From: Joel Fernandes <joelaf@google.com>
To: linux-kernel@vger.kernel.org
Cc: Joel Fernandes <joel@joelfernandes.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
byungchul.park@lge.com, kernel-team@android.com
Subject: [PATCH v3 1/4] rcu: Add comment documenting how rcu_seq_snap works
Date: Sun, 20 May 2018 21:42:17 -0700 [thread overview]
Message-ID: <20180521044220.123933-2-joel@joelfernandes.org> (raw)
In-Reply-To: <20180521044220.123933-1-joel@joelfernandes.org>
rcu_seq_snap may be tricky to decipher. Lets document how it works with
an example to make it easier.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/rcu.h | 33 ++++++++++++++++++++++++++++++++-
1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 0453a7d12b3f..d4396c96f614 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -91,7 +91,38 @@ static inline void rcu_seq_end(unsigned long *sp)
WRITE_ONCE(*sp, rcu_seq_endval(sp));
}
-/* Take a snapshot of the update side's sequence number. */
+/*
+ * rcu_seq_snap - Take a snapshot of the update side's sequence number.
+ *
+ * This function returns the earliest value of the grace-period sequence number
+ * that will indicate that a full grace period has elapsed since the current
+ * time. Once the grace-period sequence number has reached this value, it will
+ * be safe to invoke all callbacks that have been registered prior to the
+ * current time. This value is the current grace-period number plus two to the
+ * power of the number of low-order bits reserved for state, then rounded up to
+ * the next value in which the state bits are all zero.
+ *
+ * For example, since RCU_SEQ_STATE_MASK=3 and the least significant bit of
+ * the seq is used to track if a GP is in progress or not, its sufficient if we
+ * add (6+1) and mask with ~3 to get the next GP. Let's see why with an example:
+ *
+ * Say the current seq is 12 which is 0b1100 (GP is 3 and state bits are 0b00).
+ * To get to the next GP number of 4, we have to add 0b100 to this (0x1 << 2)
+ * to account for the shift due to 2 state bits. Now, if the current seq is
+ * 13 (GP is 3 and state bits are 0b01), then it means the current grace period
+ * is already in progress so the next GP that a future call back will be queued
+ * to run at is GP+2 = 5, not 4. To account for the extra +1, we just overflow
+ * the 2 lower bits by adding 0b11. Incase the lower bit was set, the overflow
+ * will cause the extra +1 to the GP, along with the usual +1 explained before.
+ * This gives us GP+2. Finally we mask the lower to bits by ~0x3 incase the
+ * overflow didn't occur. This masking is needed because incase RCU was idle
+ * (no GP in progress so lower 2 bits are 0b00), then the overflow of the lower
+ * 2 state bits wouldn't occur, so we mask to zero out those lower 2 bits.
+ *
+ * In other words, the next seq can be obtained by (0b11 + 0b100) & (~0b11)
+ * which can be generalized to:
+ * seq + (RCU_SEQ_STATE_MASK + (RCU_SEQ_STATE_MASK + 1)) & (~RCU_SEQ_STATE_MASK)
+ */
static inline unsigned long rcu_seq_snap(unsigned long *sp)
{
unsigned long s;
--
2.17.0.441.gb46fe60e1d-goog
next prev parent reply other threads:[~2018-05-21 4:42 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-21 4:42 [PATCH v3 0/4] fixes, cleanups for rcu/dev Joel Fernandes
2018-05-21 4:42 ` Joel Fernandes [this message]
2018-05-21 4:50 ` [PATCH v3 1/4] rcu: Add comment documenting how rcu_seq_snap works Randy Dunlap
2018-05-21 5:48 ` Joel Fernandes
2018-05-21 6:18 ` Randy Dunlap
2018-05-21 9:35 ` Joe Perches
2018-05-21 23:42 ` Paul E. McKenney
2018-05-21 4:42 ` [PATCH v3 2/4] rcu: Cleanup the variables used to request a new grace period Joel Fernandes
2018-05-21 23:39 ` Paul E. McKenney
2018-05-21 23:41 ` Joel Fernandes
2018-05-21 4:42 ` [PATCH v3 3/4] rcu: Use better variable names in funnel locking loop Joel Fernandes
2018-05-21 23:13 ` Paul E. McKenney
2018-05-22 0:00 ` Joel Fernandes
2018-05-22 0:19 ` Paul E. McKenney
2018-05-22 0:19 ` Joel Fernandes
2018-05-22 0:29 ` Paul E. McKenney
2018-05-21 4:42 ` [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq_needed Joel Fernandes
2018-05-21 23:25 ` Paul E. McKenney
2018-05-22 0:07 ` Joel Fernandes
2018-05-22 0:28 ` Paul E. McKenney
2018-05-22 4:16 ` Paul E. McKenney
2018-05-22 4:43 ` Joel Fernandes
2018-05-22 12:12 ` Paul E. McKenney
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