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* [PATCH] mips: select CONFIG_WEAK_REORDERING_BEYOND_LLSC from CONFIG_EYEQ
@ 2026-08-28 14:07 Théo Lebrun
  2026-08-28 15:56 ` Théo Lebrun
  0 siblings, 1 reply; 3+ messages in thread
From: Théo Lebrun @ 2026-08-28 14:07 UTC (permalink / raw)
  To: Thomas Bogendoerfer, Alexei Starovoitov, Daniel Borkmann,
	David S. Miller, Jakub Kicinski, Jesper Dangaard Brouer,
	John Fastabend, Stanislav Fomichev, Jiaxun Yang, Gregory CLEMENT
  Cc: linux-mips, linux-kernel, Vladimir Kondratiev, Benoît Monin,
	Thomas Petazzoni, stable, Théo Lebrun

On I6500 CPU cores, lld and scd give no ordering guarantees (same as all
other instructions). To respect the assumption that arch_cmpxchg() is
fully ordered, we must inject sync instructions above and below our
lld/scd loops using the already in place WEAK_REORDERING_BEYOND_LLSC
infrastructure.

Otherwise, bad things can happen:

[   34.054496] CPU 3 Unable to handle kernel paging request at virtual address 0000000000000000, epc == a80000080838e01c, ra == a80000080838dfc4
[   34.054559] Oops[#1]:
[   34.069561] CPU: 3 UID: 0 PID: 170 Comm: pipe_race Not tainted 7.2.0-rc6-01553-gb73c35220968-dirty #103 VOLUNTARY
[   34.079932] Hardware name: Mobile EyeQ5 MP5 Evaluation board
[   34.085592] $ 0   : 0000000000000000 0000000000000001 0000000000000000 0000000000000000
[   34.093616] $ 4   : a800000808ee2618 000000000b7a879d 0000000000001000 0000000000000000
[   34.101638] $ 8   : 0000000000e3f2c9 0000000000000000 a800000808a2a9f8 0000000000000000
[   34.109660] $12   : a8000008139ffcd8 ffffffff84080018 a80000080837fae0 7878787878787878
[   34.117682] $16   : a800000807e82940 0000000000001000 0000000000000000 0000000000000000
[   34.125704] $20   : a800000802920e00 a8000008139ffdf8 a800000802649400 0000000000e3f2c9
[   34.133726] $24   : 0000000000000006 00000001200406e0
[   34.141783] $28   : a8000008139fc000 a8000008139ffd10 0000000000e3f2c8 a80000080838dfc4
[   34.149837] epc   : a80000080838e01c anon_pipe_read+0xd4/0x428
[   34.155697] ra    : a80000080838dfc4 anon_pipe_read+0x7c/0x428
[   34.161549] Status: 140000e3  KX SX UX KERNEL EXL IE
[   34.166551] Cause : 40800408 (ExcCode 02)
[   34.170574] BadVA : 0000000000000000
[   34.174161] PrId  : 0001b028 (MIPS I6500)
[   34.178183] Process pipe_race (pid: 170, threadinfo=000000005ca35720, task=00000000e1013890, tls=000000014ebbb780)
[   34.188568] Stack : a800000802649400 0000000000000000 0000000000000000 a8000008139ffdd0
[   34.196623]	 0000000000000fba a800000808ee0000 0000000000000001 a8000008130c3e80
[   34.204676]	 a8000008080d1280 a8000008139ffd58 a8000008139ffd58 1dbd2b22ea1dd500
[   34.212729]	 a800000802649400 a800000808ee0000 ffffffffffffffea 0000000000000001
[   34.220783]	 0000000000001000 0000000000000000 00000001200ae518 ffffffffffffffff
[   34.228836]	 000000fffbe0e530 a80000080837edf4 000000fffbe0e530 0000000000000000
[   34.236890]	 0000000000000000 0000000000000000 000000014ebb55a0 0000000000001000
[   34.244943]	 0000000000000001 a800000802649400 0000000000000000 0000000000000000
[   34.252996]	 0000000000000000 0000400400000000 0000000000000000 1dbd2b22ea1dd500
[   34.261049]	 00000000140000e3 a800000802649400 a800000802649400 a800000808ee0000
[   34.269103]	 ...
[   34.271568] Call Trace:
[   34.274026] [<a80000080838e01c>] anon_pipe_read+0xd4/0x428
[   34.279533] [<a80000080837edf4>] vfs_read+0x25c/0x318
[   34.284607] [<a80000080837faac>] ksys_read+0x104/0x138
[   34.289763] [<a80000080802b9cc>] syscall_common+0x44/0x68
[   34.295187]
[   34.296689] Code: f84000cf  02209825  de020010 <dc420000> d8400004  02002825  0040f809  02802025  f84000c3
[   34.306504]
[   34.308099] ---[ end trace 0000000000000000 ]---

