Re: general protection fault in wb_workfn (2)

On Sun 27-05-18 09:47:54, Tetsuo Handa wrote:

Forwarding http://lkml.kernel.org/r/201805251915.FGH64517.HVFJOOLFFMQStO@I-love.SAKURA.ne.jp .

Jan Kara wrote:

quoted

quoted

void delayed_work_timer_fn(struct timer_list *t)
{
	struct delayed_work *dwork = from_timer(dwork, t, timer);

	/* should have been called from irqsafe timer with irq already off */
	__queue_work(dwork->cpu, dwork->wq, &dwork->work);
}

Then, wb_workfn() is after all scheduled even if we check for
WB_registered bit, isn't it?

It can be queued after WB_registered bit is cleared but it cannot be queued
after mod_delayed_work(bdi_wq, &wb->dwork, 0) has finished. That function
deletes the pending timer (the timer cannot be armed again because
WB_registered is cleared) and queues what should be the last round of
wb_workfn().

mod_delayed_work() deletes the pending timer but does not wait for already
invoked timer handler to complete because it is using del_timer() rather than
del_timer_sync(). Then, what happens if __queue_work() is almost concurrently
executed from two CPUs, one from mod_delayed_work(bdi_wq, &wb->dwork, 0) from
wb_shutdown() path (which is called without spin_lock_bh(&wb->work_lock)) and
the other from delayed_work_timer_fn() path (which is called without checking
WB_registered bit under spin_lock_bh(&wb->work_lock)) ?

In this case, work should still be queued only once. The synchronization in
this case should be provided by the WORK_STRUCT_PENDING_BIT. When a delayed
work is queued by mod_delayed_work(), this bit is set, and gets cleared
only once the work is started on some CPU. But admittedly this code is
rather convoluted so I may be missing something.

Also you should note that flush_delayed_work() which follows
mod_delayed_work() in wb_shutdown() does del_timer_sync() so I don't see
how anything could get past that. In fact mod_delayed_work() is in
wb_shutdown() path to make sure wb_workfn() gets executed at least once
before the bdi_writeback structure gets cleaned up so that all queued items
are finished. We do not rely on it to remove pending timers or queued
wb_workfn() executions.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR