forked from Mirror/frr
lib/seqlock: add a few more comments
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
This commit is contained in:
David Lamparter
parent
2a5e62359f
commit
30ef834ab3
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@ -36,8 +36,14 @@
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#include "seqlock.h"
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/****************************************
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* OS specific synchronization wrappers *
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****************************************/
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/*
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* Linux: sys_futex()
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*/
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#ifdef HAVE_SYNC_LINUX_FUTEX
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/* Linux-specific - sys_futex() */
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#include <sys/syscall.h>
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#include <linux/futex.h>
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@ -55,8 +61,10 @@ static long sys_futex(void *addr1, int op, int val1,
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#define wait_poke(sqlo) \
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sys_futex((int *)&sqlo->pos, FUTEX_WAKE, INT_MAX, NULL, NULL, 0)
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/*
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* OpenBSD: sys_futex(), almost the same as on Linux
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*/
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#elif defined(HAVE_SYNC_OPENBSD_FUTEX)
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/* OpenBSD variant of the above. */
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#include <sys/syscall.h>
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#include <sys/futex.h>
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@ -69,8 +77,10 @@ static long sys_futex(void *addr1, int op, int val1,
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#define wait_poke(sqlo) \
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futex((int *)&sqlo->pos, FUTEX_WAKE, INT_MAX, NULL, NULL, 0)
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/*
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* FreeBSD: _umtx_op()
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*/
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#elif defined(HAVE_SYNC_UMTX_OP)
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/* FreeBSD-specific: umtx_op() */
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#include <sys/umtx.h>
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#define wait_once(sqlo, val) \
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@ -89,9 +99,10 @@ static int wait_time(struct seqlock *sqlo, uint32_t val,
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#define wait_poke(sqlo) \
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_umtx_op((void *)&sqlo->pos, UMTX_OP_WAKE, INT_MAX, NULL, NULL)
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#else
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/* generic version. used on *BSD, Solaris and OSX.
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/*
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* generic version. used on NetBSD, Solaris and OSX. really shitty.
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*/
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#else
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#define TIME_ABS_REALTIME 1
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@ -151,6 +162,9 @@ void seqlock_wait(struct seqlock *sqlo, seqlock_val_t val)
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bool seqlock_timedwait(struct seqlock *sqlo, seqlock_val_t val,
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const struct timespec *abs_monotime_limit)
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{
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/*
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* ABS_REALTIME - used on NetBSD, Solaris and OSX
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*/
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#if TIME_ABS_REALTIME
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#define time_arg1 &abs_rt
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#define time_arg2 NULL
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@ -170,6 +184,9 @@ bool seqlock_timedwait(struct seqlock *sqlo, seqlock_val_t val,
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}
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abs_rt.tv_sec += abs_monotime_limit->tv_sec - curmono.tv_sec;
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/*
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* RELATIVE - used on OpenBSD (might get a patch to get absolute monotime)
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*/
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#elif TIME_RELATIVE
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struct timespec reltime;
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@ -183,6 +200,9 @@ bool seqlock_timedwait(struct seqlock *sqlo, seqlock_val_t val,
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reltime.tv_sec--; \
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reltime.tv_nsec += 1000000000; \
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}
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/*
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* FreeBSD & Linux: absolute time re. CLOCK_MONOTONIC
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*/
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#else
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#define time_arg1 abs_monotime_limit
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#define time_arg2 NULL
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@ -61,12 +61,22 @@ typedef _Atomic uint32_t seqlock_ctr_t;
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typedef uint32_t seqlock_val_t;
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#define seqlock_assert_valid(val) assert((val) & SEQLOCK_HELD)
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/* NB: SEQLOCK_WAITERS is only allowed if SEQLOCK_HELD is also set; can't
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* have waiters on an unheld seqlock
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*/
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#define SEQLOCK_HELD (1U << 0)
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#define SEQLOCK_WAITERS (1U << 1)
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#define SEQLOCK_VAL(n) ((n) & ~SEQLOCK_WAITERS)
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#define SEQLOCK_STARTVAL 1U
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#define SEQLOCK_INCR 4U
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/* TODO: originally, this was using "atomic_fetch_add", which is the reason
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* bit 0 is used to indicate held state. With SEQLOCK_WAITERS added, there's
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* no fetch_add anymore (cmpxchg loop instead), so we don't need to use bit 0
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* for this anymore & can just special-case the value 0 for it and skip it in
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* counting.
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*/
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struct seqlock {
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/* always used */
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seqlock_ctr_t pos;
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@ -80,10 +90,16 @@ struct seqlock {
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extern void seqlock_init(struct seqlock *sqlo);
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/* while (sqlo <= val) - wait until seqlock->pos > val, or seqlock unheld */
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/* basically: "while (sqlo <= val) wait();"
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* returns when sqlo > val || !seqlock_held(sqlo)
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*/
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extern void seqlock_wait(struct seqlock *sqlo, seqlock_val_t val);
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/* same, but time-limited (limit is an absolute CLOCK_MONOTONIC value) */
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extern bool seqlock_timedwait(struct seqlock *sqlo, seqlock_val_t val,
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const struct timespec *abs_monotime_limit);
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/* one-shot test, returns true if seqlock_wait would return immediately */
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extern bool seqlock_check(struct seqlock *sqlo, seqlock_val_t val);
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static inline bool seqlock_held(struct seqlock *sqlo)
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@ -93,12 +109,20 @@ static inline bool seqlock_held(struct seqlock *sqlo)
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/* sqlo - get seqlock position -- for the "counter" seqlock */
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extern seqlock_val_t seqlock_cur(struct seqlock *sqlo);
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/* sqlo++ - note: like x++, returns previous value, before bumping */
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/* ++sqlo (but atomic & wakes waiters) - returns value that we bumped to.
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*
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* guarantees:
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* - each seqlock_bump call bumps the position by exactly one SEQLOCK_INCR.
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* There are no skipped/missed or multiple increments.
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* - each return value is only returned from one seqlock_bump() call
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*/
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extern seqlock_val_t seqlock_bump(struct seqlock *sqlo);
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/* sqlo = val - can be used on held seqlock. */
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extern void seqlock_acquire_val(struct seqlock *sqlo, seqlock_val_t val);
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/* sqlo = ref - standard pattern: acquire relative to other seqlock */
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static inline void seqlock_acquire(struct seqlock *sqlo, struct seqlock *ref)
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{
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