On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote: > On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote: > > On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote: > > > Hi, > > > > > > Changelog-v2 > > > ============ > > > > > > - Standardize on seqcount_LOCKNAME_t as the canonical reference for > > > sequence counters with associated locks, instead of v1 > > > seqcount_LOCKTYPE_t. > > > > > > - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t > > > property accessors. > > > > > > - Touch-up the lock-unlock rationale for more clarity. Enforce writer > > > non-preemitiblity using "__seq_enforce_writer_non_preemptibility()". > > > > > > Cover letter (v1) > > > ================= > > > > > > https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@xxxxxxxxxxxxx > > > > > > Preemption must be disabled before entering a sequence counter write > > > side critical section. Otherwise the read side section can preempt the > > > write side section and spin for the entire scheduler tick. If that > > > reader belongs to a real-time scheduling class, it can spin forever and > > > the kernel will livelock. > > > > > > Disabling preemption cannot be done for PREEMPT_RT though: it can lead > > > to higher latencies, and the write side sections will not be able to > > > acquire locks which become sleeping locks (e.g. spinlock_t). > > > > > > To remain preemptible, while avoiding a possible livelock caused by the > > > reader preempting the writer, use a different technique: let the reader > > > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the > > > case, acquire then release the associated LOCKNAME writer serialization > > > lock. This will allow any possibly-preempted writer to make progress > > > until the end of its writer serialization lock critical section. > > > > > > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with > > > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t. > > > > Reverting this patchset [1] from today's linux-next fixed a splat below. The > > splat looks like a false positive anyway because the existing locking dependency > > chains from the task #1 here: > > > > &s->seqcount#2 ---> pidmap_lock > > > > [ 528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}: > > [ 528.078078][ T7867] lock_acquire+0x10c/0x560 > > [ 528.078089][ T7867] _raw_spin_lock_irqsave+0x64/0xb0 > > [ 528.078108][ T7867] free_pid+0x5c/0x160 > > free_pid at kernel/pid.c:131 > > [ 528.078127][ T7867] release_task.part.40+0x59c/0x7f0 > > __unhash_process at kernel/exit.c:76 > > (inlined by) __exit_signal at kernel/exit.c:147 > > (inlined by) release_task at kernel/exit.c:198 > > [ 528.078145][ T7867] do_exit+0x77c/0xda0 > > exit_notify at kernel/exit.c:679 > > (inlined by) do_exit at kernel/exit.c:826 > > [ 528.078163][ T7867] kthread+0x148/0x1d0 > > [ 528.078182][ T7867] ret_from_kernel_thread+0x5c/0x80 > > > > It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2 > > in read_mems_allowed_begin() is read_seqcount_begin(¤t->mems_allowed_seq), > > so there should be no deadlock? > > > > I think this happened because seqcount_##lockname##_init() is defined at > function rather than macro, so when the seqcount_init() gets expand in > that function, the lock_class_key of seqcount will be a static variable > of seqcount_##lockname##_init() function, as a result, all > seqcount_##lockname##_t in the same compile unit (in this case it's > kernel/fork.c) share the same lock class key, and lockdep thought they > are the same lock ;-) > Don't know how to fix this properly, but below is an ugly attemption, only build test, just food for thoughts. Regards, Boqun --------------->8 diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index f73c7eb68f27..938a5053def3 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -84,14 +84,18 @@ static inline void __seqcount_init(seqcount_t *s, const char *name, # define SEQCOUNT_DEP_MAP_INIT(lockname) \ .dep_map = { .name = #lockname } +# define MSIOCU 8 /* MAX SEQCOUNT IN ON COMPILE UNIT */ /** * seqcount_init() - runtime initializer for seqcount_t * @s: Pointer to the seqcount_t instance */ # define seqcount_init(s) \ do { \ - static struct lock_class_key __key; \ - __seqcount_init((s), #s, &__key); \ + static struct lock_class_key __key[MSIOCU]; \ + static int idx = 0; \ + \ + BUG_ON(idx >= MSIOCU); \ + __seqcount_init((s), #s, &__key[idx++]); \ } while (0) static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
