LoongArch: Remove arch-specific __smp_mb__{before,after}_atomic()

When running BPF selftest "test_progs -t timer_lockup" on a LoongArch
machine, the kernel intermittently experiences soft lockups and panics
in hrtimer_cancel().

The root cause is that the bpf_timer_cancel() implementation relies on
smp_mb__after_atomic() to provide a full data barrier after a relaxed
atomic_inc() to ensure the proper store-load ordering before performing
lockless dependency checks on weakly-ordered architectures, such as on
LoongArch, but the current definition of smp_mb__after_atomic() is just
a plain compiler barrier.

This allows a subsequent read to bypass the prior write due to store-
load reordering, enabling concurrent CPUs to simultaneously bypass the
software deadlock detection, enter hrtimer_cancel(), and then trigger a
severe ABBA deadlock in the hrtimer core.

Moreover, there are hundreds of smp_mb__after_atomic() call sites across
the core kernel (kernel/, drivers/, mm/, net/, fs/) that currently lack
necessary full data barriers on LoongArch, which have potential risks.

To comply with the LKMM specification, fix this by removing the arch-
specific definition of __smp_mb__after_atomic(), so that this macro can
automatically fall back to the generic definition in asm-generic that
maps to __smp_mb() and emits the required hardware DBAR instruction.

While at it, do the same thing for __smp_mb__before_atomic().

With this patch the BPF timer_lockup selftest was stressed for millions
consecutive loops on a physical LoongArch machine, without encountering
any further lockups.

Cc: stable@vger.kernel.org # 6.7+
Fixes: affef66b65889a0e ("LoongArch: Relax memory ordering for atomic operations")
Closes: https://lore.kernel.org/loongarch/CAK3+h2xOSEZUHhou7N2cRL-aGrZCNSm45g+P7thObMe+fpgYCA@mail.gmail.com/
Reported-by: Vincent Li <vincent.mc.li@gmail.com>
Reviewed-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

0 files changed, 0 insertions, 0 deletions