linux-kernel - [PATCH] lib: dhry: Fix unstable smp_processor_id(

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Message-Id: <b0d29932bb24ad82cea7f821e295c898e9657be0.1678890070.git.geert+renesas@glider.be>
Date:   Wed, 15 Mar 2023 15:25:22 +0100
From:   Geert Uytterhoeven <geert+renesas@...der.be>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Tobias Klausmann <klausman@...warzvogel.de>
Subject: [PATCH] lib: dhry: Fix unstable smp_processor_id(_) usage

When running the in-kernel Dhrystone benchmark with
CONFIG_DEBUG_PREEMPT=y:

    BUG: using smp_processor_id() in preemptible [00000000] code: bash/938

Fix this by not using smp_processor_id() directly, but instead wrapping
the whole benchmark inside a get_cpu()/put_cpu() pair.  This makes sure
the whole benchmark is run on the same CPU core, and the reported values
are consistent.

Fixes: d5528cc16893f1f6 ("lib: add Dhrystone benchmark test")
Reported-by: Tobias Klausmann <klausman@...warzvogel.de>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217179
Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
---
 lib/dhry_run.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/dhry_run.c b/lib/dhry_run.c
index f9d33efa6d090604..f15ac666e9d38bd2 100644
--- a/lib/dhry_run.c
+++ b/lib/dhry_run.c
@@ -31,6 +31,7 @@ MODULE_PARM_DESC(iterations,
 
 static void dhry_benchmark(void)
 {
+	unsigned int cpu = get_cpu();
 	int i, n;
 
 	if (iterations > 0) {
@@ -45,9 +46,10 @@ static void dhry_benchmark(void)
 	}
 
 report:
+	put_cpu();
 	if (n >= 0)
-		pr_info("CPU%u: Dhrystones per Second: %d (%d DMIPS)\n",
-			smp_processor_id(), n, n / DHRY_VAX);
+		pr_info("CPU%u: Dhrystones per Second: %d (%d DMIPS)\n", cpu,
+			n, n / DHRY_VAX);
 	else if (n == -EAGAIN)
 		pr_err("Please increase the number of iterations\n");
 	else
-- 
2.34.1