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Siewior" , LKML , "Ahmed S. Darwish" Subject: [PATCH v1 3/8] seqlock: Introduce seqcount_latch_t Date: Thu, 27 Aug 2020 13:40:39 +0200 Message-Id: <20200827114044.11173-4-a.darwish@linutronix.de> In-Reply-To: <20200827114044.11173-1-a.darwish@linutronix.de> References: <20200519214547.352050-1-a.darwish@linutronix.de> <20200827114044.11173-1-a.darwish@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Latch sequence counters are a multiversion concurrency control mechanism where the seqcount_t counter even/odd value is used to switch between two copies of protected data. 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 above plain seqcount_t: raw_write_seqcount_latch(). The read side was expected to use plain seqcount_t raw_read_seqcount(). A specialized latch read function, raw_read_seqcount_latch(), was later added. It became the standardized way for latch read paths. Due to the dependent load, it has one read memory barrier less than the plain seqcount_t raw_read_seqcount() API. Only raw_write_seqcount_latch() and raw_read_seqcount_latch() should be used with latch sequence counters. Having *unique* read and write path APIs means that latch sequence counters are actually a data type of their own -- just inappropriately overloading plain seqcount_t. Introduce seqcount_latch_t. This adds type-safety and ensures that only the correct latch-safe APIs are to be used. Not to break bisection, let the latch APIs also accept plain seqcount_t or seqcount_raw_spinlock_t. After converting all call sites to seqcount_latch_t, only that new data type will be allowed. References: 9b0fd802e8c0 ("seqcount: Add raw_write_seqcount_latch()") References: 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()") References: aadd6e5caaac ("time/sched_clock: Use raw_read_seqcount_latch()") Signed-off-by: Ahmed S. Darwish --- Documentation/locking/seqlock.rst | 18 ++++++ include/linux/seqlock.h | 104 +++++++++++++++++++++--------- 2 files changed, 91 insertions(+), 31 deletions(-) diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst index 62c5ad98c11c..a334b584f2b3 100644 --- a/Documentation/locking/seqlock.rst +++ b/Documentation/locking/seqlock.rst @@ -139,6 +139,24 @@ with the associated LOCKTYPE lock acquired. Read path: same as in :ref:`seqcount_t`. + +.. _seqcount_latch_t: + +Latch sequence counters (``seqcount_latch_t``) +---------------------------------------------- + +Latch sequence counters are a multiversion concurrency control mechanism +where the embedded seqcount_t counter even/odd value is used to switch +between two copies of protected data. This allows the sequence counter +read path to safely interrupt its own write side critical section. + +Use seqcount_latch_t when the write side sections cannot be protected +from interruption by readers. This is typically the case when the read +side can be invoked from NMI handlers. + +Check `raw_write_seqcount_latch()` for more information. + + .. _seqlock_t: Sequential locks (``seqlock_t``) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 962d9768945f..f83fbf2180db 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -587,34 +587,76 @@ static inline void write_seqcount_t_invalidate(seqcount_t *s) kcsan_nestable_atomic_end(); } -/** - * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy - * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants +/* + * Latch sequence counters (seqcount_latch_t) + * + * A sequence counter variant where the counter even/odd value is used to + * switch between two copies of protected data. This allows the read path, + * typically NMIs, to safely interrupt the write side critical section. * - * Use seqcount_t latching to switch between two storage places protected - * by a sequence counter. Doing so allows having interruptible, preemptible, - * seqcount_t write side critical sections. + * As the write sections are fully preemptible, no special handling for + * PREEMPT_RT is needed. + */ +typedef struct { + seqcount_t seqcount; +} seqcount_latch_t; + +/** + * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t + * @seq_name: Name of the seqcount_latch_t instance + */ +#define SEQCNT_LATCH_ZERO(seq_name) { \ + .seqcount = SEQCNT_ZERO(seq_name.seqcount), \ +} + +/** + * seqcount_latch_init() - runtime initializer for seqcount_latch_t + * @s: Pointer to the seqcount_latch_t instance + */ +static inline void seqcount_latch_init(seqcount_latch_t *s) +{ + seqcount_init(&s->seqcount); +} + +/** + * raw_read_seqcount_latch() - pick even/odd latch data copy + * @s: Pointer to seqcount_t, seqcount_raw_spinlock_t, or seqcount_latch_t * - * Check raw_write_seqcount_latch() for more details and a full reader and - * writer usage example. + * See raw_write_seqcount_latch() for details and a full reader/writer + * usage example. * * Return: sequence counter raw value. Use the lowest bit as an index for - * picking which data copy to read. The full counter value must then be - * checked with read_seqcount_retry(). + * picking which data copy to read. The full counter must then be checked + * with read_seqcount_latch_retry(). */ -#define raw_read_seqcount_latch(s) \ - raw_read_seqcount_t_latch(__seqcount_ptr(s)) +#define raw_read_seqcount_latch(s) \ +({ \ + /* \ + * Pairs with the first smp_wmb() in raw_write_seqcount_latch(). \ + * Due to the dependent load, a full smp_rmb() is not needed. \ + */ \ + _Generic(*(s), \ + seqcount_t: READ_ONCE(((seqcount_t *)s)->sequence), \ + seqcount_raw_spinlock_t: READ_ONCE(((seqcount_raw_spinlock_t *)s)->seqcount.sequence), \ + seqcount_latch_t: READ_ONCE(((seqcount_latch_t *)s)->seqcount.sequence)); \ +}) -static inline int raw_read_seqcount_t_latch(seqcount_t *s) +/** + * read_seqcount_latch_retry() - end a seqcount_latch_t read section + * @s: Pointer to seqcount_latch_t + * @start: count, from raw_read_seqcount_latch() + * + * Return: true if a read section retry is required, else false + */ +static inline int +read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) { - /* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */ - int seq = READ_ONCE(s->sequence); /* ^^^ */ - return seq; + return read_seqcount_retry(&s->seqcount, start); } /** - * raw_write_seqcount_latch() - redirect readers to even/odd copy - * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants + * raw_write_seqcount_latch() - redirect latch readers to even/odd copy + * @s: Pointer to seqcount_t, seqcount_raw_spinlock_t, or seqcount_latch_t * * The latch technique is a multiversion concurrency control method that allows * queries during non-atomic modifications. If you can guarantee queries never @@ -633,7 +675,7 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s) * The basic form is a data structure like:: * * struct latch_struct { - * seqcount_t seq; + * seqcount_latch_t seq; * struct data_struct data[2]; * }; * @@ -643,13 +685,13 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s) * void latch_modify(struct latch_struct *latch, ...) * { * smp_wmb(); // Ensure that the last data[1] update is visible - * latch->seq++; + * latch->seq.sequence++; * 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++; + * latch->seq.sequence++; * smp_wmb(); // Ensure that the seqcount update is visible * * modify(latch->data[1], ...); @@ -668,8 +710,8 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s) * idx = seq & 0x01; * entry = data_query(latch->data[idx], ...); * - * // read_seqcount_retry() includes needed smp_rmb() - * } while (read_seqcount_retry(&latch->seq, seq)); + * // This includes needed smp_rmb() + * } while (read_seqcount_latch_retry(&latch->seq, seq)); * * return entry; * } @@ -693,14 +735,14 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s) * When data is a dynamic data structure; one should use regular RCU * patterns to manage the lifetimes of the objects within. */ -#define raw_write_seqcount_latch(s) \ - raw_write_seqcount_t_latch(__seqcount_ptr(s)) - -static inline void raw_write_seqcount_t_latch(seqcount_t *s) -{ - smp_wmb(); /* prior stores before incrementing "sequence" */ - s->sequence++; - smp_wmb(); /* increment "sequence" before following stores */ +#define raw_write_seqcount_latch(s) \ +{ \ + smp_wmb(); /* prior stores before incrementing "sequence" */ \ + _Generic(*(s), \ + seqcount_t: ((seqcount_t *)s)->sequence++, \ + seqcount_raw_spinlock_t:((seqcount_raw_spinlock_t *)s)->seqcount.sequence++, \ + seqcount_latch_t: ((seqcount_latch_t *)s)->seqcount.sequence++); \ + smp_wmb(); /* increment "sequence" before following stores */ \ } /* -- 2.28.0