linux-kernel - [RFC PATCH 13/13] kernel: Enable waitpid() for futex2

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Message-Id: <20210215152404.250281-14-andrealmeid@collabora.com>
Date:   Mon, 15 Feb 2021 12:24:04 -0300
From:   André Almeida <andrealmeid@...labora.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     kernel@...labora.com, krisman@...labora.com,
        pgriffais@...vesoftware.com, z.figura12@...il.com,
        joel@...lfernandes.org, malteskarupke@...tmail.fm,
        linux-api@...r.kernel.org, fweimer@...hat.com,
        libc-alpha@...rceware.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, acme@...nel.org, corbet@....net,
        André Almeida <andrealmeid@...labora.com>
Subject: [RFC PATCH 13/13] kernel: Enable waitpid() for futex2

To make pthreads works as expected if they are using futex2, wake
clear_child_tid with futex2 as well. This is make applications that uses
waitpid() (and clone(CLONE_CHILD_SETTID)) wake while waiting for the
child to terminate. Given that apps should not mix futex() and futex2(),
any correct app will trigger a harmless noop wakeup on the interface
that it isn't using.

Signed-off-by: André Almeida <andrealmeid@...labora.com>
---

This commit is here for the intend to show what we need to do in order
to get a full NPTL working on top of futex2. It should be merged after
we talk to glibc folks on the details around the futex_wait() side. For
instance, we could use this as an opportunity to use private futexes or
8bit sized futexes, but both sides need to use the exactly same flags.
---
 include/linux/syscalls.h |  2 ++
 kernel/fork.c            |  2 ++
 kernel/futex2.c          | 30 ++++++++++++++++++------------
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 06823bc7ef9d..38c2fc50ada9 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1312,6 +1312,8 @@ int ksys_ipc(unsigned int call, int first, unsigned long second,
 	unsigned long third, void __user * ptr, long fifth);
 int compat_ksys_ipc(u32 call, int first, int second,
 	u32 third, u32 ptr, u32 fifth);
+long ksys_futex_wake(void __user *uaddr, unsigned long nr_wake,
+		     unsigned int flags);
 
 /*
  * The following kernel syscall equivalents are just wrappers to fs-internal
diff --git a/kernel/fork.c b/kernel/fork.c
index d66cd1014211..e39846a73a43 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1308,6 +1308,8 @@ static void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 			put_user(0, tsk->clear_child_tid);
 			do_futex(tsk->clear_child_tid, FUTEX_WAKE,
 					1, NULL, NULL, 0, 0);
+			ksys_futex_wake(tsk->clear_child_tid, 1,
+					FUTEX_32 | FUTEX_SHARED_FLAG);
 		}
 		tsk->clear_child_tid = NULL;
 	}
diff --git a/kernel/futex2.c b/kernel/futex2.c
index 8a8b45f98d3b..a810b7f5c3a0 100644
--- a/kernel/futex2.c
+++ b/kernel/futex2.c
@@ -942,18 +942,8 @@ static inline bool futex_match(struct futex_key key1, struct futex_key key2)
 		key1.offset == key2.offset);
 }
 
-/**
- * sys_futex_wake - Wake a number of futexes waiting on an address
- * @uaddr:   Address of futex to be woken up
- * @nr_wake: Number of futexes waiting in uaddr to be woken up
- * @flags:   Flags for size and shared
- *
- * Wake `nr_wake` threads waiting at uaddr.
- *
- * Returns the number of woken threads on success, error code otherwise.
- */
-SYSCALL_DEFINE3(futex_wake, void __user *, uaddr, unsigned int, nr_wake,
-		unsigned int, flags)
+long ksys_futex_wake(void __user *uaddr, unsigned long nr_wake,
+		     unsigned int flags)
 {
 	bool shared = (flags & FUTEX_SHARED_FLAG) ? true : false;
 	unsigned int size = flags & FUTEX_SIZE_MASK;
@@ -990,6 +980,22 @@ SYSCALL_DEFINE3(futex_wake, void __user *, uaddr, unsigned int, nr_wake,
 	return ret;
 }
 
+/**
+ * sys_futex_wake - Wake a number of futexes waiting on an address
+ * @uaddr:   Address of futex to be woken up
+ * @nr_wake: Number of futexes waiting in uaddr to be woken up
+ * @flags:   Flags for size and shared
+ *
+ * Wake `nr_wake` threads waiting at uaddr.
+ *
+ * Returns the number of woken threads on success, error code otherwise.
+ */
+SYSCALL_DEFINE3(futex_wake, void __user *, uaddr, unsigned int, nr_wake,
+		unsigned int, flags)
+{
+	return ksys_futex_wake(uaddr, nr_wake, flags);
+}
+
 static void futex_double_unlock(struct futex_bucket *b1, struct futex_bucket *b2)
 {
 	spin_unlock(&b1->lock);
-- 
2.30.1