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Groups > linux.kernel > #1522760
| From | Boqun Feng <boqun.feng@gmail.com> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [RFC][PATCH 7/7] kref: Implement using refcount_t |
| Date | 2016-11-15 15:20 +0100 |
| Message-ID | <sDMKC-621-1@gated-at.bofh.it> (permalink) |
| References | <sDtyh-1DM-5@gated-at.bofh.it> <sDtyi-1DM-23@gated-at.bofh.it> <sDLbQ-4VX-3@gated-at.bofh.it> <sDLET-5lG-57@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
[Multipart message — attachments visible in raw view] - view raw
On Tue, Nov 15, 2016 at 02:01:54PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 15, 2016 at 08:33:37PM +0800, Boqun Feng wrote:
> > Hi Peter,
> >
> > On Mon, Nov 14, 2016 at 06:39:53PM +0100, Peter Zijlstra wrote:
> > [...]
> > > +/*
> > > + * Similar to atomic_dec_and_test(), it will BUG on underflow and fail to
> > > + * decrement when saturated at UINT_MAX.
> > > + *
> > > + * Provides release memory ordering, such that prior loads and stores are done
> > > + * before a subsequent free.
> >
> > I'm not sure this is correct, the RELEASE semantics is for the STORE
> > part of cmpxchg, and semantically it will guarantee that memory
> > operations after cmpxchg won't be reordered upwards, for example, on
> > ARM64, the following code:
> >
> > WRITE_ONCE(x, 1)
> >
> > atomic_cmpxchg_release(&a, 1, 2);
> > r1 = ll(&a)
> > if (r1 == 1) {
> > sc_release(&a, 2);
> > }
> >
> > free()
> >
> > could be reordered as, I think:
> >
> > atomic_cmpxchg_release(&a, 1, 2);
> > r1 = ll(&a)
> > if (r1 == 1) {
> > free()
> > WRITE_ONCE(x, 1)
> > sc_release(&a, 2);
> > }
> >
> > Of course, we need to wait for Will to confirm about this. But if this
> > could happen, we'd better to use a smp_mb()+atomic_cmpxchg_relaxed()
> > here and for other refcount_dec_and_*().
>
> Can't happen I think because of the control dependency between
> dec_and_test() and free().
>
> That is, the cmpxchg_release() must complete to determine if it was
> successful or it needs a retry. The success, combined with the state of
> the variable will then determine if we call free().
>
The thing is that determination of the variable's state(i.e.
store_release() succeeds) and the actual writeback to memory are two
separate events. So yes, free() won't execute before store_release()
commits successfully, but there is no barrier here to order the memory
effects of store_release() and free().
See a similar example:
https://marc.info/?l=linux-s390&m=146604339321723&w=2
But as I said, we actually only need the pairing of orderings:
1) load part of cmpxchg -> free()
2) object accesses -> store part of cmpxchg
Ordering #1 can be achieved via control dependency as you pointed out
that free()s very much includes stores. And ordering #2 can be achieved
with RELEASE.
So the code is right, I just thought the comment may be misleading. The
reason we use cmpxchg_release() is just for achieving ordering #2, and
not to order "prior loads and stores" with "a subsequent free".
Am I missing some subtle orderings here?
Regards,
Boqun
> So I don't think we can get free() (which very much includes stores) to
> happen before the store-release.
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[RFC][PATCH 0/7] kref improvements Peter Zijlstra <peterz@infradead.org> - 2016-11-14 18:50 +0100
[RFC][PATCH 5/7] kref: Implement kref_put_lock() Peter Zijlstra <peterz@infradead.org> - 2016-11-14 18:50 +0100
Re: [RFC][PATCH 5/7] kref: Implement kref_put_lock() Kees Cook <keescook@chromium.org> - 2016-11-14 21:40 +0100
Re: [RFC][PATCH 5/7] kref: Implement kref_put_lock() Peter Zijlstra <peterz@infradead.org> - 2016-11-15 09:00 +0100
[RFC][PATCH 6/7] kref: Avoid more abuse Peter Zijlstra <peterz@infradead.org> - 2016-11-14 18:50 +0100
[RFC][PATCH 4/7] kref: Use kref_get_unless_zero() more Peter Zijlstra <peterz@infradead.org> - 2016-11-14 18:50 +0100
[RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-14 18:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Ingo Molnar <mingo@kernel.org> - 2016-11-15 09:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-15 10:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Ingo Molnar <mingo@kernel.org> - 2016-11-15 11:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-15 11:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Ingo Molnar <mingo@kernel.org> - 2016-11-15 14:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Kees Cook <keescook@chromium.org> - 2016-11-15 19:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-15 20:20 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Kees Cook <keescook@chromium.org> - 2016-11-15 20:30 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Ingo Molnar <mingo@kernel.org> - 2016-11-16 09:40 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Greg KH <gregkh@linuxfoundation.org> - 2016-11-16 10:00 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Ingo Molnar <mingo@kernel.org> - 2016-11-16 10:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Greg KH <gregkh@linuxfoundation.org> - 2016-11-16 10:30 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-16 11:20 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Kees Cook <keescook@chromium.org> - 2016-11-16 20:00 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 09:40 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Kees Cook <keescook@chromium.org> - 2016-11-17 21:00 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Kees Cook <keescook@chromium.org> - 2016-11-16 19:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-15 13:40 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-15 14:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-15 15:20 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 10:30 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-17 10:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 11:40 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 11:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Greg KH <gregkh@linuxfoundation.org> - 2016-11-17 12:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 18:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 18:30 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Will Deacon <will.deacon@arm.com> - 2016-11-17 18:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-18 09:30 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Will Deacon <will.deacon@arm.com> - 2016-11-18 11:20 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Will Deacon <will.deacon@arm.com> - 2016-11-17 18:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-17 18:30 +0100
