Re: [Boost-users] shared_ptr race condition

On Sat, Aug 16, 2008 at 5:42 PM, David Abrahams wrote:

on Sat Aug 16 2008, Mika Heiskanen http://mika.heiskanen-at-fmi.fi/> wrote:

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David Abrahams wrote:

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on Sat Aug 16 2008, "Eduardo Panisset" http://eduardo.panisset-at-gmail.com/> wrote:

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if( new_use_count == 0 ) { dispose(); weak_release(); } How Can I ensure that the release member function wouldn't dispose a pointee more than one time ? (release executing concurrently from different shared_ptr instances)

Because the reference count was 1, there can be only one shared_ptr instance that owns the pointee. No other threads can be referencing it.

But since the mutex is released, a copy could be created before the new_use_count test is reached? Or is it impossible to create a copy if use_count_ is zero?

What thread can create a copy? We know exactly what the only thread that has a reference to the owned object is doing (it's executing the very code we're concerned with).

Ok, I agree with that. I increment the reference count by copying a shared_ptr to another shared_ptr (before the shared_ptr releases its pointee). However, let me give another example: There are two threads, each one with its own shared_ptr. Thread 2 created its shared_ptr2 by copying the shared_prt1 of thread 1. Then the reference count is equal to 2. Some time later, shared_ptr1 goes out of scope and then shared_prt2 also goes out of scope. Consider the following event sequence: 1. Thread 1 executes release member function, and it is preempted before executing comparation if (new_use_count == 0) 2. Thread 2 executes release member function until the end, disposing the pointee. 3. Thread 1 is rescheduled, executes comparation if (new_use_count == 0) and also diposes the pointee! Is it possible, isn't it?