Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 9 Dec 2020 13:29:09 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t |
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On Wed, Dec 09, 2020 at 08:28:52PM +0900, Sergey Senozhatsky wrote: > On (20/12/09 12:00), Peter Zijlstra wrote: > > > So another potential re-entry path is > > > > > > atomic_foo() > > > spin_lock_irqsave(ATOMIC_HASH(v), flags) > > > printk() > > > prb() > > > atomic_foo() > > > spin_lock_irqsave(ATOMIC_HASH(v), flags) > > > > > > which can deadlock, in theory, if both atomics HASH to the same > > > key (same spin_lock). > > > > Yep, but see the 'mostly' in the 'they mostly work'. Given the > > limitiations of these architectures there's really only so much you can > > do. > > Right, agreed. > > Nevertheless TIL that lockless printk buffer is not always lockless. > Perhaps, people that work with those archs need to also know this.
Last time I broke them, they were aware they're 'special' and IIRC they're mostly just limping along on prayers.
> I haven't checked all the archs, but if, somehow, (IF) some of them > can panic the system with the atomic hash entries locked, then on > those archs new printk may not be able to flush-on-panic. Because > while printk iterates logbuf it may HASH to the atomic hash table > entry, that will never be unlocked. So there are some changes in > atomic/printk department on those archs.
Yeah, so I wouldn't put too much effort into thinking about it. Hopefully we can eventually delete these architectures and really forget they exist.
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