linux-kernel - Re: [PATCH] ia64: populate the cmpxchg header with appropriate code

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Message-ID: <CA+8MBb+LEqcyGQ+qfmnqzmPEtHeOf=XoR4rtpW9Rz74zU80Mkw@mail.gmail.com>
Date:	Thu, 12 Apr 2012 14:52:48 -0700
From:	Tony Luck <tony.luck@...el.com>
To:	Émeric Maschino <emeric.maschino@...il.com>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	Fenghua Yu <fenghua.yu@...el.com>,
	David Howells <dhowells@...hat.com>,
	Michel Lespinasse <walken@...gle.com>
Subject: Re: [PATCH] ia64: populate the cmpxchg header with appropriate code

I think that problem with the futex change is not with the ia64 inline
parts ... but
with the code that decides whether to enable them.  Try this
(white-space damaged,
but not intended for prime-time) patch:

diff --git a/kernel/futex.c b/kernel/futex.c
index e2b0fb9..9a00bf8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2722,7 +2722,9 @@ static int __init futex_init(void)
         * implementation, the non-functional ones will return
         * -ENOSYS.
         */
+#ifndef CONFIG_IA64
        if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
+#endif
                futex_cmpxchg_enabled = 1;

        for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {

IA64 doesn't return a -EFAULT when we poke at the supposedly invalid user
address of "NULL" because we map a special page to catch prefetch references
so that loops like "for (p = something; p != NULL; p = p->next) { use
p->field }" don't
do unpleasant thinks when the pre-fetcher dereferences the NULL "p".

This patch enables the futexttest suite to run ... perhaps it will help
with the other problems too?

-Tony
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