#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+/*
+ * Returns approximate total of the readers' ->seq[] values for the
+ * rank of per-CPU counters specified by idx.
+ */
+static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
+{
+ int cpu;
+ unsigned long sum = 0;
+ unsigned long t;
+
+ for_each_possible_cpu(cpu) {
+ t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
+ sum += t;
+ }
+ return sum;
+}
+
/*
* Returns approximate number of readers active on the specified rank
- * of per-CPU counters. Also snapshots each counter's value in the
- * corresponding element of sp->snap[] for later use validating
- * the sum.
+ * of the per-CPU ->c[] counters.
*/
static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
{
for_each_possible_cpu(cpu) {
t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
sum += t;
- sp->snap[cpu] = t;
}
- return sum & SRCU_REF_MASK;
+ return sum;
}
/*
- * To be called from the update side after an index flip. Returns true
- * if the modulo sum of the counters is stably zero, false if there is
- * some possibility of non-zero.
+ * Return true if the number of pre-existing readers is determined to
+ * be stably zero. An example unstable zero can occur if the call
+ * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
+ * but due to task migration, sees the corresponding __srcu_read_unlock()
+ * decrement. This can happen because srcu_readers_active_idx() takes
+ * time to sum the array, and might in fact be interrupted or preempted
+ * partway through the summation.
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
- int cpu;
+ unsigned long seq;
+
+ seq = srcu_readers_seq_idx(sp, idx);
+
+ /*
+ * The following smp_mb() A pairs with the smp_mb() B located in
+ * __srcu_read_lock(). This pairing ensures that if an
+ * __srcu_read_lock() increments its counter after the summation
+ * in srcu_readers_active_idx(), then the corresponding SRCU read-side
+ * critical section will see any changes made prior to the start
+ * of the current SRCU grace period.
+ *
+ * Also, if the above call to srcu_readers_seq_idx() saw the
+ * increment of ->seq[], then the call to srcu_readers_active_idx()
+ * must see the increment of ->c[].
+ */
+ smp_mb(); /* A */
/*
* Note that srcu_readers_active_idx() can incorrectly return
* zero even though there is a pre-existing reader throughout.
* To see this, suppose that task A is in a very long SRCU
* read-side critical section that started on CPU 0, and that
- * no other reader exists, so that the modulo sum of the counters
+ * no other reader exists, so that the sum of the counters
* is equal to one. Then suppose that task B starts executing
* srcu_readers_active_idx(), summing up to CPU 1, and then that
* task C starts reading on CPU 0, so that its increment is not
return false;
/*
- * Since the caller recently flipped ->completed, we can see at
- * most one increment of each CPU's counter from this point
- * forward. The reason for this is that the reader CPU must have
- * fetched the index before srcu_readers_active_idx checked
- * that CPU's counter, but not yet incremented its counter.
- * Its eventual counter increment will follow the read in
- * srcu_readers_active_idx(), and that increment is immediately
- * followed by smp_mb() B. Because smp_mb() D is between
- * the ->completed flip and srcu_readers_active_idx()'s read,
- * that CPU's subsequent load of ->completed must see the new
- * value, and therefore increment the counter in the other rank.
- */
- smp_mb(); /* A */
-
- /*
- * Now, we check the ->snap array that srcu_readers_active_idx()
- * filled in from the per-CPU counter values. Since
- * __srcu_read_lock() increments the upper bits of the per-CPU
- * counter, an increment/decrement pair will change the value
- * of the counter. Since there is only one possible increment,
- * the only way to wrap the counter is to have a huge number of
- * counter decrements, which requires a huge number of tasks and
- * huge SRCU read-side critical-section nesting levels, even on
- * 32-bit systems.
+ * The remainder of this function is the validation step.
+ * The following smp_mb() D pairs with the smp_mb() C in
+ * __srcu_read_unlock(). If the __srcu_read_unlock() was seen
+ * by srcu_readers_active_idx() above, then any destructive
+ * operation performed after the grace period will happen after
+ * the corresponding SRCU read-side critical section.
*
- * All of the ways of confusing the readings require that the scan
- * in srcu_readers_active_idx() see the read-side task's decrement,
- * but not its increment. However, between that decrement and
- * increment are smb_mb() B and C. Either or both of these pair
- * with smp_mb() A above to ensure that the scan below will see
- * the read-side tasks's increment, thus noting a difference in
- * the counter values between the two passes.
- *
- * Therefore, if srcu_readers_active_idx() returned zero, and
- * none of the counters changed, we know that the zero was the
- * correct sum.
- *
- * Of course, it is possible that a task might be delayed
- * for a very long time in __srcu_read_lock() after fetching
- * the index but before incrementing its counter. This
- * possibility will be dealt with in __synchronize_srcu().
+ * Note that there can be at most NR_CPUS worth of readers using
+ * the old index, which is not enough to overflow even a 32-bit
+ * integer. (Yes, this does mean that systems having more than
+ * a billion or so CPUs need to be 64-bit systems.) Therefore,
+ * the sum of the ->seq[] counters cannot possibly overflow.
+ * Therefore, the only way that the return values of the two
+ * calls to srcu_readers_seq_idx() can be equal is if there were
+ * no increments of the corresponding rank of ->seq[] counts
+ * in the interim. But the missed-increment scenario laid out
+ * above includes an increment of the ->seq[] counter by
+ * the corresponding __srcu_read_lock(). Therefore, if this
+ * scenario occurs, the return values from the two calls to
+ * srcu_readers_seq_idx() will differ, and thus the validation
+ * step below suffices.
*/
- for_each_possible_cpu(cpu)
- if (sp->snap[cpu] !=
- ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]))
- return false; /* False zero reading! */
- return true;
+ smp_mb(); /* D */
+
+ return srcu_readers_seq_idx(sp, idx) == seq;
}
/**
preempt_disable();
idx = rcu_dereference_index_check(sp->completed,
rcu_read_lock_sched_held()) & 0x1;
- ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) +=
- SRCU_USAGE_COUNT + 1;
+ ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
+ ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
preempt_enable();
return idx;
}
{
int trycount = 0;
- /*
- * If a reader fetches the index before the ->completed increment,
- * but increments its counter after srcu_readers_active_idx_check()
- * sums it, then smp_mb() D will pair with __srcu_read_lock()'s
- * smp_mb() B to ensure that the SRCU read-side critical section
- * will see any updates that the current task performed before its
- * call to synchronize_srcu(), or to synchronize_srcu_expedited(),
- * as the case may be.
- */
- smp_mb(); /* D */
-
/*
* SRCU read-side critical sections are normally short, so wait
* a small amount of time before possibly blocking.
schedule_timeout_interruptible(1);
}
}
-
- /*
- * The following smp_mb() E pairs with srcu_read_unlock()'s
- * smp_mb C to ensure that if srcu_readers_active_idx_check()
- * sees srcu_read_unlock()'s counter decrement, then any
- * of the current task's subsequent code will happen after
- * that SRCU read-side critical section.
- *
- * It also ensures the order between the above waiting and
- * the next flipping.
- */
- smp_mb(); /* E */
}
static void srcu_flip(struct srcu_struct *sp)
projects / karo-tx-linux.git / commitdiff