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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
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25
kernel/futex.c
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@@ -1182,9 +1182,10 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,
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, u32 uval,
* *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 *uaddr, 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 = handle_exit_race(uaddr, uval, p);