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futex: Provide distinct return value when owner is exiting

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attach_to_pi_owner() returns -EAGAIN for various cases:

 - Owner task is exiting
 - Futex value has changed

The caller drops the held locks (hash bucket, mmap_sem) and retries the
operation. In case of the owner task exiting this can result in a live
lock.

As a preparatory step for seperating those cases, provide a distinct return
value (EBUSY) for the owner exiting case.

No functional change.

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/20191106224556.935606117@linutronix.de
This commit is contained in:
Thomas Gleixner 2019-11-06 22:55:45 +01:00
parent 3f186d9748
commit ac31c7ff86
1 changed files with 9 additions and 7 deletions
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16
kernel/futex.c
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@ -1182,11 +1182,11 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,
u32 uval2; u32 uval2;
/* /*
* If the futex exit state is not yet FUTEX_STATE_DEAD, wait * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
* for it to finish. * caller that the alleged owner is busy.
*/ */
if (tsk && tsk->futex_state != FUTEX_STATE_DEAD) if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
return -EAGAIN; return -EBUSY;
/* /*
* Reread the user space value to handle the following situation: * Reread the user space value to handle the following situation:
@ -2092,12 +2092,13 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
if (!ret) if (!ret)
goto retry; goto retry;
goto out; goto out;
case -EBUSY:
case -EAGAIN: case -EAGAIN:
/* /*
* Two reasons for this: * Two reasons for this:
* - Owner is exiting and we just wait for the * - EBUSY: Owner is exiting and we just wait for the
* exit to complete. * exit to complete.
* - The user space value changed. * - EAGAIN: The user space value changed.
*/ */
double_unlock_hb(hb1, hb2); double_unlock_hb(hb1, hb2);
hb_waiters_dec(hb2); hb_waiters_dec(hb2);
@ -2843,12 +2844,13 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
goto out_unlock_put_key; goto out_unlock_put_key;
case -EFAULT: case -EFAULT:
goto uaddr_faulted; goto uaddr_faulted;
case -EBUSY:
case -EAGAIN: case -EAGAIN:
/* /*
* Two reasons for this: * Two reasons for this:
* - Task is exiting and we just wait for the * - EBUSY: Task is exiting and we just wait for the
* exit to complete. * exit to complete.
* - The user space value changed. * - EAGAIN: The user space value changed.
*/ */
queue_unlock(hb); queue_unlock(hb);
put_futex_key(&q.key); put_futex_key(&q.key);