Hi Tejun, I suspect this is a long-standing bug introduced by all the pool rework you did at some point, but I don't really know nor can I figure out how to fix it right now. I guess it could possibly also be a lockdep issue, or an issue in how it's used, but I definitely know that this used to work (i.e. warn) back when I introduced the lockdep checking to the WQ code. I was actually bitten by a bug like this, and erroneously dismissed it as not being the case because lockdep hadn't warned (and the actual deadlock debug output is basically not existent).
In any case, the following code really should result in a warning from lockdep, but doesn't. If you comment in the #define DEADLOCK, it will actually cause a deadlock :) #include <linux/kernel.h> #include <linux/mutex.h> #include <linux/workqueue.h> #include <linux/module.h> #include <linux/delay.h> DEFINE_MUTEX(mtx); static struct workqueue_struct *wq; static struct work_struct w1, w2; static void w1_wk(struct work_struct *w) { mutex_lock(&mtx); msleep(100); mutex_unlock(&mtx); } static void w2_wk(struct work_struct *w) { } /* * if not defined, then lockdep should warn only, * if defined, the system will really deadlock. */ //#define DEADLOCK static int init(void) { wq = create_singlethread_workqueue("test"); if (!wq) return -ENOMEM; INIT_WORK(&w1, w1_wk); INIT_WORK(&w2, w2_wk); #ifdef DEADLOCK queue_work(wq, &w1); queue_work(wq, &w2); #endif mutex_lock(&mtx); flush_work(&w2); mutex_unlock(&mtx); #ifndef DEADLOCK queue_work(wq, &w1); queue_work(wq, &w2); #endif return 0; } module_init(init); (to test, just copy it to some C file and add "obj-y += myfile.o" to the Makefile in that directory, then boot the kernel - perhaps in a VM) johannes