Merge pull request #386 from EQ-Alpha/fix_rdb_hang

add readwrite lock for forking

Author: MalavanEQAlpha

src/module.cpp

@@ -365,11 +365,7 @@ typedef struct RedisModuleCommandFilter {
 static list *moduleCommandFilters;
 
 /* Module GIL Variables */
-static int s_cAcquisitionsServer = 0;
-static int s_cAcquisitionsModule = 0;
-static std::mutex s_mutex;
-static std::condition_variable s_cv;
-static std::recursive_mutex s_mutexModule;
+static readWriteLock s_moduleGIL;
 thread_local bool g_fModuleThread = false;
 
 typedef void (*RedisModuleForkDoneHandler) (int exitcode, int bysignal, void *user_data);
@@ -5969,95 +5965,58 @@ void RM_ThreadSafeContextUnlock(RedisModuleCtx *ctx) {
 //  as the server thread acquisition is sufficient.  If we did try to lock we would deadlock
 static bool FModuleCallBackLock(bool fServerThread)
 {
-    return !fServerThread && aeThreadOwnsLock() && !g_fModuleThread && s_cAcquisitionsServer > 0;
+    return !fServerThread && aeThreadOwnsLock() && !g_fModuleThread && s_moduleGIL.hasReader();
 }
 void moduleAcquireGIL(int fServerThread, int fExclusive) {
-    std::unique_lock<std::mutex> lock(s_mutex);
-    int *pcheck = fServerThread ? &s_cAcquisitionsModule : &s_cAcquisitionsServer;
-
     if (FModuleCallBackLock(fServerThread)) {
         return;
     }
 
-    while (*pcheck > 0)
-        s_cv.wait(lock);
-
     if (fServerThread)
     {
-        ++s_cAcquisitionsServer;
+        s_moduleGIL.acquireRead();
     }
     else
     {
-        // only try to acquire the mutexModule in exclusive mode 
-        if (fExclusive){
-            // It is possible that another module thread holds the GIL (and s_mutexModule as a result). 
-            // When said thread goes to release the GIL, it will wait for s_mutex, which this thread owns. 
-            // This thread is however waiting for the GIL (and s_mutexModule) that the other thread owns.
-            // As a result, a deadlock has occured. 
-            // We release the lock on s_mutex and wait until we are able to safely acquire the GIL 
-            // in order to prevent this deadlock from occuring. 
-            while (!s_mutexModule.try_lock())
-                s_cv.wait(lock);     
-        }    
-        ++s_cAcquisitionsModule;
-        fModuleGILWlocked++;
+        s_moduleGIL.acquireWrite(fExclusive);
     }
 }
 
 int moduleTryAcquireGIL(bool fServerThread, int fExclusive) {
-    std::unique_lock<std::mutex> lock(s_mutex, std::defer_lock);
-    if (!lock.try_lock())
-        return 1;
-    int *pcheck = fServerThread ? &s_cAcquisitionsModule : &s_cAcquisitionsServer;
-
     if (FModuleCallBackLock(fServerThread)) {
         return 0;
     }
 
-    if (*pcheck > 0)
-        return 1;
-
     if (fServerThread)
     {
-        ++s_cAcquisitionsServer;
+        if (!s_moduleGIL.tryAcquireRead())
+            return 1;
     }
     else
     {
-        // only try to acquire the mutexModule in exclusive mode 
-        if (fExclusive){
-            if (!s_mutexModule.try_lock())
-                return 1;
-        }
-        ++s_cAcquisitionsModule;
-        fModuleGILWlocked++;
+        if (!s_moduleGIL.tryAcquireWrite(fExclusive))
+            return 1;
     }
     return 0;
 }
 
 void moduleReleaseGIL(int fServerThread, int fExclusive) {
-    std::unique_lock<std::mutex> lock(s_mutex);
-
     if (FModuleCallBackLock(fServerThread)) {
         return;
     }
-
+    
     if (fServerThread)
     {
-        --s_cAcquisitionsServer;
+        s_moduleGIL.releaseRead();
     }
     else
     {
-        // only try to release the mutexModule in exclusive mode
-        if (fExclusive)
-            s_mutexModule.unlock();
-        --s_cAcquisitionsModule;
-        fModuleGILWlocked--;
+        s_moduleGIL.releaseWrite(fExclusive);
     }
-    s_cv.notify_all();
 }
 
 int moduleGILAcquiredByModule(void) {
-    return fModuleGILWlocked > 0;
+    return s_moduleGIL.hasWriter();
 }
 
