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Date: Wed, 14 Feb 2018 06:47:30 +0000 (UTC)
From: "Xiaofeng Liu via cygwin" <cygwin AT cygwin DOT com>
Reply-To: Xiaofeng Liu <liuxf09 AT yahoo DOT com>
Reply-To: Xiaofeng Liu <liuxf09 AT yahoo DOT com>
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Message-ID: <1264797847.540865.1518590850864@mail.yahoo.com>
In-Reply-To: <20171201171536.GA4325@calimero.vinschen.de>
References: <1543396632 DOT 5417641 DOT 1512146709346 DOT ref AT mail DOT yahoo DOT com> <1543396632 DOT 5417641 DOT 1512146709346 AT mail DOT yahoo DOT com> <20171201171536 DOT GA4325 AT calimero DOT vinschen DOT de>
Subject: Re: mixed usage of lock protection and lock-free List template class in thread.h
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Here is the sample code that will hang in cygwin:
test-thread.cpp
to compile: g++ -std=c++0x test-thread.cpp -lpthread

#include <stdio.h>#include <stdlib.h>#include <thread>#include <vector>#include <string>#include <ctime>#include <climits>#include <cassert>#include <mutex>#include <condition_variable>
#include <deque>#include <mutex>#include <pthread.h>#include <cstdint>using namespace std;
template<class T>class Queue {    typedef std::deque<T*> Container;public:    Queue(int _capacity = std::numeric_limits<int>::max()) : capacity(_capacity), closed(false) {}    bool enqueue(T* d)    {        if (closed) return false;        std::unique_lock<std::mutex> lock(mutex);        if (d == 0) { closed = true; empty_cv.notify_all(); return true; }        while (data.size() >= capacity)            full_cv.wait(lock);        data.push_back(d);        empty_cv.notify_one();        return true;    }    T* dequeue()    {        std::unique_lock<std::mutex> lock(mutex);        while (data.empty() && !closed)            empty_cv.wait(lock);        if (data.size()) {            T* d = data.front();            data.pop_front();            full_cv.notify_one();            return d;        } else return 0;    }    size_t size() const { return data.size(); }
private:    std::mutex mutex;    std::condition_variable full_cv, empty_cv;    uint32_t capacity;    bool closed;    Container data;};
struct Node {    int data;};
struct Job {    Node* node;    Queue<Node>* recycle;};
static Queue<Job> jobs;
static void* handle_job(void* arg){    long ithr = (long)arg;    unsigned long seed = time(0) + ithr*1000000;    int NS = 1000;    while (Job* j = jobs.dequeue()) {        struct timespec ts;        ts.tv_sec = 0;        seed = seed * 1103515245 + 12345;        ts.tv_nsec = seed%NS;        nanosleep(&ts, 0);        j->recycle->enqueue(j->node);        delete j;    }}
struct Task {    Queue<Node> recycle;    int size; // number of sub jobs    Task(int N) : size(N)    {        for (int i = 0; i<N; ++i) {            Node* t = new Node();            t->data = i;            Job* job = new Job;            job->node = t;            job->recycle = &recycle;            jobs.enqueue(job);        }    }    ~Task()    {        int i = 0;        while (Node* t = recycle.dequeue()) {            delete t;            if (++i == size) break;        }    }};
static string timestamp(){    time_t t;    struct tm tmp;    char buf[80];    t = time(NULL);    localtime_r(&t, &tmp);    strftime(buf, sizeof(buf), "%d-%m-%Y %I:%M:%S", &tmp);    return buf;}
static void* create_task(void* arg){    long ithr = (long)arg;    int TASK_NUM = 1000000;    int one_percent = TASK_NUM/100;    int TASK_SUB_JOB_NUM = 1;    int NS = 1000;    unsigned long seed = time(0) + ithr*10000;    int i = 0;    for (; i < TASK_NUM; ++i) {        struct timespec ts;        ts.tv_sec = 0;        seed = seed * 1103515245 + 12345;        ts.tv_nsec = seed%NS;        nanosleep(&ts, 0);        if (i%one_percent == 0) {            fprintf(stderr, "%s: create_task[%d]: %d%% done\n", timestamp().c_str(), ithr, i/one_percent);        }        Task task(TASK_SUB_JOB_NUM);    }    fprintf(stderr, "%s: create_task[%d]: %d%% done\n", timestamp().c_str(), ithr, i/one_percent);}

