Messages in this thread | | | Date | Fri, 8 Jun 2007 09:16:11 +0200 | From | Ingo Molnar <> | Subject | Re: [patch 3/4] pi-futex: Fix exit races and locking problems |
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* Thomas Gleixner <tglx@linutronix.de> wrote:
> From: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru> > > 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@linutronix.de> > > 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@linutronix.de>
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@elte.hu>
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