Re: [Qemu-devel] [PATCH] blockdev: avoid acquiring AioContext lock twice at do_drive_backup()

[Sergio Lopez <slp@redhat.com>]

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Max Reitz <mreitz@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 <slp@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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