int main(){    int NTHR_HANDLE_JOB = 4, NTHR_CREATE_TASK = 4;    std::vector<pthread_t> threads(NTHR_HANDLE_JOB+NTHR_CREATE_TASK);    int k = 0;    for (long i = 0; i < NTHR_HANDLE_JOB; ++i) {        pthread_create(&threads[k++], NULL, handle_job, (void*)i);    }    for (long i = 0; i < NTHR_CREATE_TASK; ++i) {        pthread_create(&threads[k++], NULL, create_task, (void*)i);    }    // wait for create_task thread    for (size_t i = NTHR_HANDLE_JOB; i < threads.size(); ++i) {        pthread_join(threads[i], NULL);    }    jobs.enqueue(0);    // wait for handle_job thread    for (size_t i = 0; i < NTHR_HANDLE_JOB; ++i)        pthread_join(threads[i], NULL);}
In this code, mutex and cond need be created and destroyed very frequently, which could corrupt the static list object owned by some classes in thread.h. In my test, I have a computer of 8 threads to run cygwin, and the hang could happen when cond/mutex objects are created and destroyed for the order of 1 millions times within a few minutes.
I can also observe that the peak memory kept increasing to a few hundred MB, and I suspect there is a MEMORY LEAK in cygwin kernel. 
I hope the format will be good. If not, I will try again.
Thanks.       From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
 To: cygwin AT cygwin DOT com 
 Sent: Friday, December 1, 2017 9:15 AM
 Subject: Re: mixed usage of lock protection and lock-free List template class in thread.h
   
On Dec  1 16:45, Xiaofeng Liu via cygwin wrote:
> Lock protection and lock-free should never be mixed ! 
> ​https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/thread.h;hb=1f42dc2bcf58d3b8629eb13d53de3f69fc314b47#l110
> 
>  110 template <class list_node> inline void 111 List_insert (list_node *&head, list_node *node) 112 { 113   if (!node) 114     return; 115   do 116     node->next = head; 117   while (InterlockedCompareExchangePointer ((PVOID volatile *) &head, 118                                             node, node->next) != node->next); 119 } 120  121 template <class list_node> inline void 122 List_remove (fast_mutex &mx, list_node *&head, list_node *node) 123 { 124   if (!node) 125     return; 126   mx.lock (); 127   if (head) 128     { 129       if (InterlockedCompareExchangePointer ((PVOID volatile *) &head, 130                                              node->next, node) != node) 131         { 132           list_node *cur = head; 133  134           while (cur->next && node != cur->next) 135             cur = cur->next; 136           if (node == cur->next) 137             cur->next = cur->next->next; 138         } 139     } 140   mx.unlock (); 141 }
> The symptom I met is a job hang with the following stack:
> #0  0x000000007711c2ea in ntdll!ZwWaitForMultipleObjects () from /cygdrive/c/Windows/SYSTEM32/ntdll.dll
> #1  0x000007fefd111430 in KERNELBASE!GetCurrentProcess () from /cygdrive/c/Windows/system32/KERNELBASE.dll
> #2  0x0000000076fc06c0 in WaitForMultipleObjects () from /cygdrive/c/Windows/system32/kernel32.dll
> #3  0x00000001800458ac in cygwait(void*, _LARGE_INTEGER*, unsigned int) () from /usr/bin/cygwin1.dll
> #4  0x000000018013d029 in pthread_cond::~pthread_cond() () from /usr/bin/cygwin1.dll
> #5  0x000000018013d0dd in pthread_cond::~pthread_cond() () from /usr/bin/cygwin1.dll
> #6  0x0000000180141196 in pthread_cond_destroy () from /usr/bin/cygwin1.dll
> #7  0x0000000180116d5b in _sigfe () from /usr/bin/cygwin1.dll
> #8  0x0000000100908e38 in std::_Sp_counted_ptr_inplace<std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>, std::allocator<int>, (__gnu_cxx::_Lock_policy)2>::_M_dispose() ()
> The problem with the current implementation for concurrent insert and delete is explained at WikipediaNon-blocking linked list
> 
> My question is how to solve this ? Adding lock protection in List_insert (removing lock-freee) or implementing a complete lock-free List based on Harris's solution to use two CAS?

First of all, please, please, please fix your MUA!  Just like your
mails to cygwin-patches a couple of weeks ago, your mails are pretty
much unreadable due to broken line wrapping.

Back to business: This code is working since 2003.  So, is that just
a theoretical problem, or a practical one?  If the latter, what is
broken exactly?

However, since you're asking, a lockless implementation where appropriate
is always welcome.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                cygwin AT cygwin DOT com
Red Hat

   
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