[PATCH v3 13/13] kernel: Enable waitpid() for futex2

From: "André Almeida" <andrealmeid@collabora.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Darren Hart <dvhart@infradead.org>,
	linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: kernel@collabora.com, krisman@collabora.com,
	pgriffais@valvesoftware.com, z.figura12@gmail.com,
	joel@joelfernandes.org, malteskarupke@fastmail.fm,
	linux-api@vger.kernel.org, fweimer@redhat.com,
	libc-alpha@sourceware.org, linux-kselftest@vger.kernel.org,
	shuah@kernel.org, acme@kernel.org, corbet@lwn.net,
	"Peter Oskolkov" <posk@posk.io>,
	"André Almeida" <andrealmeid@collabora.com>
Subject: [PATCH v3 13/13] kernel: Enable waitpid() for futex2
Date: Tue, 27 Apr 2021 20:12:48 -0300	[thread overview]
Message-ID: <20210427231248.220501-14-andrealmeid@collabora.com> (raw)
In-Reply-To: <20210427231248.220501-1-andrealmeid@collabora.com>

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@collabora.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 d5a485cf52c4..59655c9317e6 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1315,6 +1315,8 @@ 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 426cd0c51f9e..9f99273b161b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1316,6 +1316,8 @@ 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 321472593e6f..901edf41d558 100644
--- a/kernel/futex2.c
+++ b/kernel/futex2.c
@@ -937,18 +937,8 @@ 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;
@@ -985,6 +975,22 @@ 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);
-- 
2.31.1

