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