 

src/networking.cpp

@@ -3863,82 +3863,37 @@ int checkClientPauseTimeoutAndReturnIfPaused(void) {
  *
  * The function returns the total number of events processed. */
 void processEventsWhileBlocked(int iel) {
-    serverAssert(GlobalLocksAcquired());
-    int iterations = 4; /* See the function top-comment. */
-
-    std::vector<client*> vecclients;
-    listIter li;
-    listNode *ln;
-    listRewind(g_pserver->clients, &li);
 
-    // All client locks must be acquired *after* the global lock is reacquired to prevent deadlocks
-    //  so unlock here, and save them for reacquisition later
-    while ((ln = listNext(&li)) != nullptr)
-    {
-        client *c = (client*)listNodeValue(ln);
-        if (c->lock.fOwnLock()) {
-            serverAssert(c->flags & CLIENT_PROTECTED);  // If the client is not protected we have no gurantee they won't be free'd in the event loop
-            c->lock.unlock();
-            vecclients.push_back(c);
+    int eventsCount = 0;
+    executeWithoutGlobalLock([&](){
+        int iterations = 4; /* See the function top-comment. */
+        try
+        {
+            ProcessingEventsWhileBlocked = 1;
+            while (iterations--) {
+                long long startval = g_pserver->events_processed_while_blocked;
+                long long ae_events = aeProcessEvents(g_pserver->rgthreadvar[iel].el,
+                    AE_FILE_EVENTS|AE_DONT_WAIT|
+                    AE_CALL_BEFORE_SLEEP|AE_CALL_AFTER_SLEEP);
+                /* Note that g_pserver->events_processed_while_blocked will also get
+                * incremeted by callbacks called by the event loop handlers. */
+                eventsCount += ae_events;
+                long long events = eventsCount - startval;
+                if (!events) break;
+            }
+            ProcessingEventsWhileBlocked = 0;
         }
-    }
-    
-    /* Since we're about to release our lock we need to flush the repl backlog queue */
-    bool fReplBacklog = g_pserver->repl_batch_offStart >= 0;
-    if (fReplBacklog) {
-        flushReplBacklogToClients();
-        g_pserver->repl_batch_idxStart = -1;
-        g_pserver->repl_batch_offStart = -1;
-    }
-
-    long long eventsCount = 0;
-    aeReleaseLock();
-    serverAssert(!GlobalLocksAcquired());
-    try
-    {
-        ProcessingEventsWhileBlocked = 1;
-        while (iterations--) {
-            long long startval = g_pserver->events_processed_while_blocked;
-            long long ae_events = aeProcessEvents(g_pserver->rgthreadvar[iel].el,
-                AE_FILE_EVENTS|AE_DONT_WAIT|
-                AE_CALL_BEFORE_SLEEP|AE_CALL_AFTER_SLEEP);
-            /* Note that g_pserver->events_processed_while_blocked will also get
-            * incremeted by callbacks called by the event loop handlers. */
-            eventsCount += ae_events;
-            long long events = eventsCount - startval;
-            if (!events) break;
-        }
-        ProcessingEventsWhileBlocked = 0;
-    }
-    catch (...)
-    {
-        // Caller expects us to be locked so fix and rethrow
-        ProcessingEventsWhileBlocked = 0;
-        AeLocker locker;
-        locker.arm(nullptr);
-        locker.release();
-        for (client *c : vecclients)
-            c->lock.lock();
-        throw;
-    }
-    
-    AeLocker locker;
-    locker.arm(nullptr);
-    locker.release();
+        catch (...)
+        {
+            ProcessingEventsWhileBlocked = 0;
+            throw;
+        }
+    });
 
     g_pserver->events_processed_while_blocked += eventsCount;
 
     whileBlockedCron();
 
