gh-96387: take_gil() resets drop request before exit (#96869) (#96941)

vstinner · web-flow · commit 6ff54716f107 · 2022-09-20T02:12:09.000+02:00
At Python exit, sometimes a thread holding the GIL can wait forever for a thread (usually a daemon thread) which requested to drop the GIL, whereas the thread already exited. To fix the race condition, the thread which requested the GIL drop now resets its request before exiting. take_gil() now calls RESET_GIL_DROP_REQUEST() before PyThread_exit_thread() if it called SET_GIL_DROP_REQUEST to fix a race condition with drop_gil(). Issue discovered and analyzed by Mingliang ZHAO. (cherry picked from commit 04f4977)
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst b/Misc/NEWS.d/next/Core and Builtins/2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst
@@ -0,0 +1,5 @@
+At Python exit, sometimes a thread holding the GIL can wait forever for a
+thread (usually a daemon thread) which requested to drop the GIL, whereas
+the thread already exited. To fix the race condition, the thread which
+requested the GIL drop now resets its request before exiting. Issue
+discovered and analyzed by Mingliang ZHAO. Patch by Victor Stinner.
diff --git a/Python/ceval_gil.h b/Python/ceval_gil.h
@@ -239,6 +239,7 @@ take_gil(PyThreadState *tstate)
         goto _ready;
     }
 
+    int drop_requested = 0;
     while (_Py_atomic_load_relaxed(&gil->locked)) {
         unsigned long saved_switchnum = gil->switch_number;
 
@@ -254,11 +255,21 @@ take_gil(PyThreadState *tstate)
         {
             if (tstate_must_exit(tstate)) {
                 MUTEX_UNLOCK(gil->mutex);
+                // gh-96387: If the loop requested a drop request in a previous
+                // iteration, reset the request. Otherwise, drop_gil() can
+                // block forever waiting for the thread which exited. Drop
+                // requests made by other threads are also reset: these threads
+                // may have to request again a drop request (iterate one more
+                // time).
+                if (drop_requested) {
+                    RESET_GIL_DROP_REQUEST(interp);
+                }
                 PyThread_exit_thread();
             }
             assert(is_tstate_valid(tstate));
 
             SET_GIL_DROP_REQUEST(interp);
+            drop_requested = 1;
         }
     }
 

Original file line number	Diff line number	Diff line change
`@@ -239,6 +239,7 @@ take_gil(PyThreadState *tstate)`
`239`	`239`	`goto _ready;`
`240`	`240`	`}`
`241`	`241`
	`242`	`+ int drop_requested = 0;`
`242`	`243`	`while (_Py_atomic_load_relaxed(&gil->locked)) {`
`243`	`244`	`unsigned long saved_switchnum = gil->switch_number;`
`244`	`245`
`@@ -254,11 +255,21 @@ take_gil(PyThreadState *tstate)`
`254`	`255`	`{`
`255`	`256`	`if (tstate_must_exit(tstate)) {`
`256`	`257`	`MUTEX_UNLOCK(gil->mutex);`
	`258`	`+ // gh-96387: If the loop requested a drop request in a previous`
	`259`	`+ // iteration, reset the request. Otherwise, drop_gil() can`
	`260`	`+ // block forever waiting for the thread which exited. Drop`
	`261`	`+ // requests made by other threads are also reset: these threads`
	`262`	`+ // may have to request again a drop request (iterate one more`
	`263`	`+ // time).`
	`264`	`+ if (drop_requested) {`
	`265`	`+ RESET_GIL_DROP_REQUEST(interp);`
	`266`	`+ }`
`257`	`267`	`PyThread_exit_thread();`
`258`	`268`	`}`
`259`	`269`	`assert(is_tstate_valid(tstate));`
`260`	`270`
`261`	`271`	`SET_GIL_DROP_REQUEST(interp);`
	`272`	`+ drop_requested = 1;`
`262`	`273`	`}`
`263`	`274`	`}`
`264`	`275`