Re: [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code

[Thomas Gleixner]

Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes:
> On 14-Feb-2020 02:39:31 PM, Thomas Gleixner wrote:
>> Replace the preempt_disable/enable() pairs with migrate_disable/enable()
>> pairs to prepare BPF to work on PREEMPT_RT enabled kernels. On a non-RT
>> kernel this maps to preempt_disable/enable(), i.e. no functional change.

...

> Having all those events randomly and silently discarded might be quite
> unexpected from a user standpoint. This turns the deadlock prevention
> mechanism into a random tracepoint-dropping facility, which is
> unsettling.

Well, it randomly drops events which might be unrelated to the syscall
target today already, this will just drop some more. Shrug.

> One alternative approach we could consider to solve this is to make
> this deadlock prevention nesting counter per-thread rather than
> per-cpu.

That should work both on !RT and RT.

> Also, I don't think using __this_cpu_inc() without preempt-disable or
> irq off is safe. You'll probably want to move to this_cpu_inc/dec
> instead, which can be heavier on some architectures.

Good catch.

Thanks,

        tglx