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    Subject[tip: locking/core] timekeeping: Use seqcount_latch_t
    The following commit has been merged into the locking/core branch of tip:

    Commit-ID: 249d053835320cb3e7c00066cf085a6ba9b1f126
    Gitweb: https://git.kernel.org/tip/249d053835320cb3e7c00066cf085a6ba9b1f126
    Author: Ahmed S. Darwish <a.darwish@linutronix.de>
    AuthorDate: Thu, 27 Aug 2020 13:40:41 +02:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Thu, 10 Sep 2020 11:19:29 +02:00

    timekeeping: Use seqcount_latch_t

    Latch sequence counters are a multiversion concurrency control mechanism
    where the seqcount_t counter even/odd value is used to switch between
    two data storage copies. This allows the seqcount_t read path to safely
    interrupt its write side critical section (e.g. from NMIs).

    Initially, latch sequence counters were implemented as a single write
    function, raw_write_seqcount_latch(), above plain seqcount_t. The read
    path was expected to use plain seqcount_t raw_read_seqcount().

    A specialized read function was later added, raw_read_seqcount_latch(),
    and became the standardized way for latch read paths. Having unique read
    and write APIs meant that latch sequence counters are basically a data
    type of their own -- just inappropriately overloading plain seqcount_t.
    The seqcount_latch_t data type was thus introduced at seqlock.h.

    Use that new data type instead of seqcount_raw_spinlock_t. This ensures
    that only latch-safe APIs are to be used with the sequence counter.

    Note that the use of seqcount_raw_spinlock_t was not very useful in the
    first place. Only the "raw_" subset of seqcount_t APIs were used at
    timekeeping.c. This subset was created for contexts where lockdep cannot
    be used. seqcount_LOCKTYPE_t's raison d'être -- verifying that the
    seqcount_t writer serialization lock is held -- cannot thus be done.

    References: 0c3351d451ae ("seqlock: Use raw_ prefix instead of _no_lockdep")
    References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
    Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20200827114044.11173-6-a.darwish@linutronix.de
    ---
    kernel/time/timekeeping.c | 10 +++++-----
    1 file changed, 5 insertions(+), 5 deletions(-)

    diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
    index 4c47f38..999c981 100644
    --- a/kernel/time/timekeeping.c
    +++ b/kernel/time/timekeeping.c
    @@ -64,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
    * See @update_fast_timekeeper() below.
    */
    struct tk_fast {
    - seqcount_raw_spinlock_t seq;
    + seqcount_latch_t seq;
    struct tk_read_base base[2];
    };

    @@ -81,13 +81,13 @@ static struct clocksource dummy_clock = {
    };

    static struct tk_fast tk_fast_mono ____cacheline_aligned = {
    - .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
    + .seq = SEQCNT_LATCH_ZERO(tk_fast_mono.seq),
    .base[0] = { .clock = &dummy_clock, },
    .base[1] = { .clock = &dummy_clock, },
    };

    static struct tk_fast tk_fast_raw ____cacheline_aligned = {
    - .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
    + .seq = SEQCNT_LATCH_ZERO(tk_fast_raw.seq),
    .base[0] = { .clock = &dummy_clock, },
    .base[1] = { .clock = &dummy_clock, },
    };
    @@ -467,7 +467,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
    tk_clock_read(tkr),
    tkr->cycle_last,
    tkr->mask));
    - } while (read_seqcount_retry(&tkf->seq, seq));
    + } while (read_seqcount_latch_retry(&tkf->seq, seq));

    return now;
    }
    @@ -533,7 +533,7 @@ static __always_inline u64 __ktime_get_real_fast_ns(struct tk_fast *tkf)
    tk_clock_read(tkr),
    tkr->cycle_last,
    tkr->mask));
    - } while (read_seqcount_retry(&tkf->seq, seq));
    + } while (read_seqcount_latch_retry(&tkf->seq, seq));

    return now;
    }
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