Thread/warp local lock in cuda

Question

I want to implement critical sections in cuda. I read many questions and answers on this subject, and answers often involve atomicCAS and atomicExch.

However, this doesn

Answer 1

Atomics can be made to work at the warp level. The hazard with intra-warp negotiation for the lock has to do with how the lock is acquired in relation to how it is released. Questions such as this and this and this point out this hazard, but you should not conflate this as being a limitation of using atomics. The hazard arises as a result of how conditional code is executed on a GPU(*), and dependency between the 2 paths in the conditional construct.

In order to avoid the above hazard within a warp, it's necessary to break the dependency between the 2 paths in the conditional construct. The acquisition of the lock must be able to proceed independently from the critical section and the release of the lock.

However, since you desire just coordination within a warp, atomics are overkill. We already have a number of powerful warp communication and coordination mechanisms such as __ballot() that you mention.

What follows then is a worked example of using __ballot() in a mechanism to ensure that only 1 thread (per warp) will "acquire a lock". Since this is all happening only at the warp level, we once again can leverage this to entirely dispense with the release of the lock. We can know the lock is released when the critical section is complete (since we only need to track completion across a warp).

This example performs stream compaction at the warp level, using a __ballot() mechanism to ensure that only 1 thread proceeds into the "critical section" at a time:

$ cat t398.cu
#include 
#include 

const int num_threads = 1024;  // should be 1024 or less for this demo

// returns true if the thread acquired the lock
// will return true for at most 1 thread per warp

__device__ bool warp_lock(int req){
  return ((__ffs(__ballot(req))) == ((threadIdx.x & 31)+1));
}

__global__ void test_lock(int *pattern, int *result){

  __shared__ int done;
  __shared__ int warp_index[32];
  int myreq = pattern[threadIdx.x];
  done = 1;
  if ((threadIdx.x & 31) == 0) warp_index[threadIdx.x>>5] = 0;
  do {
    done = 1;
    __syncthreads();
    // attempt to  "acquire lock"
    bool mylock = warp_lock(myreq);
    // if lock acquired, do "critical section"
    if (mylock){
      done = 0;
      int my_index = (warp_index[threadIdx.x>>5]++) + ((threadIdx.x>>5)*32);
      result[my_index] = threadIdx.x;
      myreq = 0;
      }
    __syncthreads();
    } while (!done);
}

