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

[Emanuele Giuseppe Esposito]

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,