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Date: Wed, 18 Mar 2015 14:29:35 +0000 (UTC)
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, rusty@...tcorp.com.au, oleg@...hat.com,
paulmck@...ux.vnet.ibm.com, torvalds@...ux-foundation.org,
linux-kernel@...r.kernel.org, andi@...stfloor.org,
rostedt@...dmis.org, tglx@...utronix.de,
David Woodhouse <David.Woodhouse@...el.com>,
Rik van Riel <riel@...hat.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Michel Lespinasse <walken@...gle.com>
Subject: Re: [PATCH 5/8] seqlock: Better document raw_write_seqcount_latch()
----- Original Message -----
> Improve the documentation of the latch technique as used in the
> current timekeeping code, such that it can be readily employed
> elsewhere.
>
> Borrow from the comments in timekeeping and replace those with a
> reference to this more generic comment.
>
> Cc: David Woodhouse <David.Woodhouse@...el.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Acked-by: Michel Lespinasse <walken@...gle.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> include/linux/seqlock.h | 77
> +++++++++++++++++++++++++++++++++++++++++++++-
> kernel/time/timekeeping.c | 27 ----------------
> 2 files changed, 77 insertions(+), 27 deletions(-)
>
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -233,9 +233,84 @@ static inline void raw_write_seqcount_en
> s->sequence++;
> }
>
> -/*
> +/**
> * raw_write_seqcount_latch - redirect readers to even/odd copy
> * @s: pointer to seqcount_t
> + *
> + * The latch technique is a multiversion concurrency control method that
> allows
> + * queries during non atomic modifications. If you can guarantee queries
> never
> + * interrupt the modification -- e.g. the concurrency is strictly between
> CPUs
> + * -- you most likely do not need this.
> + *
> + * Where the traditional RCU/lockless data structures rely on atomic
> + * modifications to ensure queries observe either the old or the new state
> the
> + * latch allows the same for non atomic updates. The trade-off is doubling
> the
> + * cost of storage; we have to maintain two copies of the entire data
> + * structure.
> + *
> + * Very simply put: we first modify one copy and then the other. This
> ensures
> + * there is always one copy in a stable state, ready to give us an answer.
> + *
> + * The basic form is a data structure like:
> + *
> + * struct latch_struct {
> + * seqcount_t seq;
> + * struct data_struct data[2];
> + * };
> + *
> + * Where a modification, which is assumed to be externally serialized, does
> the
> + * following:
> + *
> + * void latch_modify(struct latch_struct *latch, ...)
> + * {
> + * smp_wmb(); <- Ensure that the last data[1] update is visible
> + * latch->seq++;
> + * smp_wmb(); <- Ensure that the seqcount update is visible
> + *
> + * modify(latch->data[0], ...);
> + *
> + * smp_wmb(); <- Ensure that the data[0] update is visible
> + * latch->seq++;
> + * smp_wmb(); <- Ensure that the seqcount update is visible
> + *
> + * modify(latch->data[1], ...);
> + * }
> + *
> + * The query will have a form like:
> + *
> + * struct entry *latch_query(struct latch_struct *latch, ...)
> + * {
> + * struct entry *entry;
> + * unsigned seq;
> + * int idx;
very minor nit: why is seq unsigned, but idx a signed int ?
Could we do:
unsigned seq, idx; instead ?
Other than that:
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> + *
> + * do {
> + * seq = latch->seq;
> + * smp_rmb();
> + *
> + * idx = seq & 0x01;
> + * entry = data_query(latch->data[idx], ...);
> + *
> + * smp_rmb();
> + * } while (seq != latch->seq);
> + *
> + * return entry;
> + * }
> + *
> + * So during the modification, queries are first redirected to data[1]. Then
> we
> + * modify data[0]. When that is complete, we redirect queries back to
> data[0]
> + * and we can modify data[1].
> + *
> + * NOTE: The non-requirement for atomic modifications does _NOT_ include
> + * the publishing of new entries in the case where data is a dynamic
> + * data structure.
> + *
> + * An iteration might start in data[0] and get suspended long enough
> + * to miss an entire modification sequence, once it resumes it might
> + * observe the new entry.
> + *
> + * NOTE: When data is a dynamic data structure; one should use regular RCU
> + * patterns to manage the lifetimes of the objects within.
> */
> static inline void raw_write_seqcount_latch(seqcount_t *s)
> {
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -339,32 +339,7 @@ static inline s64 timekeeping_get_ns_raw
> * We want to use this from any context including NMI and tracing /
> * instrumenting the timekeeping code itself.
> *
> - * So we handle this differently than the other timekeeping accessor
> - * functions which retry when the sequence count has changed. The
> - * update side does:
> - *
> - * smp_wmb(); <- Ensure that the last base[1] update is visible
> - * tkf->seq++;
> - * smp_wmb(); <- Ensure that the seqcount update is visible
> - * update(tkf->base[0], tkr);
> - * smp_wmb(); <- Ensure that the base[0] update is visible
> - * tkf->seq++;
> - * smp_wmb(); <- Ensure that the seqcount update is visible
> - * update(tkf->base[1], tkr);
> - *
> - * The reader side does:
> - *
> - * do {
> - * seq = tkf->seq;
> - * smp_rmb();
> - * idx = seq & 0x01;
> - * now = now(tkf->base[idx]);
> - * smp_rmb();
> - * } while (seq != tkf->seq)
> - *
> - * As long as we update base[0] readers are forced off to
> - * base[1]. Once base[0] is updated readers are redirected to base[0]
> - * and the base[1] update takes place.
> + * Employ the latch technique; see @raw_write_seqcount_latch.
> *
> * So if a NMI hits the update of base[0] then it will use base[1]
> * which is still consistent. In the worst case this can result is a
>
>
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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