From: Roland Dreier <rdreier@cisco.com>
To: "Matt Turner" <mattst88@gmail.com>
Cc: linux-kernel@vger.kernel.org, "Ivan Kokshaysky" <ink@jurassic.park.msu.ru>,
        rth@twiddle.net, "Jay Estabrook" <jay.estabrook@hp.com>
Subject: Re: questions about native alpha futex implementation
References: <b4198de60812171113r72d8d04ao67cde38c62185bcc@mail.gmail.com>
Date: Wed, 17 Dec 2008 11:26:47 -0800
In-Reply-To: <b4198de60812171113r72d8d04ao67cde38c62185bcc@mail.gmail.com>
	(Matt Turner's message of "Wed, 17 Dec 2008 14:13:15 -0500")
Message-ID: <adaprjqzjpk.fsf@cisco.com>
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 > Alpha uses a generic futex implementation, which causes some problems [1].

Summarizing this: the generic futex implementation leaves
futex_atomic_cmpxchg_inatomic() as a stub that just returns -ENOSYS, and
glibc robust futex code can't handle this return value, which leads to
glibc tests hanging.

 > I've read through the code, and it appears as if the implementation
 > could be done by using the ldq_l/stq_c instructions, relatively easy I
 > might add. I'm definitely interested in implementing this, but first...
 > 
 > I have only a few questions.
 > 
 > 1) What are the benefits of a native futex implementation, other than
 > fixing the glibc test failures?

Presumably native implementations can use optimized assembly, which
would be somewhat faster than a generic implementation in C.

 > 2) Is there a technical reason it hasn't been implemented on Alpha?

I don't know for sure, but I would guess it's just that no one has cared
enough about optimizing futexes on alpha.

A native implementation for alpha is probably an amusing project, but
also figuring out a way of implementing the missing operations for
generic futexes would be nice too (given that glibc uses them now).
Although a generic futex_atomic_cmpxchg_inatomic() that works for SMP
might be tricky.

 - R.
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