int main(){

  int *h_pattern, *d_pattern, *h_result, *d_result;
  h_pattern = (int *)malloc(num_threads*sizeof(int));
  h_result  = (int *)malloc(num_threads*sizeof(int));
  cudaMalloc(&d_pattern, num_threads*sizeof(int));
  cudaMalloc(&d_result, num_threads*sizeof(int));
  for (int i = 0; i < num_threads; i++) {h_pattern[i] = rand()&1;}
  cudaMemcpy(d_pattern, h_pattern, num_threads*sizeof(int), cudaMemcpyHostToDevice);
  cudaMemset(d_result, 0, num_threads*sizeof(int));
  test_lock<<<1, num_threads>>>(d_pattern, d_result);
  cudaMemcpy(h_result, d_result, num_threads*sizeof(int), cudaMemcpyDeviceToHost);
  printf("index, in, out\n");
  for (int i = 0; i < num_threads; i++)
    printf("%4d, %d, %4d\n", i, h_pattern[i], h_result[i]);
  return 0;
}
$ nvcc -arch=sm_61 -o t398 t398.cu
$ cuda-memcheck ./t398
========= CUDA-MEMCHECK
index, in, out
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   1, 0,    2
   2, 1,    3
   3, 1,    4
   4, 1,    5
   5, 1,    8
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 843, 1,  852
 844, 1,  853
 845, 0,  856
 846, 0,  857
 847, 1,  859
 848, 0,  863
 849, 1,    0
 850, 1,    0
 851, 1,    0
 852, 1,    0
 853, 1,    0
 854, 0,    0
 855, 0,    0
 856, 1,    0
 857, 1,    0
 858, 0,    0
 859, 1,    0
 860, 0,    0
 861, 0,    0
 862, 0,    0
 863, 1,    0
 864, 1,  864
 865, 1,  865
 866, 0,  867
 867, 1,  868
 868, 1,  869
 869, 1,  871
 870, 0,  873
 871, 1,  874
 872, 0,  875
 873, 1,  877
 874, 1,  881
 875, 1,  882
 876, 0,  885
 877, 1,  887
 878, 0,  888
 879, 0,  890
 880, 0,  891
 881, 1,  893
 882, 1,  894
 883, 0,    0
 884, 0,    0
 885, 1,    0
 886, 0,    0
 887, 1,    0
 888, 1,    0
 889, 0,    0
 890, 1,    0
 891, 1,    0
 892, 0,    0
 893, 1,    0
 894, 1,    0
 895, 0,    0
 896, 0,  897
 897, 1,  898
 898, 1,  899
 899, 1,  902
 900, 0,  904
 901, 0,  907
 902, 1,  908
 903, 0,  910
 904, 1,  913
 905, 0,  919
 906, 0,  921
 907, 1,  922
 908, 1,  927
 909, 0,    0
 910, 1,    0
 911, 0,    0
 912, 0,    0
 913, 1,    0
 914, 0,    0
 915, 0,    0
 916, 0,    0
 917, 0,    0
 918, 0,    0
 919, 1,    0
 920, 0,    0
 921, 1,    0
 922, 1,    0
 923, 0,    0
 924, 0,    0
 925, 0,    0
 926, 0,    0
 927, 1,    0
 928, 1,  928
 929, 0,  931
 930, 0,  933
 931, 1,  935
 932, 0,  936
 933, 1,  938
 934, 0,  939
 935, 1,  942
 936, 1,  944
 937, 0,  945
 938, 1,  946
 939, 1,  948
 940, 0,  949
 941, 0,  950
 942, 1,  951
 943, 0,  954
 944, 1,  958
 945, 1,  959
 946, 1,    0
 947, 0,    0
 948, 1,    0
 949, 1,    0
 950, 1,    0
 951, 1,    0
 952, 0,    0
 953, 0,    0
 954, 1,    0
 955, 0,    0
 956, 0,    0
 957, 0,    0
 958, 1,    0
 959, 1,    0
 960, 0,  962
 961, 0,  964
 962, 1,  965
 963, 0,  966
 964, 1,  967
 965, 1,  968
 966, 1,  971
 967, 1,  972
 968, 1,  973
 969, 0,  977
 970, 0,  979
 971, 1,  985
 972, 1,  987
 973, 1,  988
 974, 0,  991
 975, 0,    0
 976, 0,    0
 977, 1,    0
 978, 0,    0
 979, 1,    0
 980, 0,    0
 981, 0,    0
 982, 0,    0
 983, 0,    0
 984, 0,    0
 985, 1,    0
 986, 0,    0
 987, 1,    0
 988, 1,    0
 989, 0,    0
 990, 0,    0
 991, 1,    0
 992, 0,  993
 993, 1,  994
 994, 1,  995
 995, 1,  997
 996, 0,  999
 997, 1, 1000
 998, 0, 1002
 999, 1, 1004
1000, 1, 1005
1001, 0, 1006
1002, 1, 1007
1003, 0, 1009
1004, 1, 1012
1005, 1, 1018
1006, 1, 1019
1007, 1, 1021
1008, 0, 1022
1009, 1,    0
1010, 0,    0
1011, 0,    0
1012, 1,    0
1013, 0,    0
1014, 0,    0
1015, 0,    0
1016, 0,    0
1017, 0,    0
1018, 1,    0
1019, 1,    0
1020, 0,    0
1021, 1,    0
1022, 1,    0
1023, 0,    0
========= ERROR SUMMARY: 0 errors
$

Note that all threads can proceed thru the "acquire lock" section without any inter-thread dependency (although only one thread at a time will "win"), and note that all threads can proceed thru the "critical section" without any inter-thread dependency.

(*) Note that this situation is mitigated in Volta