http: Join worker threads before deleting work queue

laanwj commented at 6:32 pm on February 6, 2018: member

This prevents a potential race condition if control flow ends up in ShutdownHTTPServer before the thread gets to queue->Run(), deleting the work queue while workers are still going to use it.

Meant to fix #12362.

laanwj added the label RPC/REST/ZMQ on Feb 6, 2018

laanwj added the label Bug on Feb 6, 2018

laanwj added this to the milestone 0.16.0 on Feb 6, 2018

in src/httpserver.cpp:452 in 2fefa3077e outdated

448@@ -449,6 +449,7 @@ bool UpdateHTTPServerLogging(bool enable) {
449 
450 std::thread threadHTTP;
451 std::future<bool> threadResult;
452+static std::vector<std::thread> threadHTTPWorkers;

ryanofsky commented at 6:44 pm on February 6, 2018:

Could follow g_thread_http_workers naming convention.

laanwj commented at 6:50 pm on February 6, 2018:

Can we please first make sure that this fixes the issue before jumping on naming issues

ryanofsky commented at 6:47 pm on February 6, 2018: member

utACK 2fefa3077e69dfdbccbd010cda5627787ec92bb8. Untested, but change makes sense. Detached threads are always a little suspicious.

in src/httpserver.cpp:464 in 2fefa3077e outdated

460@@ -460,8 +461,7 @@ bool StartHTTPServer()
461     threadHTTP = std::thread(std::move(task), eventBase, eventHTTP);
462 
463     for (int i = 0; i < rpcThreads; i++) {
464-        std::thread rpc_worker(HTTPWorkQueueRun, workQueue);
465-        rpc_worker.detach();
466+        threadHTTPWorkers.emplace_back(std::thread(HTTPWorkQueueRun, workQueue));

TheBlueMatt commented at 7:05 pm on February 6, 2018:

No need to create the temporary just to move it into a new std::thread constructor.

laanwj commented at 7:09 pm on February 6, 2018:

Right

TheBlueMatt commented at 7:06 pm on February 6, 2018: member

Tested ACK 2fefa3077e69dfdbccbd010cda5627787ec92bb8 - my previous sleeps and valgrind are no longer able to find any issue here.

Would love to see @jnewbery confirm he cannot reproduce anymore as well.

http: Join worker threads before deleting work queue

This prevents a potential race condition if control flow ends up in
`ShutdownHTTPServer` before the thread gets to `queue->Run()`,
deleting the work queue while workers are still going to use it.

Meant to fix #12362.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>

b1c2370dde

http: Remove WaitExit from WorkQueue

This function, which waits for all threads to exit, is no longer needed
now that threads are joined instead.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>

f94665466e

in src/httpserver.cpp:489 in 2fefa3077e outdated

486@@ -487,6 +487,10 @@ void StopHTTPServer()
487     if (workQueue) {
488         LogPrint(BCLog::HTTP, "Waiting for HTTP worker threads to exit\n");
489         workQueue->WaitExit();

laanwj commented at 7:14 pm on February 6, 2018:

I think we don’t need WaitExit() anymore with this?

TheBlueMatt commented at 7:19 pm on February 6, 2018:

It would seem not, indeed. Testing shows no difference with it removed.

laanwj commented at 7:30 pm on February 6, 2018:

Ok, will remove it in a separate commit.

laanwj force-pushed on Feb 6, 2018

laanwj commented at 7:35 pm on February 6, 2018: member

ok, renamed the variable and added a commit to remove WaitExit()

jnewbery commented at 7:36 pm on February 6, 2018: member

I can confirm that I’m not able to reproduce the error when running with commit f94665466ed50e868c98b1a1c708ad5767727bb6.

laanwj added the label Needs backport on Feb 6, 2018

in src/httpserver.cpp:148 in f94665466e outdated

139@@ -141,13 +140,6 @@ class WorkQueue
140         running = false;
141         cond.notify_all();
142     }
143-    /** Wait for worker threads to exit */
144-    void WaitExit()
145-    {
146-        std::unique_lock<std::mutex> lock(cs);
147-        while (numThreads > 0)

ryanofsky commented at 7:51 pm on February 6, 2018:

