This patch adds srcu_read_lock_lite() and srcu_read_unlock_lite(), which
dispense with the read-side smp_mb() but also are restricted to code
regions that RCU is watching. If a given srcu_struct structure uses
srcu_read_lock_lite() and srcu_read_unlock_lite(), it is not permitted
to use any other SRCU read-side marker, before, during, or after.
Another price of light-weight readers is heavier weight grace periods.
Such readers mean that SRCU grace periods on srcu_struct structures
used by light-weight readers will incur at least two calls to
synchronize_rcu(). In addition, normal SRCU grace periods for
light-weight-reader srcu_struct structures never auto-expedite.
Note that expedited SRCU grace periods for light-weight-reader
srcu_struct structures still invoke synchronize_rcu(), not
synchronize_srcu_expedited(). Something about wishing to keep
the IPIs down to a dull roar.
The srcu_read_lock_lite() and srcu_read_unlock_lite() functions may not
(repeat, *not*) be used from NMI handlers, but if this is needed, an
additional flavor of SRCU reader can be added by some future commit.
[ paulmck: Apply Alexei Starovoitov expediting feedback. ]
[ paulmck: Apply kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Tested-by: kernel test robot <oliver.sang@xxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: <bpf@xxxxxxxxxxxxxxx>
---
include/linux/srcu.h | 51 ++++++++++++++++++++++++-
include/linux/srcutree.h | 1 +
kernel/rcu/srcutree.c | 82 ++++++++++++++++++++++++++++++++++------
3 files changed, 122 insertions(+), 12 deletions(-)
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 84daaa33ea0ab..4ba96e2cfa405 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -56,6 +56,13 @@ void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
void cleanup_srcu_struct(struct srcu_struct *ssp);
int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
+#ifdef CONFIG_TINY_SRCU
+#define __srcu_read_lock_lite __srcu_read_lock
+#define __srcu_read_unlock_lite __srcu_read_unlock
+#else // #ifdef CONFIG_TINY_SRCU
+int __srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp);
+void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) __releases(ssp);
+#endif // #else // #ifdef CONFIG_TINY_SRCU
void synchronize_srcu(struct srcu_struct *ssp);
#define SRCU_GET_STATE_COMPLETED 0x1
@@ -179,7 +186,7 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU)
void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
#else
-static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) { }
+#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)
#endif
@@ -249,6 +256,32 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
return retval;
}
+/**
+ * srcu_read_lock_lite - register a new reader for an SRCU-protected structure.
+ * @ssp: srcu_struct in which to register the new reader.
+ *
+ * Enter an SRCU read-side critical section, but for a light-weight
+ * smp_mb()-free reader. See srcu_read_lock() for more information.
+ *
+ * If srcu_read_lock_lite() is ever used on an srcu_struct structure,
+ * then none of the other flavors may be used, whether before, during,
+ * or after. Note that grace-period auto-expediting is disabled for _lite
+ * srcu_struct structures because auto-expedited grace periods invoke
+ * synchronize_rcu_expedited(), IPIs and all.
+ *
+ * Note that srcu_read_lock_lite() can be invoked only from those contexts
+ * where RCU is watching. Otherwise, lockdep will complain.
+ */
+static inline int srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp)
+{
+ int retval;
+
+ srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE);
+ retval = __srcu_read_lock_lite(ssp);
+ rcu_try_lock_acquire(&ssp->dep_map);
+ return retval;
+}
+
/**
* srcu_read_lock_nmisafe - register a new reader for an SRCU-protected structure.
* @ssp: srcu_struct in which to register the new reader.
@@ -325,6 +358,22 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
__srcu_read_unlock(ssp, idx);
}
+/**
+ * srcu_read_unlock_lite - unregister a old reader from an SRCU-protected structure.
+ * @ssp: srcu_struct in which to unregister the old reader.
+ * @idx: return value from corresponding srcu_read_lock().
+ *
+ * Exit a light-weight SRCU read-side critical section.
+ */
+static inline void srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
+ __releases(ssp)
+{
+ WARN_ON_ONCE(idx & ~0x1);
+ srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE);
+ srcu_lock_release(&ssp->dep_map);
+ __srcu_read_unlock(ssp, idx);
+}
+
/**
* srcu_read_unlock_nmisafe - unregister a old reader from an SRCU-protected structure.
* @ssp: srcu_struct in which to unregister the old reader.
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 79ad809c7f035..8074138cbd624 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -46,6 +46,7 @@ struct srcu_data {
/* Values for ->srcu_reader_flavor. */
#define SRCU_READ_FLAVOR_NORMAL 0x1 // srcu_read_lock().
#define SRCU_READ_FLAVOR_NMI 0x2 // srcu_read_lock_nmisafe().
+#define SRCU_READ_FLAVOR_LITE 0x4 // srcu_read_lock_lite().
/*
* Node in SRCU combining tree, similar in function to rcu_data.
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 4c51be484b48a..bf51758cf4a64 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -429,20 +429,29 @@ static bool srcu_gp_is_expedited(struct srcu_struct *ssp)
}
/*
- * Returns approximate total of the readers' ->srcu_lock_count[] values
- * for the rank of per-CPU counters specified by idx.
