From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Thu, 27 Aug 2020 13:40:41 +0200
Subject: timekeeping: Use seqcount_latch_t
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Patch-mainline: v5.10-rc1
Git-commit: 249d053835320cb3e7c00066cf085a6ba9b1f126
References: bsc#1176564 bsc#1162702
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'ĂȘtre -- verifying that the
seqcount_t writer serialization lock is held -- cannot thus be done.
References: 0c3351d451ae ("seqlock: Use raw_ prefix instead of _no_lockdep")
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
Acked-by: Daniel Wagner <dwagner@suse.de>
---
kernel/time/timekeeping.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -64,7 +64,7 @@ static struct timekeeper shadow_timekeep
* 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_f
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_r
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;
}