Re: [PATCH] mips: select CONFIG_WEAK_REORDERING_BEYOND_LLSC from CONFIG_EYEQ

["Théo Lebrun" <theo.lebrun@bootlin.com>]

First off, sorry to net maintainers for being in Cc of this patch.
get_maintainer.pl added you and the MLs because the commit message
containing "xdp". I deleted the MLs in Cc but forgot the individuals.

On Fri Aug 28, 2026 at 4:07 PM CEST, Théo Lebrun wrote:
> 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:

Simpler reproducer than the C program given initially. It takes a few
seconds to trigger.

   dd if=/dev/zero ibs=20M obs=70 | dd bs=4096 > /dev/null

Goal is to spam write(2) on one side and read(2) on the other.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com