linux-kernel - RE: Is gcc thread-unsafe?

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Message-ID: <MDEHLPKNGKAHNMBLJOLKMEEDHMAC.davids@webmaster.com>
Date:	Thu, 25 Oct 2007 14:42:56 -0700
From:	"David Schwartz" <davids@...master.com>
To:	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: RE: Is gcc thread-unsafe?


	I asked a collection of knowledgeable people I know about the issue. The
consensus is that the optimization is not permitted in POSIX code but that
it is permitted in pure C code. The basic argument goes like this:

	To make POSIX-compliant code even possible, surely optimizations that add
writes to variables must be prohibited. That is -- if POSIX prohibits
writing to a variable in certain cases only the programmer can detect, then
a POSIX-compliant compiler cannot write to a variable except where
explicitly told to do so. Any optimization that *adds* a write to a variable
that would not otherwise occur *must* be prohibited.

	Otherwise, it is literally impossible to comply with the POSIX requirement
that concurrent modifications and reads to shared variables take place while
holding a mutex.

	The simplest solution is simply to ditch the optimization. If it really
isn't even an optimization, then that's an easy way out.

	DS


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