mirror of https://gitee.com/openkylin/linux.git
111 lines
4.7 KiB
Plaintext
111 lines
4.7 KiB
Plaintext
Lockdep-RCU was added to the Linux kernel in early 2010
|
|
(http://lwn.net/Articles/371986/). This facility checks for some common
|
|
misuses of the RCU API, most notably using one of the rcu_dereference()
|
|
family to access an RCU-protected pointer without the proper protection.
|
|
When such misuse is detected, an lockdep-RCU splat is emitted.
|
|
|
|
The usual cause of a lockdep-RCU slat is someone accessing an
|
|
RCU-protected data structure without either (1) being in the right kind of
|
|
RCU read-side critical section or (2) holding the right update-side lock.
|
|
This problem can therefore be serious: it might result in random memory
|
|
overwriting or worse. There can of course be false positives, this
|
|
being the real world and all that.
|
|
|
|
So let's look at an example RCU lockdep splat from 3.0-rc5, one that
|
|
has long since been fixed:
|
|
|
|
=============================
|
|
WARNING: suspicious RCU usage
|
|
-----------------------------
|
|
block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage!
|
|
|
|
other info that might help us debug this:
|
|
|
|
|
|
rcu_scheduler_active = 1, debug_locks = 0
|
|
3 locks held by scsi_scan_6/1552:
|
|
#0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>]
|
|
scsi_scan_host_selected+0x5a/0x150
|
|
#1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>]
|
|
elevator_exit+0x22/0x60
|
|
#2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>]
|
|
cfq_exit_queue+0x43/0x190
|
|
|
|
stack backtrace:
|
|
Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17
|
|
Call Trace:
|
|
[<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0
|
|
[<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120
|
|
[<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190
|
|
[<ffffffff812a5046>] elevator_exit+0x36/0x60
|
|
[<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60
|
|
[<ffffffff8145cc09>] scsi_free_queue+0x9/0x10
|
|
[<ffffffff81460944>] __scsi_remove_device+0x84/0xd0
|
|
[<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10
|
|
[<ffffffff817da069>] ? error_exit+0x29/0xb0
|
|
[<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80
|
|
[<ffffffff8145e722>] __scsi_scan_target+0x112/0x680
|
|
[<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c
|
|
[<ffffffff817da069>] ? error_exit+0x29/0xb0
|
|
[<ffffffff812bcc60>] ? kobject_del+0x40/0x40
|
|
[<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0
|
|
[<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150
|
|
[<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90
|
|
[<ffffffff8145f170>] do_scan_async+0x20/0x160
|
|
[<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90
|
|
[<ffffffff810975b6>] kthread+0xa6/0xb0
|
|
[<ffffffff817db154>] kernel_thread_helper+0x4/0x10
|
|
[<ffffffff81066430>] ? finish_task_switch+0x80/0x110
|
|
[<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe
|
|
[<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70
|
|
[<ffffffff817db150>] ? gs_change+0xb/0xb
|
|
|
|
Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows:
|
|
|
|
if (rcu_dereference(ioc->ioc_data) == cic) {
|
|
|
|
This form says that it must be in a plain vanilla RCU read-side critical
|
|
section, but the "other info" list above shows that this is not the
|
|
case. Instead, we hold three locks, one of which might be RCU related.
|
|
And maybe that lock really does protect this reference. If so, the fix
|
|
is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to
|
|
take the struct request_queue "q" from cfq_exit_queue() as an argument,
|
|
which would permit us to invoke rcu_dereference_protected as follows:
|
|
|
|
if (rcu_dereference_protected(ioc->ioc_data,
|
|
lockdep_is_held(&q->queue_lock)) == cic) {
|
|
|
|
With this change, there would be no lockdep-RCU splat emitted if this
|
|
code was invoked either from within an RCU read-side critical section
|
|
or with the ->queue_lock held. In particular, this would have suppressed
|
|
the above lockdep-RCU splat because ->queue_lock is held (see #2 in the
|
|
list above).
|
|
|
|
On the other hand, perhaps we really do need an RCU read-side critical
|
|
section. In this case, the critical section must span the use of the
|
|
return value from rcu_dereference(), or at least until there is some
|
|
reference count incremented or some such. One way to handle this is to
|
|
add rcu_read_lock() and rcu_read_unlock() as follows:
|
|
|
|
rcu_read_lock();
|
|
if (rcu_dereference(ioc->ioc_data) == cic) {
|
|
spin_lock(&ioc->lock);
|
|
rcu_assign_pointer(ioc->ioc_data, NULL);
|
|
spin_unlock(&ioc->lock);
|
|
}
|
|
rcu_read_unlock();
|
|
|
|
With this change, the rcu_dereference() is always within an RCU
|
|
read-side critical section, which again would have suppressed the
|
|
above lockdep-RCU splat.
|
|
|
|
But in this particular case, we don't actually dereference the pointer
|
|
returned from rcu_dereference(). Instead, that pointer is just compared
|
|
to the cic pointer, which means that the rcu_dereference() can be replaced
|
|
by rcu_access_pointer() as follows:
|
|
|
|
if (rcu_access_pointer(ioc->ioc_data) == cic) {
|
|
|
|
Because it is legal to invoke rcu_access_pointer() without protection,
|
|
this change would also suppress the above lockdep-RCU splat.
|