Hi, On 11/30/2022 10:47 AM, 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. Use the computed hash will be better than hash bucket address, because the hash buckets are allocated sequentially. > > * 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; The only purpose of trylock here is to make lockdep happy and it may lead to unnecessary -EBUSY error for htab operations in NMI context. I still prefer add a virtual lock-class for map_locked to fix the lockdep warning. So could you use separated patches to fix the potential dead-lock and the lockdep warning ? It will be better you can also add a bpf selftests for deadlock problem as said before. Thanks, Tao > } 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 >>> . >
