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; } else { raw_spin_lock_irqsave(&b->raw_lock, flags); } *pflags = flags; return 0; } > lockdep deduces that may be a dead-lock. I have already tried to use the same > map_locked for keys with the same bucket, the dead-lock is gone, but still got > lockdep warning. > > > > Hao > > . > -- Best regards, Tonghao
