On 3/18/2014 7:47 AM, Alex wrote: > On 03/18/2014 01:26 PM, Dimitry Sibiryakov wrote: >> Back to the question: >> >> 17.03.2014 12:30, Alex Peshkoff wrote: >>>> Besides, storing reference to object already increment its >>>>> usage counter, so nobody can free the object during wrapper's life time. >>> But using something like commit for transaction can nullify next pointer. >> next is a reference counting pointer, it cannot be nullified as long as >> Y-object is >> alive. So, what is the point to use YEntry.next() in Y-object methods? >> > Let me talk about attachment - it's using same approach but sample is > more realistic. > > IAttachment* userHandle points to class YAttachment, containing (among > others) reference counter and reference pointer to IAttachment next. > Let's take into an account that on successful detach next is nullified, > and if other references are present (i.e. YAttachment is not destroyed > due to reference counter > 0) any attempt to use it should raise > 'invalid handle' error. > > Let N (where N > 1) threads to work with this attachment (i.e. reference > counter is N). Thread T1 wants to execute statement - it calls > YAttachment::execute, which checks for non-null next pointer and (at > this time T1 is swapped out, T2 calls detach, next is nullified ...) > invokes next function: next->execute(...). And gets segfault. Absolutely > without mutex this is not solvable, but we want to keep it locked for as > small time as possible. Therefore under locked mutex we copy next to > local variable, addRef() to it, unlock mutex (you see it's locked for a > very small timeslice) and later work with this variable. In the sample > above we will invoke next->execute() and get 'invalid handle' message > from next provider. IMHO it's better than segfault. > > Do you really use a mutex to when walking a structure???? Why not a read/write lock? With a mutex, readers block readers, which is a total waste of cycles, thread stalls, thread switches, etc.
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