Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 12 Nov 2010 08:39:17 -0800 | Subject | Re: [patches] seqlock: add barrier-less special cases for seqcounts |
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On Thu, Nov 11, 2010 at 12:00 AM, Nick Piggin <npiggin@kernel.dk> wrote: > Add branch annotations for seqlock read fastpath, and introduce > __read_seqcount_begin and __read_seqcount_end functions, that can avoid > the smp_rmb() if it is provided with some other barrier. Read barriers > have non trivial cost on many architectures. > > These will be used by store-free path walking algorithm, where > performance is critical and seqlocks are widely used.
A couple of questions:
- what are the barriers in question? IOW, describe some normal use.
- do we really want the "repeat until seqlock is even" code in the __read_seqcount_begin() code for those kinds of internal cases?
That second one is very much a question for the use-case like the pathname walk where you have a fall-back that uses "real" locking rather than the optimistic sequence locks. I have a suspicion that if seq_locks are used as an "optimistic lockless path with a locking fallback", then if we see an odd value at the beginning we should consider it a hint that the sequence lock is contended and the optimistic path should be aborted early.
In other words, I kind of suspect that anybody that wants to use some internal sequence lock function like __read_seqcount_begin() would also want to do its own decision about what happens when the seqlock is already in the middle of having an active writer.
So the interface seems a bit broken: if we really want to expose these kinds of internal helper functions, then I suspect not only the smp_rmb(), but also the whole "loop until even" should be in the normal "read_seqcount_begin()" function, and __read_seqcount_begin() would _literally_ just do the single sequence counter access.
I dunno. Just a gut feel. Added Al, Ingo and Thomas to the Cc - the whole "loop in begin" was added by Ingo and Thomas a few years ago to avoid a live-lock, but that live-lock issue really isn't an issue if you end up falling back on a locking algorithm and have a "early failure" case for the __read_seqcount_begin() the same way we have the final failure case for [__]read_seqcount_retry().
Linus
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