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futex: Replace PF_EXITPIDONE with a state

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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
This commit is contained in:
Thomas Gleixner 2019-11-06 22:55:37 +01:00
parent ba31c1a485
commit 3d4775df0a
4 changed files with 49 additions and 29 deletions
Download patch file Download diff file Expand all files Collapse all files

33
include/linux/futex.h
View file

@ -50,6 +50,10 @@ 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)
{
@ -59,6 +63,34 @@ 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);
@ -68,6 +100,7 @@ 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) { }
static inline long do_futex(u32 __user *uaddr, int op, u32 val,
ktime_t *timeout, u32 __user *uaddr2,
u32 val2, u32 val3)