Max Reitz <mre...@redhat.com> writes: > On 12.09.19 18:16, Sergio Lopez wrote: >> do_drive_backup() acquires the AioContext lock of the corresponding >> BlockDriverState. This is not a problem when it's called from >> qmp_drive_backup(), but drive_backup_prepare() also acquires the lock >> before calling it. >> >> This change adds a BlockDriverState argument to do_drive_backup(), >> which is used to signal that the context lock is already acquired and >> to save a couple of redundant calls. > > But those redundant calls don’t really hurt (it’s just bdrv_lookup_bs(), > as far as I can tell). Wouldn’t it be simpler to just release the > context lock in drive_backup_prepare() before calling do_drive_backup()? > The BDS is drained anyway.
Redundant calls rarely hurt, they're just redundant ;-)
> On top of that, do_backup_common() calls bdrv_try_set_aio_context() to
> bring the target into the source’s AioContext. However, this function
> must be called with the old AioContext held, and the new context not held.
Is this documented somewhere? I see nothing in the function declaration
nor definition.
I'm starting to get the feeling that the block layer is riddled with
unwritten rules and assumptions that makes every change a lot harder
than it should be.
> Currently, it’s called exactly the other way around: With the new
> context held, but the old one not held.
>
> So I think it indeed actually makes more sense to release the AioContext
> before calling do_drive_backup(), and to move the
> bdrv_try_set_aio_context() call for target_bs to the callers of
> do_backup_common() (where they have not yet taken the AioContext lock).
OK. I see this also happens in external_snapshot_prepare() and
qmp_drive_mirror() too. I guess we should fix these too.
In qmp_drive_mirror(), would it be safe to delay the acquisition of any
context until just before the blockdev_mirror_common()?
> Max
>
>> Signed-off-by: Sergio Lopez <s...@redhat.com>
>> ---
>> blockdev.c | 54 ++++++++++++++++++++++++++++++++++++++----------------
>> 1 file changed, 38 insertions(+), 16 deletions(-)
>>
>> diff --git a/blockdev.c b/blockdev.c
>> index fbef6845c8..0cc6c69ceb 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -1762,8 +1762,10 @@ typedef struct DriveBackupState {
>> BlockJob *job;
>> } DriveBackupState;
>>
>> -static BlockJob *do_drive_backup(DriveBackup *backup, JobTxn *txn,
>> - Error **errp);
>> +static BlockJob *do_drive_backup(DriveBackup *backup,
>> + BlockDriverState *backup_bs,
>> + JobTxn *txn,
>> + Error **errp);
>>
>> static void drive_backup_prepare(BlkActionState *common, Error **errp)
>> {
>> @@ -1781,6 +1783,11 @@ static void drive_backup_prepare(BlkActionState
>> *common, Error **errp)
>> return;
>> }
>>
>> + if (!bs->drv) {
>> + error_setg(errp, "Device has no medium");
>> + return;
>> + }
>> +
>> aio_context = bdrv_get_aio_context(bs);
>> aio_context_acquire(aio_context);
>>
>> @@ -1789,7 +1796,9 @@ static void drive_backup_prepare(BlkActionState
>> *common, Error **errp)
>>
>> state->bs = bs;
>>
>> - state->job = do_drive_backup(backup, common->block_job_txn, &local_err);
>> + state->job = do_drive_backup(backup, bs,
>> + common->block_job_txn,
>> + &local_err);
>> if (local_err) {
>> error_propagate(errp, local_err);
>> goto out;
>> @@ -3607,7 +3616,9 @@ static BlockJob *do_backup_common(BackupCommon *backup,
>> return job;
>> }
>>
>> -static BlockJob *do_drive_backup(DriveBackup *backup, JobTxn *txn,
>> +static BlockJob *do_drive_backup(DriveBackup *backup,
>> + BlockDriverState *backup_bs,
>> + JobTxn *txn,
>> Error **errp)
>> {
>> BlockDriverState *bs;
>> @@ -3625,18 +3636,27 @@ static BlockJob *do_drive_backup(DriveBackup
>> *backup, JobTxn *txn,
>> backup->mode = NEW_IMAGE_MODE_ABSOLUTE_PATHS;
>> }
>>
>> - bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>> - if (!bs) {
>> - return NULL;
>> - }
>> + if (backup_bs) {
>> + bs = backup_bs;
>> + /*
>> + * If the caller passes us a BDS, we assume it has already
>> + * acquired the context lock.
>> + */
>> + aio_context = bdrv_get_aio_context(bs);
>> + } else {
>> + bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>> + if (!bs) {
>> + return NULL;
>> + }
>>
>> - if (!bs->drv) {
>> - error_setg(errp, "Device has no medium");
>> - return NULL;
>> - }
>> + if (!bs->drv) {
>> + error_setg(errp, "Device has no medium");
>> + return NULL;
>> + }
>>
>> - aio_context = bdrv_get_aio_context(bs);
>> - aio_context_acquire(aio_context);
>> + aio_context = bdrv_get_aio_context(bs);
>> + aio_context_acquire(aio_context);
>> + }
>>
>> if (!backup->has_format) {
>> backup->format = backup->mode == NEW_IMAGE_MODE_EXISTING ?
>> @@ -3713,7 +3733,9 @@ static BlockJob *do_drive_backup(DriveBackup *backup,
>> JobTxn *txn,
>> unref:
>> bdrv_unref(target_bs);
>> out:
>> - aio_context_release(aio_context);
>> + if (!backup_bs) {
>> + aio_context_release(aio_context);
>> + }
>> return job;
>> }
>>
>> @@ -3721,7 +3743,7 @@ void qmp_drive_backup(DriveBackup *arg, Error **errp)
>> {
>>
>> BlockJob *job;
>> - job = do_drive_backup(arg, NULL, errp);
>> + job = do_drive_backup(arg, NULL, NULL, errp);
>> if (job) {
>> job_start(&job->job);
>> }
>>
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