> From: Philip Martin
> Sent: 29 May 2002 23:59
> "Sander Striker" <striker@apache.org> writes:
>> There is a difference between debug and production. Your patch
>> only addressed the debug version.
>
> I don't understand. There is no bug in the production version, the
> mutex in the pool is not even created.
Look at svn_pool_create in libsvn_subr. We create a mutex in the
top level subversion pool, for its own allocator. Clear that pool
and reuse it... BOOM.
> [snip]
>>> This patch recreates the mutex. That works fine in Subversion but
>>> then Subversion only has one thread! What is this mutex for? It's
>>> used in apr_pool_walk_tree so that threads can safely access pools
>>> used in other threads. Now consider what happens when a pool gets
>>> cleared, the pool cleanup handlers destroy the mutex and the pool
>>> free's the memory occupied by the mutex. Nowhere does the code
>>> consider that another thread might have the mutex locked. Nowhere
>>> does the code consider that another thread might be blocked waiting to
>>> lock the mutex. I don't understand how this can work. Until the pool
>>> has been removed from its parent's list of children the mutex cannot
>>> be destroyed.
>>
>> This is all correct. I thought it was general knowledge that if you
>> go around clearing/destroying a pool in one thread and using it in the
>> other, you are asking for trouble. I think we need to document this,
>> but I don't see it as something we should fix.
>
> Again I don't understand. By "using it" are you referring to the
> debug only functions? The ones mentioned in the comment above the
> mutex creation? What is this mutex for if not to allow multiple
> threads access to the pool tree?
There is a difference between accessing the 'pool tree' and 'using
a pool'. If you are doing clear/destroy in one thread and p[c]alloc
in another on the same pool, this is very likely to blow up.
The mutex is there so that multiple threads can create/destroy child
pools on a shared pool.
In case it isn't obvious by now: pools aren't thread safe. You should
never share a pool between threads if the threads aren't synchronized.
Sander
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Received on Sat Jun 1 14:22:54 2002