_setg(errp, "Block replication is running or done");
> -        aio_context_release(aio_context);
>           return;
>       }
>   
>       if (s->mode != mode) {
>           error_setg(errp, "The parameter mode's value is invalid, needs %d,"
>                      " but got %d", s->mode, mode);
> -        aio_context_release(aio_context);
>           return;
>       }
>   
> @@ -492,21 +490,18 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>           if (!s->active_disk || !s->active_disk->bs ||
>                                       !s->active_disk->bs->backing) {
>               error_setg(errp, "Active disk doesn't have backing file");
> -            aio_context_release(aio_context);
>               return;
>           }
>   
>           s->hidden_disk = s->active_disk->bs->backing;
>           if (!s->hidden_disk->bs || !s->hidden_disk->bs->backing) {
>               error_setg(errp, "Hidden disk doesn't have backing file");
> -            aio_context_release(aio_context);
>               return;
>           }
>   
>           s->secondary_disk = s->hidden_disk->bs->backing;
>           if (!s->secondary_disk->bs || !bdrv_has_blk(s->secondary_disk->bs)) {
>               error_setg(errp, "The secondary disk doesn't have block backend");
> -            aio_context_release(aio_context);
>               return;
>           }
>   
> @@ -518,7 +513,6 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>               active_length != hidden_length || hidden_length != disk_length) {
>               error_setg(errp, "Active disk, hidden disk, secondary disk's length"
>                          " are not the same");
> -            aio_context_release(aio_context);
>               return;
>           }
>   
> @@ -529,7 +523,6 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>               !s->hidden_disk->bs->drv->bdrv_make_empty) {
>               error_setg(errp,
>                          "Active disk or hidden disk doesn't support make_empty");
> -            aio_context_release(aio_context);
>               return;
>           }
>   
> @@ -537,7 +530,6 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>           reopen_backing_file(bs, true, &local_err);
>           if (local_err) {
>               error_propagate(errp, local_err);
> -            aio_context_release(aio_context);
>               return;
>           }
>   
> @@ -550,7 +542,6 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>               !check_top_bs(top_bs, bs)) {
>               error_setg(errp, "No top_bs or it is invalid");
>               reopen_backing_file(bs, false, NULL);
> -            aio_context_release(aio_context);
>               return;
>           }
>           bdrv_op_block_all(top_bs, s->blocker);
> @@ -566,13 +557,11 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>           if (local_err) {
>               error_propagate(errp, local_err);
>               backup_job_cleanup(bs);
> -            aio_context_release(aio_context);
>               return;
>           }
>           job_start(&s->backup_job->job);
>           break;
>       default:
> -        aio_context_release(aio_context);
>           abort();
>       }
>   
> @@ -582,18 +571,15 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
>           secondary_do_checkpoint(s, errp);
>       }
>   
> -    s->error = 0;
> -    aio_context_release(aio_context);
> +    qatomic_set(&s->error, 0);
>   }
>   
> +/* Called with no I/O pending.  */
>   static void replication_do_checkpoint(ReplicationState *rs, Error **errp)
>   {
>       BlockDriverState *bs = rs->opaque;
>       BDRVReplicationState *s;
> -    AioContext *aio_context;
>   
> -    aio_context = bdrv_get_aio_context(bs);
> -    aio_context_acquire(aio_context);
>       s = bs->opaque;
>   
>       if (s->stage == BLOCK_REPLICATION_DONE ||
> @@ -603,38 +589,30 @@ static void replication_do_checkpoint(ReplicationState *rs, Error **errp)
>            * Ignore the request because the secondary side of replication
>            * doesn't have to do anything anymore.
>            */
> -        aio_context_release(aio_context);
>           return;
>       }
>   
>       if (s->mode == REPLICATION_MODE_SECONDARY) {
>           secondary_do_checkpoint(s, errp);
>       }
> -    aio_context_release(aio_context);
>   }
>   
>   static void replication_get_error(ReplicationState *rs, Error **errp)
>   {
>       BlockDriverState *bs = rs->opaque;
>       BDRVReplicationState *s;
> -    AioContext *aio_context;
>   
> -    aio_context = bdrv_get_aio_context(bs);
> -    aio_context_acquire(aio_context);
>       s = bs->opaque;
>   
>       if (s->stage == BLOCK_REPLICATION_NONE) {
>           error_setg(errp, "Block replication is not running");
> -        aio_context_release(aio_context);
>           return;
>       }
>   
> -    if (s->error) {
> +    if (qatomic_read(&s->error)) {
>           error_setg(errp, "I/O error occurred");
> -        aio_context_release(aio_context);
>           return;
>       }
> -    aio_context_release(aio_context);
>   }
>   
>   static void replication_done(void *opaque, int ret)
> @@ -648,10 +626,10 @@ static void replication_done(void *opaque, int ret)
>           s->active_disk = NULL;
>           s->secondary_disk = NULL;
>           s->hidden_disk = NULL;
> -        s->error = 0;
> +        qatomic_set(&s->error, 0);
>       } else {
>           s->stage = BLOCK_REPLICATION_FAILOVER_FAILED;
> -        s->error = -EIO;
> +        qatomic_set(&s->error, -EIO);
>       }
>   }
>   
> @@ -659,10 +637,7 @@ static void replication_stop(ReplicationState *rs, bool failover, Error **errp)
>   {
>       BlockDriverState *bs = rs->opaque;
>       BDRVReplicationState *s;
> -    AioContext *aio_context;
>   
> -    aio_context = bdrv_get_aio_context(bs);
> -    aio_context_acquire(aio_context);
>       s = bs->opaque;
>   
>       if (s->stage == BLOCK_REPLICATION_DONE ||
> @@ -672,20 +647,18 @@ static void replication_stop(ReplicationState *rs, bool failover, Error **errp)
>            * Ignore the request because the secondary side of replication
>            * doesn't have to do anything anymore.
>            */
> -        aio_context_release(aio_context);
>           return;
>       }
>   
>       if (s->stage != BLOCK_REPLICATION_RUNNING) {
>           error_setg(errp, "Block replication is not running");
> -        aio_context_release(aio_context);
>           return;
>       }
>   
>       switch (s->mode) {
>       case REPLICATION_MODE_PRIMARY:
>           s->stage = BLOCK_REPLICATION_DONE;
> -        s->error = 0;
> +        qatomic_set(&s->error, 0);
>           break;
>       case REPLICATION_MODE_SECONDARY:
>           /*
> @@ -700,7 +673,6 @@ static void replication_stop(ReplicationState *rs, bool failover, Error **errp)
>           if (!failover) {
>               secondary_do_checkpoint(s, errp);
>               s->stage = BLOCK_REPLICATION_DONE;
> -            aio_context_release(aio_context);
>               return;
>           }
>   
> @@ -711,10 +683,8 @@ static void replication_stop(ReplicationState *rs, bool failover, Error **errp)
>                               NULL, replication_done, bs, true, errp);
>           break;
>       default:
> -        aio_context_release(aio_context);
>           abort();
>       }
> -    aio_context_release(aio_context);
>   }
>   
>   static const char *const replication_strong_runtime_opts[] = {
>