linux-kernel - Questions about wake_up[_interruptible][

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Message-ID: <b1f38f6b-9d14-46cb-81f1-6bc5f92e7c65@samba.org>
Date: Wed, 13 Aug 2025 22:28:08 +0200
From: Stefan Metzmacher <metze@...ba.org>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Steve French <smfrench@...il.com>, Namjae Jeon <linkinjeon@...nel.org>
Cc: "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Questions about wake_up[_interruptible][_all]

Hi,

there are several cases where wait queues are used in:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/smb/client/smbdirect.c
and
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/smb/server/transport_rdma.c

I'm a bit confused because we the use mixed use of
wake_up(), wake_up_interruptible() and wake_up_interruptible_all()

On the wait side the following are used
wait_event(), wait_event_interruptible() and wait_event_interruptible_timeout()

The documentation of all wait_event_* macros say 'wake_up()' should be used.
And there's no documentation on the various wake_up_* macros.

I guess I understand the difference between
wait_event() and wait_event_interruptible(),
the first ignores any signal even kill and the
2nd returns -ERESTARTSYS on any signal.

I'm wondering if using wait_event_killable() should
be preferred instead of wait_event() in order to prevent
processes in state D hanging forever.

For some wait queues it would be desired that only
a single waiter is woken, so it can make good forward progress,
so maybe some wait_event_*_exclusive() would be useful for this.

As far as I understand the difference between
wake_up() and wake_up_all() is that the first
stops after the first waiter with WQ_FLAG_EXCLUSIVE.
and wake_up_all() wakes all waiters (useful for error conditions,
which all waiters should handle immediately.

But I don't understand the difference between
wake_up() and wake_up_interruptible().
My best guess would be that wake_up_interruptible()
would not wake waiters using wait_event(), but only
waiters using wait_event_interruptible() or any other
version that includes TASK_INTERRUPTIBLE.

So I guess we never want to use wake_up_interruptible(),
but always wake_up() or wake_up_all() instead...

It would be great if the documentation of
the wake_up macros and their interaction with
the wait_event macros could be improved.

Any hints are highly welcome :-)

Thanks!
metze