Re: [patch 3/4] pi-futex: Fix exit races and locking problems

From: Ingo Molnar
Date: Fri Jun 08 2007 - 03:17:47 EST

* Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> From: Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>
> 1. New entries can be added to tsk->pi_state_list after task completed
> exit_pi_state_list(). The result is memory leakage and deadlocks.
> 2. handle_mm_fault() is called under spinlock. The result is obvious.
> 3. results in self-inflicted deadlock inside glibc.
> Sometimes futex_lock_pi returns -ESRCH, when it is not expected
> and glibc enters to for(;;) sleep() to simulate deadlock. This problem
> is quite obvious and I think the patch is right. Though it looks like
> each "if" in futex_lock_pi() got some stupid special case "else if". :-)
> 4. sometimes futex_lock_pi() returns -EDEADLK,
> when nobody has the lock. The reason is also obvious (see comment
> in the patch), but correct fix is far beyond my comprehension.
> I guess someone already saw this, the chunk:
> if (rt_mutex_trylock(&q.pi_state->pi_mutex))
> ret = 0;
> is obviously from the same opera. But it does not work, because the
> rtmutex is really taken at this point: wake_futex_pi() of previous
> owner reassigned it to us. My fix works. But it looks very stupid.
> I would think about removal of shift of ownership in wake_futex_pi()
> and making all the work in context of process taking lock.
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Fix 1) Avoid the tasklist lock variant of the exit race fix by adding
> an additional state transition to the exit code.
> This fixes also the issue, when a task with recursive segfaults
> is not able to release the futexes.
> Fix 2) Cleanup the lookup_pi_state() failure path and solve the -ESRCH
> problem finally.
> Fix 3) Solve the fixup_pi_state_owner() problem which needs to do the fixup
> in the lock protected section by using the in_atomic userspace access
> functions.
> This removes also the ugly lock drop / unqueue inside of fixup_pi_state()
> Fix 4) Fix a stale lock in the error path of futex_wake_pi()
> Added some error checks for verification.
> The -EDEADLK problem is solved by the rtmutex fixups.
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

i've read the patch and it looks all good - but obviously testing will
tell us what's up, this stuff is ... complex.

Acked-by: Ingo Molnar <mingo@xxxxxxx>

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