Revert "module, async: async_synchronize_full() on module init iff async is used"

This reverts commit 774a122.

We need to finish all async code before the module init sequence is
done.  In the reverted commit the PF_USED_ASYNC flag was added to mark a
thread that called async_schedule().  Then the PF_USED_ASYNC flag was
used to determine whether or not async_synchronize_full() needs to be
invoked.  This works when modprobe thread is calling async_schedule(),
but it does not work if module dispatches init code to a worker thread
which then calls async_schedule().

For example, PCI driver probing is invoked from a worker thread based on
a node where device is attached:

	if (cpu < nr_cpu_ids)
		error = work_on_cpu(cpu, local_pci_probe, &ddi);
	else
		error = local_pci_probe(&ddi);

We end up in a situation where a worker thread gets the PF_USED_ASYNC
flag set instead of the modprobe thread.  As a result,
async_synchronize_full() is not invoked and modprobe completes without
waiting for the async code to finish.

The issue was discovered while loading the pm80xx driver:
(scsi_mod.scan=async)

modprobe pm80xx                      worker
...
  do_init_module()
  ...
    pci_call_probe()
      work_on_cpu(local_pci_probe)
                                     local_pci_probe()
                                       pm8001_pci_probe()
                                         scsi_scan_host()
                                           async_schedule()
                                           worker->flags |= PF_USED_ASYNC;
                                     ...
      < return from worker >
  ...
  if (current->flags & PF_USED_ASYNC) <--- false
  	async_synchronize_full();

Commit 21c3c5d ("block: don't request module during elevator init")
fixed the deadlock issue which the reverted commit 774a122
("module, async: async_synchronize_full() on module init iff async is
used") tried to fix.

Since commit 0fdff3e ("async, kmod: warn on synchronous
request_module() from async workers") synchronous module loading from
async is not allowed.

Given that the original deadlock issue is fixed and it is no longer
allowed to call synchronous request_module() from async we can remove
PF_USED_ASYNC flag to make module init consistently invoke
async_synchronize_full() unless async module probe is requested.

Signed-off-by: Igor Pylypiv <ipylypiv@google.com>
Reviewed-by: Changyuan Lyu <changyuanl@google.com>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: ipylypiv

include/linux/sched.h

@@ -1680,7 +1680,6 @@ extern struct pid *cad_pid;
 #define PF_MEMALLOC		0x00000800	/* Allocating memory */
 #define PF_NPROC_EXCEEDED	0x00001000	/* set_user() noticed that RLIMIT_NPROC was exceeded */
 #define PF_USED_MATH		0x00002000	/* If unset the fpu must be initialized before use */
-#define PF_USED_ASYNC		0x00004000	/* Used async_schedule*(), used by module init */
 #define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */
 #define PF_FROZEN		0x00010000	/* Frozen for system suspend */
 #define PF_KSWAPD		0x00020000	/* I am kswapd */

kernel/async.c

@@ -205,9 +205,6 @@ async_cookie_t async_schedule_node_domain(async_func_t func, void *data,
 	atomic_inc(&entry_count);
 	spin_unlock_irqrestore(&async_lock, flags);
 
-	/* mark that this task has queued an async job, used by module init */
-	current->flags |= PF_USED_ASYNC;
-
 	/* schedule for execution */
 	queue_work_node(node, system_unbound_wq, &entry->work);
 

kernel/module.c

@@ -3725,12 +3725,6 @@ static noinline int do_init_module(struct module *mod)
 	}
 	freeinit->module_init = mod->init_layout.base;
 
-	/*
-	 * We want to find out whether @mod uses async during init.  Clear
-	 * PF_USED_ASYNC.  async_schedule*() will set it.
-	 */
-	current->flags &= ~PF_USED_ASYNC;
-
 	do_mod_ctors(mod);
 	/* Start the module */
 	if (mod->init != NULL)
@@ -3756,22 +3750,13 @@ static noinline int do_init_module(struct module *mod)
 
 	/*
 	 * We need to finish all async code before the module init sequence
-	 * is done.  This has potential to deadlock.  For example, a newly
-	 * detected block device can trigger request_module() of the
-	 * default iosched from async probing task.  Once userland helper
-	 * reaches here, async_synchronize_full() will wait on the async
-	 * task waiting on request_module() and deadlock.
-	 *
-	 * This deadlock is avoided by perfomring async_synchronize_full()
-	 * iff module init queued any async jobs.  This isn't a full
-	 * solution as it will deadlock the same if module loading from
-	 * async jobs nests more than once; however, due to the various
-	 * constraints, this hack seems to be the best option for now.
-	 * Please refer to the following thread for details.
+	 * is done. This has potential to deadlock if synchronous module
+	 * loading is requested from async (which is not allowed!).
 	 *
-	 * http://thread.gmane.org/gmane.linux.kernel/1420814
+	 * See commit 0fdff3ec6d87 ("async, kmod: warn on synchronous
+	 * request_module() from async workers") for more details.
 	 */
-	if (!mod->async_probe_requested && (current->flags & PF_USED_ASYNC))
+	if (!mod->async_probe_requested)
 		async_synchronize_full();
 
 	ftrace_free_mem(mod, mod->init_layout.base, mod->init_layout.base +