In commit “http: Remove WaitExit from WorkQueue”

It looks like the numThreads variable is unused now, so it might be worth removing as well.

laanwj commented at 8:55 am on February 7, 2018:

Thanks, I removed those in a new commit.

ryanofsky commented at 7:53 pm on February 6, 2018: member

utACK f94665466ed50e868c98b1a1c708ad5767727bb6. Only changes were variable rename and temp std::thread / WaitExit remove suggestions above.

in src/httpserver.cpp:482 in f94665466e outdated

477@@ -486,7 +478,10 @@ void StopHTTPServer()
478     LogPrint(BCLog::HTTP, "Stopping HTTP server\n");
479     if (workQueue) {
480         LogPrint(BCLog::HTTP, "Waiting for HTTP worker threads to exit\n");
481-        workQueue->WaitExit();
482+        for (auto& thread: g_thread_http_workers) {
483+            thread.join();

promag commented at 10:47 pm on February 6, 2018:

From the documentation, I don’t think this is needed, clear() below is enough.

TheBlueMatt commented at 10:52 pm on February 6, 2018:

No, if you delete a thread without either calling detach() or join(), terminate() is called. join() is not automatically called for you.

promag commented at 10:48 pm on February 6, 2018: member

utACK.

sipa commented at 3:22 am on February 7, 2018: member

utACK f94665466ed50e868c98b1a1c708ad5767727bb6. This is easier to read as well.

dcousens approved

dcousens commented at 3:46 am on February 7, 2018: contributor

utACK

http: Remove numThreads and ThreadCounter

The HTTP worker thread counter, as well as the RAII object that was used
to maintain it, is unused now, so can be removed.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>

11e01515fe

dcousens approved

in src/httpserver.cpp:79 in 11e01515fe

 95 
 96 public:
 97     explicit WorkQueue(size_t _maxDepth) : running(true),
 98-                                 maxDepth(_maxDepth),
 99-                                 numThreads(0)
100+                                 maxDepth(_maxDepth)

promag commented at 9:03 pm on February 7, 2018:

Nit, dunno about correct indentation here..

in src/httpserver.cpp:460 in 11e01515fe

456@@ -486,7 +457,10 @@ void StopHTTPServer()
457     LogPrint(BCLog::HTTP, "Stopping HTTP server\n");
458     if (workQueue) {
459         LogPrint(BCLog::HTTP, "Waiting for HTTP worker threads to exit\n");
460-        workQueue->WaitExit();
461+        for (auto& thread: g_thread_http_workers) {

promag commented at 9:06 pm on February 7, 2018:

Nit, space before :.

in src/httpserver.cpp:422 in 11e01515fe

419@@ -449,6 +420,7 @@ bool UpdateHTTPServerLogging(bool enable) {
420 
421 std::thread threadHTTP;
422 std::future<bool> threadResult;

promag commented at 9:08 pm on February 7, 2018:

Nit, this and the above could be static too.

laanwj commented at 7:39 am on February 8, 2018:

Yes, but out of scope of this PR to change that.

promag commented at 9:12 pm on February 7, 2018: member

utACK 11e0151.

meshcollider commented at 9:52 pm on February 7, 2018: contributor

Nice cleanup! utACK https://github.com/bitcoin/bitcoin/pull/12366/commits/11e01515fe0fbc7823d4111ad6e016a02c485a78

laanwj commented at 7:55 am on February 8, 2018: member

Nice cleanup!

FWIW I wrote HTTPWorkQueue with the idea that it could likely be replaced with a 1-liner after c++11, I think there’s scope for more code removal. Out of scope for a 0.16.0 fix ofc.

laanwj merged this on Feb 8, 2018

laanwj closed this on Feb 8, 2018

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http: Join worker threads before deleting work queue #12366