My initial reproducer was the xdp-tools test suite. A standalone
reproducer would be an lld/scd loop that, when the read is reordered by
the CPU, triggers a fault. We can achieve this from userspace by
stressing an anonymous pipe, which uses a mutex. Program used:

	// SPDX-License-Identifier: GPL-2.0
	// pipe_race.c - reproducer for MIPS LL/SC reordering vs fs/pipe.c
	//
	// Two userspace processes on an anonymous pipe:
	//   parent  = writer: tight write() loop
	//   child   = reader: tight read() loop

	#define _GNU_SOURCE
	#include <assert.h>
	#include <errno.h>
	#include <sched.h>
	#include <signal.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/types.h>
	#include <sys/wait.h>
	#include <time.h>
	#include <unistd.h>

	int main(void)
	{
		long wrsize = 70;	// bytes per write()
		long rdsize = 4096;	// bytes per read()
		int pfd[2];
		char *buf;
		pid_t pid;
		int ret;

		ret = pipe(pfd);
		assert(!ret);

		pid = fork();
		assert(pid >= 0);

		if (pid == 0) {			/* reader */
			close(pfd[1]);
			buf = malloc(rdsize);
			assert(buf);
			for (;;) {
				ssize_t n = read(pfd[0], buf, rdsize);

				if (n < 0 && errno == EINTR)
					continue;
				if (n <= 0)
					_exit(n < 0 ? 1 : 0);
			}
		}

		close(pfd[0]);			/* writer */
		buf = malloc(wrsize);
		assert(buf);
		memset(buf, 'x', wrsize);

		for (;;) {
			ssize_t n = write(pfd[1], buf, wrsize);

			if (n < 0 && errno == EINTR)
				continue;
			if (n != wrsize)
				break;
		}

		kill(pid, SIGKILL);
		wait(NULL);
		return 0;
	}

Fixes: fbe0fae601b7 ("MIPS: mobileye: Add EyeQ6H support")
Cc: stable@vger.kernel.org
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 arch/mips/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index e2eb9627bd14..5713a7e325dc 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -661,6 +661,7 @@ config EYEQ
 	select USB_UHCI_BIG_ENDIAN_MMIO if CPU_BIG_ENDIAN
 	select USE_OF
 	select HOTPLUG_PARALLEL if HOTPLUG_CPU
+	select WEAK_REORDERING_BEYOND_LLSC
 	help
 	  Select this to build a kernel supporting EyeQ SoC from Mobileye.
 

---
base-commit: 1b78070aaef63512688aebfbc82365ef9d6660f1
change-id: 20260828-eyeq-reordering-llsc-6c61298b2e47

Best regards,
--  
Théo Lebrun <theo.lebrun@bootlin.com>


^ permalink raw reply	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-08-30  3:05 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2026-08-28 14:07 [PATCH] mips: select CONFIG_WEAK_REORDERING_BEYOND_LLSC from CONFIG_EYEQ Théo Lebrun
2026-08-28 15:56 ` Théo Lebrun
2026-08-30  3:05   ` Jiaxun Yang

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