[PATCH] functionfs: Avoid aio locking problem

[John Stultz]

The functionfs aio logic seems broken. When using functionfs,
I was seeing frequent hangs, and enabling spinlock debugging,
I got:

g_ffs gadget: g_ffs ready
ci_hdrc ci_hdrc.0: CI_HDRC_CONTROLLER_RESET_EVENT received
BUG: spinlock lockup suspected on CPU#0, adbd/2791
 lock: 0xe7764880, .magic: e7764880, .owner: <none>/-1, .owner_cpu: -407539900
CPU: 0 PID: 2791 Comm: adbd Not tainted 4.1.0-rc1-00032-g359b12f #147
Hardware name: Qualcomm (Flattened Device Tree)
[<c0216ac8>] (unwind_backtrace) from [<c02136a8>] (show_stack+0x10/0x14)
[<c02136a8>] (show_stack) from [<c075d9fc>] (dump_stack+0x70/0xbc)
[<c075d9fc>] (dump_stack) from [<c026ef90>] (do_raw_spin_lock+0x114/0x1a0)
[<c026ef90>] (do_raw_spin_lock) from [<c0764cb8>] (_raw_spin_lock_irqsave+0x50/0x5c)
[<c0764cb8>] (_raw_spin_lock_irqsave) from [<c037c1a0>] (kiocb_set_cancel_fn+0x1c/0x60)
[<c037c1a0>] (kiocb_set_cancel_fn) from [<c05ae568>] (ffs_epfile_read_iter+0x8c/0x140)
[<c05ae568>] (ffs_epfile_read_iter) from [<c0332018>] (__vfs_read+0xb0/0xd4)
[<c0332018>] (__vfs_read) from [<c0332ef8>] (vfs_read+0x7c/0x100)
[<c0332ef8>] (vfs_read) from [<c0332fbc>] (SyS_read+0x40/0x8c)
[<c0332fbc>] (SyS_read) from [<c020ff20>] (ret_fast_syscall+0x0/0x4c)
INFO: rcu_preempt detected stalls on CPUs/tasks:
 0: (1 GPs behind) idle=805/140000000000000/0 softirq=7187/7189 fqs=2601
 (detected by 3, t=2603 jiffies, g=3028, c=3027, q=474)
Task dump for CPU 0:
adbd            R running      0  2791      1 0x00000002
[<c075f234>] (__schedule) from [<ffffffff>] (0xffffffff)

Looking at the code, the __vfs_read() calls new_sync_read(),
which allocates a struct kiocb kiocb on the stack and passes
it to the ffs_epfile_read_iter() funciton. That then calls
kiocb_set_cancel_fn() passing a pointer to that kiocb. However,
kiocb_set_cancel_fn() assumes the kiocb is a sub-element of a
struct aio_kiocb, and it tries to grab the kioctx from that
parent structure. However it seems there is no aio_kiocb
structure here, so the spin_lock_irqsave hangs trying to lock
random data on the stack.

This patch avoids the issue, by only calling kiocb_set_cancel_fn
if the aio flag is set.

Cc: Felipe Balbi <balbi@xxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrzej Pietrasiewicz <andrzej.p@xxxxxxxxxxx>
Cc: Krzysztof Opasiak <k.opasiak@xxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Robert Baldyga <r.baldyga@xxxxxxxxxxx>
Cc: linux-usb@xxxxxxxxxxxxxxx
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
 drivers/usb/gadget/function/f_fs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 3507f88..d2434c9 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -924,7 +924,8 @@ static ssize_t ffs_epfile_write_iter(struct kiocb *kiocb, struct iov_iter *from)
 
 	kiocb->private = p;
 
-	kiocb_set_cancel_fn(kiocb, ffs_aio_cancel);
+	if (p->aio)
+		kiocb_set_cancel_fn(kiocb, ffs_aio_cancel);
 
 	res = ffs_epfile_io(kiocb->ki_filp, p);
 	if (res == -EIOCBQUEUED)
@@ -968,7 +969,8 @@ static ssize_t ffs_epfile_read_iter(struct kiocb *kiocb, struct iov_iter *to)
 
 	kiocb->private = p;
 
-	kiocb_set_cancel_fn(kiocb, ffs_aio_cancel);
+	if (p->aio)
+		kiocb_set_cancel_fn(kiocb, ffs_aio_cancel);
 
 	res = ffs_epfile_io(kiocb->ki_filp, p);
 	if (res == -EIOCBQUEUED)
-- 
1.9.1

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