LoongArch: Fix calling smp_processor_id() in preemptible code#11259
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Fix the warning:
BUG: using smp_processor_id() in preemptible [00000000] code: sys
temd/1
caller is larch_insn_text_copy+0x40/0xf0
Simply changing it to raw_smp_processor_id() is not enough: if preempt
and CPU hotplug happens after raw_smp_processor_id() but before
stop_machine(), the CPU where raw_smp_processor_id() has run may be
offline when stop_machine() and no CPU will run copy_to_kernel_nofault()
in text_copy_cb(). Thus guard the larch_insn_text_copy() calls with
cpus_read_lock() and change stop_machine() to stop_machine_cpuslocked()
to prevent this.
I've considered moving the locks inside larch_insn_text_copy() but
doing so seems not an easy hack. In bpf_arch_text_poke() obviously the
memcpy() call must be guarded by text_mutex, so we have to leave the
acquire of text_mutex out of larch_insn_text_copy. But in the entire
kernel the acquire of mutexes is always after cpus_read_lock(), so we
cannot put cpus_read_lock() into larch_insn_text_copy() while leaving
the text_mutex acquire out (or we risk a deadlock due to inconsistent
lock acquire order). So let's fix the bug first and leave the
posssible refactor as future work.
Fixes: 9fbd18c ("LoongArch: BPF: Add dynamic code modification support")
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
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Pull request for series with
subject: LoongArch: Fix calling smp_processor_id() in preemptible code
version: 3
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1060482