[PATCH 5.10 042/121] Revert "module, async: async_synchronize_full() on module init iff async is used"

Greg Kroah-Hartman
2022-02-21T09:16:35Z

From: Igor Pylypiv <ipylypiv@google.com>

[ Upstream commit 67d6212afda218d564890d1674bab28e8612170f ]

This reverts commit 774a1221e862b343388347bac9b318767336b20b.

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 21c3c5d28007 ("block: don't request module during elevator init")
fixed the deadlock issue which the reverted commit 774a1221e862
("module, async: async_synchronize_full() on module init iff async is
used") tried to fix.

Since commit 0fdff3ec6d87 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/sched.h | 1 -
kernel/async.c | 3 ---
kernel/module.c | 25 +++++--------------------
3 files changed, 5 insertions(+), 24 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b85b26d9ccefe..f996d1f343bb7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1544,7 +1544,6 @@ 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 */
diff --git a/kernel/async.c b/kernel/async.c
index 33258e6e20f83..1746cd65e271b 100644
--- a/kernel/async.c
+++ b/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);

diff --git a/kernel/module.c b/kernel/module.c
index 185b2655bc206..5f4403198f04b 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3714,12 +3714,6 @@ 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)
@@ -3745,22 +3739,13 @@ 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 +
--
2.34.1