From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20191106224556.149449274@xxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Lee Jones <lee.jones@xxxxxxxxxx> --- include/linux/futex.h | 34 ++++++++++++++++++++++++++++++++++ include/linux/sched.h | 2 +- kernel/exit.c | 18 ++---------------- kernel/futex.c | 17 ++++++++--------- 4 files changed, 45 insertions(+), 26 deletions(-) diff --git a/include/linux/futex.h b/include/linux/futex.h index 63d353cedfcde..a0de6fe28e00b 100644 --- a/include/linux/futex.h +++ b/include/linux/futex.h @@ -55,6 +55,11 @@ union futex_key { #define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = 0ULL } } #ifdef CONFIG_FUTEX +enum { + FUTEX_STATE_OK, + FUTEX_STATE_DEAD, +}; + static inline void futex_init_task(struct task_struct *tsk) { tsk->robust_list = NULL; @@ -63,6 +68,34 @@ static inline void futex_init_task(struct task_struct *tsk) #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); @@ -72,5 +105,6 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, #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 diff --git a/include/linux/sched.h b/include/linux/sched.h index 1872d4e9acbe1..4de48b251447f 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1815,6 +1815,7 @@ 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]; @@ -2276,7 +2277,6 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, * Per process flags */ #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 */ diff --git a/kernel/exit.c b/kernel/exit.c index f9943ef23fa82..969e1468f2538 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -785,16 +785,7 @@ 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(); } @@ -876,12 +867,7 @@ 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); diff --git a/kernel/futex.c b/kernel/futex.c index 156b23f4b9aac..51bbe57bb14ac 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1099,19 +1099,18 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, } /* - * 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 = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN; + int ret = (p->futex_state = FUTEX_STATE_DEAD) ? -ESRCH : -EAGAIN; raw_spin_unlock_irq(&p->pi_lock); put_task_struct(p); -- 2.25.1
