Date: Mon, 30 Jan 2017 12:53:45 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Justin Hibbits <jhibbits@FreeBSD.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r312973 - head/sys/powerpc/include Message-ID: <20170130105345.GL3018@kib.kiev.ua> In-Reply-To: <201701300215.v0U2Fsl0006455@repo.freebsd.org> References: <201701300215.v0U2Fsl0006455@repo.freebsd.org>
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> +#ifdef __GNUCLIKE_ASM > + __asm __volatile ( > + "1:\tlwarx %0, 0, %3\n\t" /* load old value */ > + "cmplw %4, %0\n\t" /* compare */ > + "bne 2f\n\t" /* exit if not equal */ > + "stwcx. %5, 0, %3\n\t" /* attempt to store */ > + "bne- 1b\n\t" /* spin if failed */ > + "li %0, 1\n\t" /* success - retval = 1 */ > + "b 3f\n\t" /* we've succeeded */ > + "2:\n\t" > + "stwcx. %0, 0, %3\n\t" /* clear reservation (74xx) */ > + "stwx %0, 0, %7\n\t" > + "li %0, 0\n\t" /* failure - retval = 0 */ > + "3:\n\t" > + : "=&r" (ret), "=m" (*p), "=m" (*cmpval) > + : "r" (p), "r" (*cmpval), "r" (newval), "m" (*p), "r"(cmpval) > + : "cr0", "memory"); > +#endif It seems that in case of failed conditional store, the code retries. Note that this is not incorrect but also not a desirable behaviour with fcmpset: the function should return error and leave the retry to the caller. There is no point in having embedded loop.
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