[v3 PATCH 1/2] dwrite: Protect cached fontface list when accessed from multiple threads

[Nikolay Sivov]

On 10.08.2017 11:14, Sebastian Lackner wrote:
> On 10.08.2017 08:45, Nikolay Sivov wrote:
>> Based on patch by Anton Romanov <theli.ua at gmail.com>
>>
>> Signed-off-by: Nikolay Sivov <nsivov at codeweavers.com>
>> ---
>>  dlls/dwrite/dwrite_private.h |  6 ++++--
>>  dlls/dwrite/font.c           | 24 ++++++++++++---------
>>  dlls/dwrite/main.c           | 50 +++++++++++++++++++++++++++++++++-----------
>>  3 files changed, 56 insertions(+), 24 deletions(-)
>>
> 
> Isn't it also necessary to protect factory_cache_fontface()?

It is.

> 
> By the way: An alternative method to implement proper synchronization (which does
> not require locking critical sections before InterlockedDecrement) would be to do
> something like:
> 
> In the destructor:
> 
> --- snip ---
>     if (!ref)
>     {
>         EnterCriticalSection(...);
>         list_remove(...);
>         LeaveCriticalSection(...);
> 
>         ...
>     }
> --- snip ---

Yes, that's better I guess, thanks.

> 
> And when querying cached font faces:
> 
> --- snip ---
>     EnterCriticalSection(...);
>     LIST_FOR_EACH_ENTRY(...)
>     {
>         ...
> 
>         /* the following code could also be hidden in QueryInterface */
>         if (InterlockedIncrement( &obj->refcount ) == 1)
>         {
>             InterlockedDecrement( &obj->refcount );
>             continue;
>         }
> 
>         /* return it */
>     }
>     LeaveCriticalSection(...);
> --- snip ---

In my case, to avoid exposing structure, it's enough to AddRef() == 1,
and release otherwise I think. It's not important to decrement when
we're in a process of destroying the object.

> 
> This is also what is done in Wines threadpool implementation [1], and has the
> advantage that the CS is not needed for every Release, only when the object is
> really destroyed. But in the end it is a matter of taste, for rpcrt4 Jacek
> also picked the "hold CS during InterlockedDecrement" method.
> 
> [1] https://source.winehq.org/git/wine.git/blob/HEAD:/dlls/ntdll/threadpool.c#l2564
> 
> Best regards,
> Sebastian
> 
>