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[PATCH] Fix __raw_read_trylock() to allow multiple readers

To: linux-mips@linux-mips.org, Ralf Baechle <ralf@linux-mips.org>
Subject: [PATCH] Fix __raw_read_trylock() to allow multiple readers
From: Dave Johnson <djohnson+linux-mips@sw.starentnetworks.com>
Date: Mon, 5 Mar 2007 20:50:27 -0500
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
Code is from linux-mips.org's git tree at 2.6.18 plus some patches
from 2.6.18.7.

I've run into a deadlock that occurs with the following config:

CONFIG_SMP
CONFIG_PREEMPT

An individual call to __raw_read_trylock() will just return failure if
there is already a reader.

Any code that tries to take a read lock by looping around
__raw_read_trylock() can deadlock if there is already a reader and the
lock in question has a mix of irq and non-irq readers.

This is the case in the _read_lock() from BUILD_LOCK_OPS() in
kernel/spinlock.c.

Perhaps someone can confirm this bug by a code inspection, but I
think the below patch will fix the issue.

Patch should apply ok to 2.6.18.7 (and a similar one to >= 2.6.19)

-- 
Dave Johnson
Starent Networks

========================  PATCH FOLLOWS  ========================

Fix __raw_read_trylock() to allow multiple readers.

A deadlock can occur for mixed irq and non-irq rwlock readers if
a 2nd reader attempts to take lock by looping around __raw_read_trylock().

Signed-off-by: Dave Johnson <djohnson+linux-mips@sw.starentnetworks.com>

--- old/include/asm-mips/spinlock.h     2007-03-05 20:34:51.000000000 -0500
+++ new/include/asm-mips/spinlock.h     2007-03-05 20:35:18.000000000 -0500
@@ -249,7 +249,7 @@
                "       .set    noreorder       # __raw_read_trylock    \n"
                "       li      %2, 0                                   \n"
                "1:     ll      %1, %3                                  \n"
-               "       bnez    %1, 2f                                  \n"
+               "       bltz    %1, 2f                                  \n"
                "        addu   %1, 1                                   \n"
                "       sc      %1, %0                                  \n"
                "       beqzl   %1, 1b                                  \n"
@@ -267,7 +267,7 @@
                "       .set    noreorder       # __raw_read_trylock    \n"
                "       li      %2, 0                                   \n"
                "1:     ll      %1, %3                                  \n"
-               "       bnez    %1, 2f                                  \n"
+               "       bltz    %1, 2f                                  \n"
                "        addu   %1, 1                                   \n"
                "       sc      %1, %0                                  \n"
                "       beqz    %1, 1b                                  \n"


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