On Wed, Nov 30, 2022 at 11:07 AM Waiman Long <longman@xxxxxxxxxx> wrote: > > On 11/29/22 21:47, Tonghao Zhang wrote: > > On Wed, Nov 30, 2022 at 9:50 AM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: > >> Hi Hao, > >> > >> On 11/30/2022 3:36 AM, Hao Luo wrote: > >>> On Tue, Nov 29, 2022 at 9:32 AM Boqun Feng <boqun.feng@xxxxxxxxx> wrote: > >>>> Just to be clear, I meant to refactor htab_lock_bucket() into a try > >>>> lock pattern. Also after a second thought, the below suggestion doesn't > >>>> work. I think the proper way is to make htab_lock_bucket() as a > >>>> raw_spin_trylock_irqsave(). > >>>> > >>>> Regards, > >>>> Boqun > >>>> > >>> The potential deadlock happens when the lock is contended from the > >>> same cpu. When the lock is contended from a remote cpu, we would like > >>> the remote cpu to spin and wait, instead of giving up immediately. As > >>> this gives better throughput. So replacing the current > >>> raw_spin_lock_irqsave() with trylock sacrifices this performance gain. > >>> > >>> I suspect the source of the problem is the 'hash' that we used in > >>> htab_lock_bucket(). The 'hash' is derived from the 'key', I wonder > >>> whether we should use a hash derived from 'bucket' rather than from > >>> 'key'. For example, from the memory address of the 'bucket'. Because, > >>> different keys may fall into the same bucket, but yield different > >>> hashes. If the same bucket can never have two different 'hashes' here, > >>> the map_locked check should behave as intended. Also because > >>> ->map_locked is per-cpu, execution flows from two different cpus can > >>> both pass. > >> The warning from lockdep is due to the reason the bucket lock A is used in a > >> no-NMI context firstly, then the same bucke lock is used a NMI context, so > > Yes, I tested lockdep too, we can't use the lock in NMI(but only > > try_lock work fine) context if we use them no-NMI context. otherwise > > the lockdep prints the warning. > > * for the dead-lock case: we can use the > > 1. hash & min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1) > > 2. or hash bucket address. > > > > * for lockdep warning, we should use in_nmi check with map_locked. > > > > BTW, the patch doesn't work, so we can remove the lock_key > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c50eb518e262fa06bd334e6eec172eaf5d7a5bd9 > > > > static inline int htab_lock_bucket(const struct bpf_htab *htab, > > struct bucket *b, u32 hash, > > unsigned long *pflags) > > { > > unsigned long flags; > > > > hash = hash & min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1); > > > > preempt_disable(); > > if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) { > > __this_cpu_dec(*(htab->map_locked[hash])); > > preempt_enable(); > > return -EBUSY; > > } > > > > if (in_nmi()) { > > if (!raw_spin_trylock_irqsave(&b->raw_lock, flags)) > > return -EBUSY; > That is not right. You have to do the same step as above by decrementing > the percpu count and enable preemption. So you may want to put all these > busy_out steps after the return 0 and use "goto busy_out;" to jump there. Yes, thanks Waiman, I should add the busy_out label. > > } else { > > raw_spin_lock_irqsave(&b->raw_lock, flags); > > } > > > > *pflags = flags; > > return 0; > > } > > BTW, with that change, I believe you can actually remove all the percpu > map_locked count code. there are some case, for example, we run the bpf_prog A B in task context on the same cpu. bpf_prog A update map X htab_lock_bucket raw_spin_lock_irqsave() lookup_elem_raw() // bpf prog B is attached on lookup_elem_raw() bpf prog B update map X again and update the element htab_lock_bucket() // dead-lock raw_spinlock_irqsave() > Cheers, > Longman > -- Best regards, Tonghao
