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Date: Tue, 04 Sep 2012 15:47:55 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/6] libqblock APIs
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To: Eric Blake <eblake@redhat.com>
Cc: kwolf@redhat.com, stefanha@gmail.com, aliguori@us.ibm.com, Wenchao Xia <xiawenc@linux.vnet.ibm.com>, qemu-devel@nongnu.org

Il 04/09/2012 13:35, Eric Blake ha scritto:
>> >   No, it should be only called once, any other thread should not call
>> > it again, will document it. About the multiple thread user case, qemu
>> > block layer can't support that now, will fix that later.
> What a shame.  That makes libraries much harder to use.  It is much
> nicer to design a library where the initialization is idempotent and
> thread-safe, to be called from multiple threads.  Consider:
> 
> app links against liba and libb;
> liba links against libqb
> libb links against libqb
> 
> How am I supposed to write liba and libb to guarantee only one single
> race-free call to libqblock_init, unless libqblock_init() is idempotent?

I agree, libqblock_init should use pthread_once (or we can add QemuOnce
to qemu-thread-*.c).

Paolo