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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Yury Norov <ynorov@caviumnetworks.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Fengguang Wu <fengguang.wu@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.16 227/279] lib/find_bit_benchmark.c: avoid soft lockup in test_find_first_bit()
Date:   Mon, 18 Jun 2018 10:13:32 +0200
Message-Id: <20180618080618.216781441@linuxfoundation.org>
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4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yury Norov <ynorov@caviumnetworks.com>

[ Upstream commit 4ba281d5bd9907355e6b79fb72049c9ed50cc670 ]

test_find_first_bit() is intentionally sub-optimal, and may cause soft
lockup due to long time of run on some systems.  So decrease length of
bitmap to traverse to avoid lockup.

With the change below, time of test execution doesn't exceed 0.2 seconds
on my testing system.

Link: http://lkml.kernel.org/r/20180420171949.15710-1-ynorov@caviumnetworks.com
Fixes: 4441fca0a27f5 ("lib: test module for find_*_bit() functions")
Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 lib/find_bit_benchmark.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/lib/find_bit_benchmark.c
+++ b/lib/find_bit_benchmark.c
@@ -132,7 +132,12 @@ static int __init find_bit_test(void)
 	test_find_next_bit(bitmap, BITMAP_LEN);
 	test_find_next_zero_bit(bitmap, BITMAP_LEN);
 	test_find_last_bit(bitmap, BITMAP_LEN);
-	test_find_first_bit(bitmap, BITMAP_LEN);
+
+	/*
+	 * test_find_first_bit() may take some time, so
+	 * traverse only part of bitmap to avoid soft lockup.
+	 */
+	test_find_first_bit(bitmap, BITMAP_LEN / 10);
 	test_find_next_and_bit(bitmap, bitmap2, BITMAP_LEN);
 
 	pr_err("\nStart testing find_bit() with sparse bitmap\n");