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Message-ID: <455063b99060bfd0d01aff1666ebb3892e001256.camel@gmail.com>
Date: Fri, 31 Jan 2025 10:31:46 +0100
From: Daniele Personal <d.dario76@...il.com>
To: Florian Weimer <fweimer@...hat.com>, Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: pthread_mutex_t shared between processes with different
pid namespaces
On Tue, 2025-01-28 at 19:24 +0100, Florian Weimer wrote:
> * Rich Felker:
>
> > Yes, the pid namespace boundary is your problem. Process-shared
> > mutexes only work on the same logical system with a unique set of
> > thread identifiers. If you're trying to share them across different
> > pid namespaces, the same pid/tid may refer to different
> > processes/threads in different ones, and it's not usable as a mutex
> > ownership identity.
>From what I see, the problem seems to happen only in case of contention
of the mutex.
int __pthread_mutex_timedlock(pthread_mutex_t *restrict m, const struct
timespec *restrict at)
{
if ((m->_m_type&15) == PTHREAD_MUTEX_NORMAL
&& !a_cas(&m->_m_lock, 0, EBUSY))
return 0;
int type = m->_m_type;
int r, t, priv = (type & 128) ^ 128;
r = __pthread_mutex_trylock(m);
if (r != EBUSY) return r;
IIUC, if it is not locked, the __pthread_mutex_timedlock will acquire
it and return 0 (don't understand if with the first check or with the
__pthread_mutex_trylock) and everything works.
If instead it is locked the problem arises only inside the container.
If it was a pthread_mutex_lock it waits forever, if it was a timed lock
it exits after the timeout and you can retry.
Is this correct?
>
> Is this required for implementing the unlock-if-not-owner error code
> on
> mutex unlock?
No, I don't see problems related to EOWNERDEAD.
>
> By the way, there is a proposal to teach the kernel to rewrite the
> ownership list of task exit:
>
> [PATCH v2 0/4] futex: Drop ROBUST_LIST_LIMIT
>
> <https://lore.kernel.org/linux-kernel/20250127202608.223864-1-andreal
> meid@...lia.com/>
>
> I'm worried about the compatibility impact.
>
> Thanks,
> Florian
>
Thanks,
Daniele.
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