Messages in this thread | | | From | "Bernard Normier" <> | Subject | Concurrent access to /dev/urandom | Date | Sat, 27 Nov 2004 15:45:49 -0500 |
| |
I use /dev/urandom to generate UUIDs by reading 16 random bytes from /dev/urandom (very much like e2fsprogs' libuuid).
As long as I serialize access to /dev/urandom, I get different values. However, with concurrent access to /dev/urandom, I quickly get duplicate values. libuuid uses one shared file descriptor for all threads, while the test case below uses a file descriptor per read; it does not appear to change anything ... duplicates appear very quickly (within seconds). I tried on 2.4.20-27.9smp, 2.6.8-1.521smp, 2.6.9-1.6_FC2smp.
Within one process, I could easily serialize access to /dev/urandom to avoid this problem. But I suspect the same would happen with concurrent access from different processes, and I'd rather avoid interprocess synchronization. Is this the expected behavior of /dev/urandom? Any suggestions on how to get unique values?
Thanks, Bernard
#include <pthread.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h>
#include <iostream> #include <set>
using namespace std;
static pthread_mutex_t staticMutex = PTHREAD_MUTEX_INITIALIZER;
struct Key { long long high; long long low; };
struct KeyComp { inline bool operator()(const Key& lhs, const Key& rhs) { return lhs.high < rhs.high || (lhs.high == rhs.high && lhs.low < rhs.low); } };
static set<Key, KeyComp> keySet; static int keyCount;
void* threadFailed() { return reinterpret_cast<void*>(-1); }
extern "C" void* readRandom(void* d) { int threadId = reinterpret_cast<int>(d);
for(int i = 0; i < keyCount; ++i) { int fd = open("/dev/urandom", O_RDONLY);
if (fd == -1) { cerr << "could not open /dev/urandom" << endl; return threadFailed(); }
int reads = 0; size_t index = 0; char buffer[16];
while(reads <= 20 && index != sizeof(Key)) { ssize_t bytesRead = read(fd, buffer + index, sizeof(Key) - index);
if(bytesRead == -1) { if(errno != EINTR) { int err = errno; cerr << "Reading /dev/urandom returned " << strerror(err) << endl; close(fd); return threadFailed(); } } else { index += bytesRead; reads++; } }
if(index != sizeof(buffer)) { close(fd); cerr << "Giving up after 20 reads!" << endl; return threadFailed(); }
close(fd);
int err = pthread_mutex_lock(&staticMutex); if(err != 0) { cerr << "pthread_mutex_lock failed" << endl; return threadFailed(); }
Key& key = reinterpret_cast<Key&>(buffer);
pair<set<Key, KeyComp>::iterator, bool> result = keySet.insert(key); if(!result.second) { cerr << "******** Found duplicate!! **********" << endl; struct AsInts { unsigned int x1; unsigned int x2; unsigned int x3; unsigned int x4; };
AsInts& ints = reinterpret_cast<AsInts&>(buffer); cerr << hex << ints.x1 << "-" << ints.x2 << "-" << ints.x3 << "-" << ints.x4 << endl;
pthread_mutex_unlock(&staticMutex); return threadFailed(); }
if(i > 0 && (i % 100000 == 0)) { cout << "Thread " << threadId << ": read " << i << " keys" << endl; }
err = pthread_mutex_unlock(&staticMutex); if(err != 0) { cerr << "pthread_mutex_unlock failed" << endl; return threadFailed(); } }
return 0; }
int main(int argc, char* argv[]) { if(argc != 3) { cerr << "Usage: " << argv[0] << " [number of keys to read] [number of threads]" << endl; return -1; }
int howMany = atoi(argv[1]); int threadCount = atoi(argv[2]); keyCount = howMany / threadCount;
pthread_t* threads = new pthread_t[threadCount];
for(int i = 0; i < threadCount; ++i) { int err = pthread_create(&threads[i], 0, readRandom, reinterpret_cast<void*>(i)); if(err != 0) { cerr << "pthread_create failed" << endl; return -1; } }
for(int i = 0; i < threadCount; ++i) { void* threadStatus; int err = pthread_join(threads[i], &threadStatus); if(err != 0) { cerr << "pthread_join failed" << endl; return -1; } if(threadStatus != 0) { cerr << "Thread " << i << " failed" << endl; return -1; } }
delete[] threads; return 0; }
// build with g++ -D_REENTRANT -o utest utest.cpp -lpthread
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |