Re: [PATCH 1/2] bcache: ignore pending signals in bcache_device_init()

Michal Hocko
2020-03-04T12:41:44Z

On Wed 04-03-20 13:33:42, Oleg Nesterov wrote:
> On 03/04, Michal Hocko wrote:
> >
> > On Wed 04-03-20 13:13:25, Oleg Nesterov wrote:
> > > On 03/04, Michal Hocko wrote:
> > > >
> > > > So what would be a legit usecase to drop all signals while explicitly
> > > > calling allow_signal?
> > >
> > > Not sure I understand...
> >
> > flush_signals will simply drop all pending signals on the floor so there
> > is no way to handle them, right? I am asking when is still really a
> > desirable thing to do when you allow_signal for the kthread. The only
> > one I can imagine is that the kthread allows a single signal so it is
> > quite clear which signal is flushed.
>
> Yes. This is what I meant when I said "they should do the same if kthread
> allows a single signal".

OK, good that we are at the same page. I have clearly misread your
earlier email.

> > kernel_dequeue_signal on the other hand will give you a signal and so
> > the code can actually handle it in some way.
>
> Yes.

diff --git a/kernel/signal.c b/kernel/signal.c
index 9ad8dea93dbb..959adc2a5a3d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -465,7 +465,18 @@ void flush_sigqueue(struct sigpending *queue)
}

/*
- * Flush all pending signals for this kthread.
+ * Flush all pending signals for this kthread. Please note that this interface
+ * shouldn't be and if there is a need for it then it should be clearly
+ * documented why.
+ *
+ * Existing users should be double checked because most of them are likely
+ * obsolete. Kernel threads are not on the receiving end of signal delivery
+ * unless they explicitly request that by allow_signal() and in that case
+ * flush_signals is almost always a bug because signal should be processed
+ * by kernel_dequeue_signal rather than dropping them on the floor. The only
+ * exception when flush_signals could be used is a micro-optimization when
+ * only a single signal is allowed when retreiving the specific signal number
+ * is not needed. Please document this usage.
*/
void flush_signals(struct task_struct *t)
{

--
Michal Hocko
SUSE Labs