[PATCH 4.14 188/209] futex: Replace PF_EXITPIDONE with a state

Greg Kroah-Hartman
2019-12-04T18:09:33Z

From: Thomas Gleixner <tglx@linutronix.de>

commit 3d4775df0a89240f671861c6ab6e8d59af8e9e41 upstream.

The futex exit handling relies on PF_ flags. That's suboptimal as it
requires a smp_mb() and an ugly lock/unlock of the exiting tasks pi_lock in
the middle of do_exit() to enforce the observability of PF_EXITING in the
futex code.

Add a futex_state member to task_struct and convert the PF_EXITPIDONE logic
over to the new state. The PF_EXITING dependency will be cleaned up in a
later step.

This prepares for handling various futex exit issues later.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.149449274@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/futex.h | 33 +++++++++++++++++++++++++++++++++
include/linux/sched.h | 2 +-
kernel/exit.c | 18 ++----------------
kernel/futex.c | 25 +++++++++++++------------
4 files changed, 49 insertions(+), 29 deletions(-)

--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -53,6 +53,10 @@ union futex_key {
#define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = NULL } }

#ifdef CONFIG_FUTEX
+enum {
+ FUTEX_STATE_OK,
+ FUTEX_STATE_DEAD,
+};

static inline void futex_init_task(struct task_struct *tsk)
{
@@ -62,6 +66,34 @@ static inline void futex_init_task(struc
#endif
INIT_LIST_HEAD(&tsk->pi_state_list);
tsk->pi_state_cache = NULL;
+ tsk->futex_state = FUTEX_STATE_OK;
+}
+
+/**
+ * futex_exit_done - Sets the tasks futex state to FUTEX_STATE_DEAD
+ * @tsk: task to set the state on
+ *
+ * Set the futex exit state of the task lockless. The futex waiter code
+ * observes that state when a task is exiting and loops until the task has
+ * actually finished the futex cleanup. The worst case for this is that the
+ * waiter runs through the wait loop until the state becomes visible.
+ *
+ * This has two callers:
+ *
+ * - futex_mm_release() after the futex exit cleanup has been done
+ *
+ * - do_exit() from the recursive fault handling path.
+ *
+ * In case of a recursive fault this is best effort. Either the futex exit
+ * code has run already or not. If the OWNER_DIED bit has been set on the
+ * futex then the waiter can take it over. If not, the problem is pushed
+ * back to user space. If the futex exit code did not run yet, then an
+ * already queued waiter might block forever, but there is nothing which
+ * can be done about that.
+ */
+static inline void futex_exit_done(struct task_struct *tsk)
+{
+ tsk->futex_state = FUTEX_STATE_DEAD;
}

void futex_mm_release(struct task_struct *tsk);
@@ -71,6 +103,7 @@ long do_futex(u32 __user *uaddr, int op,
#else
static inline void futex_init_task(struct task_struct *tsk) { }
static inline void futex_mm_release(struct task_struct *tsk) { }
+static inline void futex_exit_done(struct task_struct *tsk) { }
#endif

#endif
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -959,6 +959,7 @@ struct task_struct {
#endif
struct list_head pi_state_list;
struct futex_pi_state *pi_state_cache;
+ unsigned int futex_state;
#endif
#ifdef CONFIG_PERF_EVENTS
struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts];
@@ -1334,7 +1335,6 @@ extern struct pid *cad_pid;
*/
#define PF_IDLE 0x00000002 /* I am an IDLE thread */
#define PF_EXITING 0x00000004 /* Getting shut down */
-#define PF_EXITPIDONE 0x00000008 /* PI exit done on shut down */
#define PF_VCPU 0x00000010 /* I'm a virtual CPU */
#define PF_WQ_WORKER 0x00000020 /* I'm a workqueue worker */
#define PF_FORKNOEXEC 0x00000040 /* Forked but didn't exec */
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -803,16 +803,7 @@ void __noreturn do_exit(long code)
*/
if (unlikely(tsk->flags & PF_EXITING)) {
pr_alert("Fixing recursive fault but reboot is needed!\n");
- /*
- * We can do this unlocked here. The futex code uses
- * this flag just to verify whether the pi state
- * cleanup has been done or not. In the worst case it
- * loops once more. We pretend that the cleanup was
- * done as there is no way to return. Either the
- * OWNER_DIED bit is set by now or we push the blocked
- * task into the wait for ever nirwana as well.
- */
- tsk->flags |= PF_EXITPIDONE;
+ futex_exit_done(tsk);
set_current_state(TASK_UNINTERRUPTIBLE);
schedule();
}
@@ -902,12 +893,7 @@ void __noreturn do_exit(long code)
* Make sure we are holding no locks:
*/
debug_check_no_locks_held();
- /*
- * We can do this unlocked here. The futex code uses this flag
- * just to verify whether the pi state cleanup has been done
- * or not. In the worst case it loops once more.
- */
- tsk->flags |= PF_EXITPIDONE;
+ futex_exit_done(tsk);

if (tsk->io_context)
exit_io_context(tsk);
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1182,9 +1182,10 @@ static int handle_exit_race(u32 __user *
u32 uval2;

/*
- * If PF_EXITPIDONE is not yet set, then try again.
+ * If the futex exit state is not yet FUTEX_STATE_DEAD, wait
+ * for it to finish.
*/
- if (tsk && !(tsk->flags & PF_EXITPIDONE))
+ if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
return -EAGAIN;

/*
@@ -1203,8 +1204,9 @@ static int handle_exit_race(u32 __user *
* *uaddr = 0xC0000000; tsk = get_task(PID);
* } if (!tsk->flags & PF_EXITING) {
* ... attach();
- * tsk->flags |= PF_EXITPIDONE; } else {
- * if (!(tsk->flags & PF_EXITPIDONE))
+ * tsk->futex_state = } else {
+ * FUTEX_STATE_DEAD; if (tsk->futex_state !=
+ * FUTEX_STATE_DEAD)
* return -EAGAIN;
* return -ESRCH; <--- FAIL
* }
@@ -1260,17 +1262,16 @@ static int attach_to_pi_owner(u32 __user
}

/*
- * We need to look at the task state flags to figure out,
- * whether the task is exiting. To protect against the do_exit
- * change of the task flags, we do this protected by
- * p->pi_lock:
+ * We need to look at the task state to figure out, whether the
+ * task is exiting. To protect against the change of the task state
+ * in futex_exit_release(), we do this protected by p->pi_lock:
*/
raw_spin_lock_irq(&p->pi_lock);
- if (unlikely(p->flags & PF_EXITING)) {
+ if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
/*
- * The task is on the way out. When PF_EXITPIDONE is
- * set, we know that the task has finished the
- * cleanup:
+ * The task is on the way out. When the futex state is
+ * FUTEX_STATE_DEAD, we know that the task has finished
+ * the cleanup:
*/
int ret = handle_exit_race(uaddr, uval, p);