+ * Computes approximate total of the readers' ->srcu_lock_count[] values
+ * for the rank of per-CPU counters specified by idx, and returns true if
+ * the caller did the proper barrier (gp), and if the count of the locks
+ * matches that of the unlocks passed in.
*/
-static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
+static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, unsigned long unlocks)
{
int cpu;
+ unsigned long mask = 0;
unsigned long sum = 0;
for_each_possible_cpu(cpu) {
struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
sum += atomic_long_read(&sdp->srcu_lock_count[idx]);
+ if (IS_ENABLED(CONFIG_PROVE_RCU))
+ mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
}
- return sum;
+ WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
+ "Mixed reader flavors for srcu_struct at %ps.\n", ssp);
+ if (mask & SRCU_READ_FLAVOR_LITE && !gp)
+ return false;
+ return sum == unlocks;
}
/*
@@ -473,6 +482,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
{
+ bool did_gp = !!(raw_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE);
unsigned long unlocks;
unlocks = srcu_readers_unlock_idx(ssp, idx);
@@ -482,13 +492,16 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
* unlock is counted. Needs to be a smp_mb() as the read side may
* contain a read from a variable that is written to before the
* synchronize_srcu() in the write side. In this case smp_mb()s
- * A and B act like the store buffering pattern.
+ * A and B (or X and Y) act like the store buffering pattern.
*
- * This smp_mb() also pairs with smp_mb() C to prevent accesses
- * after the synchronize_srcu() from being executed before the
- * grace period ends.
+ * This smp_mb() also pairs with smp_mb() C (or, in the case of X,
+ * Z) to prevent accesses after the synchronize_srcu() from being
+ * executed before the grace period ends.
*/
- smp_mb(); /* A */
+ if (!did_gp)
+ smp_mb(); /* A */
+ else
+ synchronize_rcu(); /* X */
/*
* If the locks are the same as the unlocks, then there must have
@@ -546,7 +559,7 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
* which are unlikely to be configured with an address space fully
* populated with memory, at least not anytime soon.
*/
- return srcu_readers_lock_idx(ssp, idx) == unlocks;
+ return srcu_readers_lock_idx(ssp, idx, did_gp, unlocks);
}
/**
@@ -750,6 +763,47 @@ void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock);
+/*
+ * Counts the new reader in the appropriate per-CPU element of the
+ * srcu_struct. Returns an index that must be passed to the matching
+ * srcu_read_unlock_lite().
+ *
+ * Note that this_cpu_inc() is an RCU read-side critical section either
+ * because it disables interrupts, because it is a single instruction,
+ * or because it is a read-modify-write atomic operation, depending on
+ * the whims of the architecture.
+ */
+int __srcu_read_lock_lite(struct srcu_struct *ssp)
+{
+ int idx;
+
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_lite().");
+ idx = READ_ONCE(ssp->srcu_idx) & 0x1;
+ this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter); /* Y */
+ barrier(); /* Avoid leaking the critical section. */
+ return idx;
+}
+EXPORT_SYMBOL_GPL(__srcu_read_lock_lite);
+
+/*
+ * Removes the count for the old reader from the appropriate
+ * per-CPU element of the srcu_struct. Note that this may well be a
+ * different CPU than that which was incremented by the corresponding
+ * srcu_read_lock_lite(), but it must be within the same task.
+ *
+ * Note that this_cpu_inc() is an RCU read-side critical section either
+ * because it disables interrupts, because it is a single instruction,
+ * or because it is a read-modify-write atomic operation, depending on
+ * the whims of the architecture.
+ */
+void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
+{
+ barrier(); /* Avoid leaking the critical section. */
+ this_cpu_inc(ssp->sda->srcu_unlock_count[idx].counter); /* Z */
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_lite().");
+}
+EXPORT_SYMBOL_GPL(__srcu_read_unlock_lite);
+
#ifdef CONFIG_NEED_SRCU_NMI_SAFE
/*
@@ -1134,6 +1188,8 @@ static void srcu_flip(struct srcu_struct *ssp)
* it stays until either (1) Compilers learn about this sort of
* control dependency or (2) Some production workload running on
* a production system is unduly delayed by this slowpath smp_mb().
+ * Except for _lite() readers, where it is inoperative, which
+ * means that it is a good thing that it is redundant.
*/
smp_mb(); /* E */ /* Pairs with B and C. */
@@ -1152,7 +1208,8 @@ static void srcu_flip(struct srcu_struct *ssp)
/*
* If SRCU is likely idle, in other words, the next SRCU grace period
- * should be expedited, return true, otherwise return false.
+ * should be expedited, return true, otherwise return false. Except that
+ * in the presence of _lite() readers, always return false.
*
* Note that it is OK for several current from-idle requests for a new
* grace period from idle to specify expediting because they will all end
@@ -1181,6 +1238,9 @@ static bool srcu_should_expedite(struct srcu_struct *ssp)
unsigned long tlast;
check_init_srcu_struct(ssp);
+ /* If _lite() readers, don't do unsolicited expediting. */
+ if (this_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE)
+ return false;
/* If the local srcu_data structure has callbacks, not idle. */
sdp = raw_cpu_ptr(ssp->sda);
spin_lock_irqsave_rcu_node(sdp, flags);
--
2.40.1
