From: Florian Weimer <fweimer@redhat.com>
To: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: libc-alpha@sourceware.org
Subject: Re: [PATCH 3/3] Refactor atfork handlers
Date: Fri, 23 Feb 2018 11:41:15 +0100 [thread overview]
Message-ID: <6e23cc90-c734-3261-f89d-de9b83bb0a6b@redhat.com> (raw)
In-Reply-To: <6d6bc007-7418-4667-bf2f-0ba2256cdbec@linaro.org>
On 02/20/2018 03:23 PM, Adhemerval Zanella wrote:
> Aside of the two scenarios (callbacks issuing fork/pthread_atfork), the only
> other scenario I see which might trigger a deadlock in this case is a signal
> handler issuing fork/pthread_atfork.
>
> Former is BZ#4737 and my understanding is this should be a EWONTFIX due
> indication future POSIX specification to interpret fork as async-signal-unsafe
> (comment #19 and I am not sure if fork could be made async-signal-safe with
> ticket locks as Rich stated in comment #21).
>
> Regarding later I think pthread_atfork is inherent async-signal-unsafe due
> it might return ENOMEM indicating it might allocate memory and our malloc
> is also async-signal-unsafe.
>
> Am I missing a scenario you might be considering?
I looked at the acquired locks during fork, and you are right, the
corner cases where a deadlock can happen in the upstream sources are
quite obscure. However, we do not currently acquire any ld.so locks,
and I think I've seen patches which change that (because upstream is
buggy and crash in the new child process). If any ld.so locks are
acquired around fork, then we have a lock ordering conflict in case an
ELF constructor calls pthread_register_atfork (which is an extremely
natural thing to do), like this:
Fork:
pthread_register_atfork lock
rtld load lock
dlopen:
rtld load lock
calling ELF constructors, and then:
pthread_register_atfork lock
The older lock-free code avoids this. You could do the same even with
locks if you created a copy of the handler list on the heap.
Thanks,
Florian
next prev parent reply other threads:[~2018-02-23 10:39 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-07 13:09 [PATCH 1/3] Refactor Linux ARCH_FORK implementation Adhemerval Zanella
2018-02-07 13:09 ` [PATCH 2/3] dynarray: Implement remove function Adhemerval Zanella
2018-02-07 14:48 ` Alexander Monakov
2018-02-07 16:06 ` Adhemerval Zanella
2018-02-07 13:09 ` [PATCH 3/3] Refactor atfork handlers Adhemerval Zanella
2018-02-07 15:07 ` Florian Weimer
2018-02-07 17:16 ` Adhemerval Zanella
2018-02-08 8:32 ` Florian Weimer
2018-02-08 12:50 ` Adhemerval Zanella
2018-02-20 11:29 ` Florian Weimer
2018-02-20 13:00 ` Adhemerval Zanella
2018-02-20 13:05 ` Florian Weimer
2018-02-20 13:27 ` Adhemerval Zanella
2018-02-20 13:42 ` Florian Weimer
2018-02-20 13:48 ` Adhemerval Zanella
2018-02-20 13:58 ` Florian Weimer
2018-02-20 14:23 ` Adhemerval Zanella
2018-02-23 10:41 ` Florian Weimer [this message]
2018-02-23 12:10 ` Adhemerval Zanella
2018-02-27 8:25 ` Florian Weimer
2018-03-07 16:51 ` [PATCH 1/3] Refactor Linux ARCH_FORK implementation Adhemerval Zanella
2018-03-08 12:05 ` Florian Weimer
2018-03-08 12:58 ` Adhemerval Zanella
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