RE: [RFC][PATCH 7/7] kref: Implement using refcount_t "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-18 11:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-18 12:40 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Will Deacon <will.deacon@arm.com> - 2016-11-18 18:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-18 20:00 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-21 05:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Ingo Molnar <mingo@kernel.org> - 2016-11-21 08:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-21 09:40 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-21 09:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-21 10:10 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Boqun Feng <boqun.feng@gmail.com> - 2016-11-21 10:40 +0100
RE: [RFC][PATCH 7/7] kref: Implement using refcount_t "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-18 11:50 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-18 12:00 +0100
RE: [RFC][PATCH 7/7] kref: Implement using refcount_t "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-18 18:00 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-18 20:00 +0100
RE: [RFC][PATCH 7/7] kref: Implement using refcount_t "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-19 08:20 +0100
Re: [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra <peterz@infradead.org> - 2016-11-19 12:50 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Christoph Hellwig <hch@infradead.org> - 2016-11-14 19:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Greg KH <gregkh@linuxfoundation.org> - 2016-11-15 08:30 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-15 08:50 +0100
[PATCH] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' Ingo Molnar <mingo@kernel.org> - 2016-11-15 09:40 +0100
[PATCH v2] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' Ingo Molnar <mingo@kernel.org> - 2016-11-15 09:50 +0100
Re: [PATCH v2] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' Peter Zijlstra <peterz@infradead.org> - 2016-11-15 10:30 +0100
[PATCH v3] printk, locking/atomics, kref: Introduce new %pAa and %pAk format string options for atomic_t and 'struct kref' Ingo Molnar <mingo@kernel.org> - 2016-11-15 10:50 +0100
Re: [PATCH v2] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' kbuild test robot <lkp@intel.com> - 2016-11-15 11:10 +0100
Re: [PATCH] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' Linus Torvalds <torvalds@linux-foundation.org> - 2016-11-15 17:50 +0100
Re: [PATCH] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' Ingo Molnar <mingo@kernel.org> - 2016-11-16 09:20 +0100
Re: [RFC][PATCH 0/7] kref improvements Greg KH <gregkh@linuxfoundation.org> - 2016-11-15 08:30 +0100
Re: [RFC][PATCH 0/7] kref improvements Ingo Molnar <mingo@kernel.org> - 2016-11-15 08:50 +0100
Re: [RFC][PATCH 0/7] kref improvements Greg KH <gregkh@linuxfoundation.org> - 2016-11-15 16:10 +0100
Re: [RFC][PATCH 0/7] kref improvements Peter Zijlstra <peterz@infradead.org> - 2016-11-15 08:50 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Greg KH <gregkh@linuxfoundation.org> - 2016-11-15 08:40 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-15 09:10 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Kees Cook <keescook@chromium.org> - 2016-11-15 22:00 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Greg KH <gregkh@linuxfoundation.org> - 2016-11-16 09:30 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-16 11:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Greg KH <gregkh@linuxfoundation.org> - 2016-11-16 11:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Greg KH <gregkh@linuxfoundation.org> - 2016-11-16 11:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Daniel Borkmann <daniel@iogearbox.net> - 2016-11-16 11:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-16 11:10 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Kees Cook <keescook@chromium.org> - 2016-11-16 20:00 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-17 09:40 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() David Windsor <dave@progbits.org> - 2016-11-17 13:50 +0100
RE: [RFC][PATCH 2/7] kref: Add kref_read() "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-17 15:40 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-17 18:10 +0100
RE: [RFC][PATCH 2/7] kref: Add kref_read() "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-17 18:10 +0100
RE: [RFC][PATCH 2/7] kref: Add kref_read() "Reshetova, Elena" <elena.reshetova@intel.com> - 2016-11-17 19:10 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-17 20:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-17 20:40 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra <peterz@infradead.org> - 2016-11-17 18:20 +0100
Re: [RFC][PATCH 2/7] kref: Add kref_read() Kees Cook <keescook@chromium.org> - 2016-11-17 20:40 +0100
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