Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage_[get|delete]

[Song Liu]

> On Feb 23, 2021, at 3:06 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Mon, Feb 22, 2021 at 05:20:10PM -0800, Song Liu wrote:
>> BPF helpers bpf_task_storage_[get|delete] could hold two locks:
>> bpf_local_storage_map_bucket->lock and bpf_local_storage->lock. Calling
>> these helpers from fentry/fexit programs on functions in bpf_*_storage.c
>> may cause deadlock on either locks.
>> 
>> Prevent such deadlock with a per cpu counter, bpf_task_storage_busy, which
>> is similar to bpf_prog_active. We need this counter to be global, because
> 
> So bpf_prog_active is one of the biggest turds around, and now you're
> making it worse ?!

bpf_prog_active is a distraction here. We are trying to enable task local 
storage for fentry/fext programs, which do not use bpf_prog_active.

bpf_task_storage_busy counter is introduced to protect against a specific 
pattern of deadlocks (attaching fentry/fexit on bpf_task_storage_[get|delete] 
helpers, then let the programs call these two helpers again). 

Thanks,
Song