This is the mail archive of the libstdc++@gcc.gnu.org mailing list for the libstdc++ project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: debug mode performance patch


Ok I will integrate a test showing the deadlock issue with current patch based on your code and then will fix the patch.

Thanks for the code !

On 11/10/2010 11:02 PM, Jonathan Wakely wrote:
On 10 November 2010 21:55, François Dumont wrote:
Well, this is not something my change introduce. For the moment the debug
mode is using a single mutex for all synchronizations. To keep binary
compatibility I haven't been able to add a mutex instance to the safe
sequence so I introduce a small mutex pool to limit contention.
Sorry if my review sounded negative, I think the general idea is very
good.  But with a single mutex there is no chance for deadlock, it
just means only one swap can take place at a time.

By introducing multiple mutexes without some checking you make it easy
to deadlock.  Slow and correct is better than fast and deadlock  ;-)

There are two problems with _M_swap in your patch:

1) if two objects are swapped whih share the same mutex then it will
try to lock the same mutex twice.  You should be able to reproduce
this like so (untested):

__gnu_debug::vector<int>    v[17];
swap(v[0], v[16]);

If I'm not mistaken, v[0] and v[16] should get the same mutex from
get_mutex and so should lock it twice. That's bad, even in a
single-threaded program.

Yes, this is why I asked if mutex are guarantied to be reentrant even if I
agree that even if it is so it is still bad practice.
When your mail arrived I had just finished some examples to
demonstrate the problems, maybe these should go in the testsuite.

This deadlocks for me:

#include<mutex>
#include<iostream>

std::mutex&  get_mutex(void* p)
{
   const uintptr_t mask = 0xf;
   static std::mutex mutexes[ mask + 1 ];
   uintptr_t index = (reinterpret_cast<uintptr_t>(p) / sizeof(void*))&  mask;
   std::cerr<<  index<<  std::endl;
   return mutexes[ index ];
}

struct A { void* p; };

void swap(A&  a, A&  b)
{
   std::lock_guard<std::mutex>  l1(get_mutex(&a));
   std::lock_guard<std::mutex>  l2(get_mutex(&b));
}

int main()
{
   A a[17];
   swap(a[0], a[16]);  // distinct objects, but "share" a mutex
}

2) consider four sequences, A, B, C and D. They happen to have
addresses such that:

&A.get_mutex() ==&C.get_mutex()
and
&B.get_mutex() ==&D.get_mutex()

now, in my multithreaded program I do this concurrently:
swap(A, B);   swap(D, C);
I'm not accessing any single object in more than one thread, but it
will deadlock.

Indeed
e.g this deadlocks for me:

// same definition of A, get_mutex and swap:
#include<thread>

A a[32];

void f(int i, int j) { for (int k=0; k<10; ++k) swap(a[i], a[j]); }

int main()
{
   // swap distinct objects in two threads
   std::thread t1(f, 0, 1);
   std::thread t2(f, 17, 16);
   t1.join();
   t2.join();
}


The solution is to ensure that you don't lock the same mutex twice and
you always lock in a consistent order

e.g. this fixes both deadlocks

void swap(A&  a, A&  b)
{
   std::mutex&  m1 = get_mutex(&a);
   std::mutex&  m2 = get_mutex(&b);
   if (&m1<  &m2)
     std::lock_guard<std::mutex>  l(m1);
   else
   {
     std::lock_guard<std::mutex>  l1(&m1<  &m2 ? m1 : m2);
     std::lock_guard<std::mutex>  l2(&m1<  &m2 ? m2 : m1);
   }
}

Ok, so except fixing this lock issue you seem to agree on the modified debug
mode lock model, great. I will fix this issue and submit a new proposition.
Great - thanks for working on this!

Jonathan


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]