Re: [patch 0/5] lightweight robust futexes: -V1

[Johannes Stezenbach]

On Wed, Feb 15, 2006, Ingo Molnar wrote:
> "Robustness" is about dealing with crashes while holding a lock: if a 
> process exits prematurely while holding a pthread_mutex_t lock that is 
> also shared with some other process (e.g. yum segfaults while holding a 
> pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need 
> to be notified that the last owner of the lock exited in some irregular 
> way.
...
> At the heart of this new approach there is a per-thread private list of 
> robust locks that userspace is holding (maintained by glibc) - which 
> userspace list is registered with the kernel via a new syscall [this 
> registration happens at most once per thread lifetime]. At do_exit() 
> time, the kernel checks this user-space list: are there any robust futex 
> locks to be cleaned up?
...
> i've tested the new syscalls on x86 and x86_64, and have made sure the 
> parsing of the userspace list is robust [ ;-) ] even if the list is 
> deliberately corrupted.

I've no knowledge about all this, and maybe I didn't get your
description, so forgive me if I'm talking garbage.

Anyway: If a process can trash its robust futext list and then
die with a segfault, why are the futexes still robust?
In this case the kernel has no way to wake up waiters with
FUTEX_OWNER_DEAD, or does it?


Johannes
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