-    // Restore it so the calling code is not confused
-    if (fReplBacklog) {
-        g_pserver->repl_batch_idxStart = g_pserver->repl_backlog_idx;
-        g_pserver->repl_batch_offStart = g_pserver->master_repl_offset;
-    }
-
-    for (client *c : vecclients)
-        c->lock.lock();
-
     // If a different thread processed the shutdown we need to abort the lua command or we will hang
     if (serverTL->el->stop)
         throw ShutdownException();

src/readwritelock.h

@@ -0,0 +1,113 @@
+#pragma once
+#include <condition_variable>
+
+class readWriteLock {
+    std::mutex m_readLock;
+    std::recursive_mutex m_writeLock;
+    std::condition_variable m_cv;
+    int m_readCount = 0;
+    int m_writeCount = 0;
+    bool m_writeWaiting = false;
+public:
+    void acquireRead() {
+        std::unique_lock<std::mutex> rm(m_readLock);
+        while (m_writeCount > 0 || m_writeWaiting)
+            m_cv.wait(rm);
+        m_readCount++;
+    }
+    
+    bool tryAcquireRead() {
+        std::unique_lock<std::mutex> rm(m_readLock, std::defer_lock);
+        if (!rm.try_lock())
+            return false;
+        if (m_writeCount > 0 || m_writeWaiting)
+            return false;
+        m_readCount++;
+        return true;
+    }
+
+    void acquireWrite(bool exclusive = true) {
+        std::unique_lock<std::mutex> rm(m_readLock);
+        m_writeWaiting = true;
+        while (m_readCount > 0)
+            m_cv.wait(rm);
+        if (exclusive) {
+            /* Another thread might have the write lock while we have the read lock
+               but won't be able to release it until they can acquire the read lock
+               so release the read lock and try again instead of waiting to avoid deadlock */
+            while(!m_writeLock.try_lock())
+                m_cv.wait(rm);
+        }
+        m_writeCount++;
+        m_writeWaiting = false;
+    }
+
+    void upgradeWrite(bool exclusive = true) {
+        std::unique_lock<std::mutex> rm(m_readLock);
+        m_writeWaiting = true;
+        while (m_readCount > 1)
+            m_cv.wait(rm);
+        if (exclusive) {
+            /* Another thread might have the write lock while we have the read lock
+               but won't be able to release it until they can acquire the read lock
+               so release the read lock and try again instead of waiting to avoid deadlock */
+            while(!m_writeLock.try_lock())
+                m_cv.wait(rm);
+        }
+        m_writeCount++;
+        m_readCount--;
+        m_writeWaiting = false;
+    }
+
+    bool tryAcquireWrite(bool exclusive = true) {
+        std::unique_lock<std::mutex> rm(m_readLock, std::defer_lock);
+        if (!rm.try_lock())
+            return false;
+        if (m_readCount > 0)
+            return false;
+        if (exclusive)
+            if (!m_writeLock.try_lock())
+                return false;
+        m_writeCount++;
+        return true;
+    }
+
+    void releaseRead() {
+        std::unique_lock<std::mutex> rm(m_readLock);
+        serverAssert(m_readCount > 0);
+        m_readCount--;
+        m_cv.notify_all();
+    }
+
+    void releaseWrite(bool exclusive = true) {
+        std::unique_lock<std::mutex> rm(m_readLock);
+        serverAssert(m_writeCount > 0);
+        if (exclusive)
+            m_writeLock.unlock();
+        m_writeCount--;
+        m_cv.notify_all();
+    }
+
+    void downgradeWrite(bool exclusive = true) {
+        std::unique_lock<std::mutex> rm(m_readLock);
+        serverAssert(m_writeCount > 0);
+        if (exclusive)
+            m_writeLock.unlock();
+        m_writeCount--;
+        while (m_writeCount > 0 || m_writeWaiting)
+            m_cv.wait(rm);
+        m_readCount++;
+    }
+
+    bool hasReader() {
+        return m_readCount > 0;
+    }
+
+    bool hasWriter() {
+        return m_writeCount > 0;
+    }
+
+    bool writeWaiting() {
+        return m_writeWaiting;
+    }
+};