Re: [PATCH v4 3/6] block-copy: improve comments of BlockCopyTask and BlockCopyState types and functions

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

On 19/06/2021 17:23, Vladimir Sementsov-Ogievskiy wrote:
> 14.06.2021 10:33, Emanuele Giuseppe Esposito wrote:
>> As done in BlockCopyCallState, categorize BlockCopyTask
>> and BlockCopyState in IN, State and OUT fields.
>> This is just to understand which field has to be protected with a lock.
>>
>> .sleep_state is handled in the series "coroutine: new sleep/wake API"
>> and thus here left as TODO.
>>
>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
>> ---
>>   block/block-copy.c | 49 +++++++++++++++++++++++++++++-----------------
>>   1 file changed, 31 insertions(+), 18 deletions(-)
>>
>> diff --git a/block/block-copy.c b/block/block-copy.c
>> index 3f26be8ddc..5ff7764e87 100644
>> --- a/block/block-copy.c
>> +++ b/block/block-copy.c
>> @@ -52,29 +52,35 @@ typedef struct BlockCopyCallState {
>>       /* Coroutine where async block-copy is running */
>>       Coroutine *co;
>> -    /* To reference all call states from BlockCopyState */
>> -    QLIST_ENTRY(BlockCopyCallState) list;
>> -
>>       /* State */
>> -    int ret;
>>       bool finished;
>> -    QemuCoSleep sleep;
>> -    bool cancelled;
>> +    QemuCoSleep sleep; /* TODO: protect API with a lock */
>> +
>> +    /* To reference all call states from BlockCopyState */
>> +    QLIST_ENTRY(BlockCopyCallState) list;
>>       /* OUT parameters */
>> +    bool cancelled;
>>       bool error_is_read;
>> +    int ret;
>>   } BlockCopyCallState;
>>   typedef struct BlockCopyTask {
>>       AioTask task;
>> +    /*
>> +     * IN parameters. Initialized in block_copy_task_create()
>> +     * and never changed.
>> +     */
> 
> That's just not true for method field :(

You're right, because it is modified later in the while loop of 
block_copy_dirty_clusters, but the task is already in the list.
Will move it to state field.

> 
>>       BlockCopyState *s;
>>       BlockCopyCallState *call_state;
>>       int64_t offset;
>> -    int64_t bytes;
>>       BlockCopyMethod method;
>> -    QLIST_ENTRY(BlockCopyTask) list;
>> +
>> +    /* State */
>>       CoQueue wait_queue; /* coroutines blocked on this task */
>> +    int64_t bytes;
>> +    QLIST_ENTRY(BlockCopyTask) list;
>>   } BlockCopyTask;
>>   static int64_t task_end(BlockCopyTask *task)
>> @@ -90,15 +96,25 @@ typedef struct BlockCopyState {
>>        */
>>       BdrvChild *source;
>>       BdrvChild *target;
>> -    BdrvDirtyBitmap *copy_bitmap;
>> +
>> +    /* State */
>>       int64_t in_flight_bytes;
>> -    int64_t cluster_size;
>>       BlockCopyMethod method;
>> -    int64_t max_transfer;
>> -    uint64_t len;
>>       QLIST_HEAD(, BlockCopyTask) tasks; /* All tasks from all 
>> block-copy calls */
>>       QLIST_HEAD(, BlockCopyCallState) calls;
>> +    /* State fields that use a thread-safe API */
>> +    BdrvDirtyBitmap *copy_bitmap;
>> +    ProgressMeter *progress;
>> +    SharedResource *mem;
>> +    RateLimit rate_limit;
>> +    /*
>> +     * IN parameters. Initialized in block_copy_state_new()
>> +     * and never changed.
>> +     */
>> +    int64_t cluster_size;
>> +    int64_t max_transfer;
>> +    uint64_t len;
>>       BdrvRequestFlags write_flags;
>>       /*
>> @@ -114,14 +130,11 @@ typedef struct BlockCopyState {
>>        * In this case, block_copy() will query the source’s allocation 
>> status,
>>        * skip unallocated regions, clear them in the copy_bitmap, and 
>> invoke
>>        * block_copy_reset_unallocated() every time it does.
>> +     *
>> +     * This field is set in backup_run() before coroutines are run,
>> +     * therefore is an IN.
> 
> That's not true: it is modified from backup_run, when block-copy already 
> initialized, and block_copy() calls may be done from backup-top filter.
> 

Ok, I am not very familiar with the backup code, so I did not see it.
This means we need to protect this field too.

At this point, I think we can increase the granularity of the lock in 
block_copy_dirty_clusters:
instead of having in each while loop

block_copy_task_create(); // locks and releases internally
block_copy_block_status(); // no lock used, but uses skip_unallocated so 
it will need one
if()
	block_copy_task_shrink(); // locks and releases internally
if(s->skip_unallocated) // will need lock
	block_copy_task_end(); // locks and releases internally
	[..]
if()
	task->method = COPY_WRITE_ZEROS; // needs lock
[..]

we can just lock the while section and eventually create _locked() 
version of task_end and similar functions that are also used in 
non-locked code blocks.


Emanuele

>>        */
>>       bool skip_unallocated;
>> -
>> -    ProgressMeter *progress;
>> -
>> -    SharedResource *mem;
>> -
>> -    RateLimit rate_limit;
>>   } BlockCopyState;
>>   static BlockCopyTask *find_conflicting_task(BlockCopyState *s,
>>
> 
>