as follows:
<pre>
- 1 unsigned long gpnum;
- 2 unsigned long completed;
+ 1 unsigned long gp_seq;
</pre>
<p>RCU grace periods are numbered, and
-the <tt>->gpnum</tt> field contains the number of the grace
-period that started most recently.
-The <tt>->completed</tt> field contains the number of the
-grace period that completed most recently.
-If the two fields are equal, the RCU grace period that most recently
-started has already completed, and therefore the corresponding
-flavor of RCU is idle.
-If <tt>->gpnum</tt> is one greater than <tt>->completed</tt>,
-then <tt>->gpnum</tt> gives the number of the current RCU
-grace period, which has not yet completed.
-Any other combination of values indicates that something is broken.
-These two fields are protected by the root <tt>rcu_node</tt>'s
+the <tt>->gp_seq</tt> field contains the current grace-period
+sequence number.
+The bottom two bits are the state of the current grace period,
+which can be zero for not yet started or one for in progress.
+In other words, if the bottom two bits of <tt>->gp_seq</tt> are
+zero, the corresponding flavor of RCU is idle.
+Any other value in the bottom two bits indicates that something is broken.
+This field is protected by the root <tt>rcu_node</tt> structure's
<tt>->lock</tt> field.
-</p><p>There are <tt>->gpnum</tt> and <tt>->completed</tt> fields
+</p><p>There are <tt>->gp_seq</tt> fields
in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
as well.
The fields in the <tt>rcu_state</tt> structure represent the
-most current values, and those of the other structures are compared
-in order to detect the start of a new grace period in a distributed
+most current value, and those of the other structures are compared
+in order to detect the beginnings and ends of grace periods in a distributed
fashion.
The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
as follows:
<pre>
- 1 unsigned long gpnum;
- 2 unsigned long completed;
+ 1 unsigned long gp_seq;
+ 2 unsigned long gp_seq_needed;
</pre>
-<p>These fields are the counterparts of the fields of the same name in
-the <tt>rcu_state</tt> structure.
-They each may lag up to one behind their <tt>rcu_state</tt>
-counterparts.
-If a given <tt>rcu_node</tt> structure's <tt>->gpnum</tt> and
-<tt>->complete</tt> fields are equal, then this <tt>rcu_node</tt>
+<p>The <tt>rcu_node</tt> structures' <tt>->gp_seq</tt> fields are
+the counterparts of the field of the same name in the <tt>rcu_state</tt>
+structure.
+They each may lag up to one step behind their <tt>rcu_state</tt>
+counterpart.
+If the bottom two bits of a given <tt>rcu_node</tt> structure's
+<tt>->gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
structure believes that RCU is idle.
-Otherwise, as with the <tt>rcu_state</tt> structure,
-the <tt>->gpnum</tt> field will be one greater than the
-<tt>->complete</tt> fields, with <tt>->gpnum</tt>
-indicating which grace period this <tt>rcu_node</tt> believes
-is still being waited for.
+</p><p>The <tt>>gp_seq</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning and the end
+of each grace period.
+
+<p>The <tt>->gp_seq_needed</tt> fields record the
+furthest-in-the-future grace period request seen by the corresponding
+<tt>rcu_node</tt> structure. The request is considered fulfilled when
+the value of the <tt>->gp_seq</tt> field equals or exceeds that of
+the <tt>->gp_seq_needed</tt> field.
-</p><p>The <tt>>gpnum</tt> field of each <tt>rcu_node</tt>
-structure is updated at the beginning
-of each grace period, and the <tt>->completed</tt> fields are
-updated at the end of each grace period.
+<table>
+<tr><th> </th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+ Suppose that this <tt>rcu_node</tt> structure doesn't see
+ a request for a very long time.
+ Won't wrapping of the <tt>->gp_seq</tt> field cause
+ problems?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+ No, because if the <tt>->gp_seq_needed</tt> field lags behind the
+ <tt>->gp_seq</tt> field, the <tt>->gp_seq_needed</tt> field
+ will be updated at the end of the grace period.
+ Modulo-arithmetic comparisons therefore will always get the
+ correct answer, even with wrapping.
+</font></td></tr>
+<tr><td> </td></tr>
+</table>
<h5>Quiescent-State Tracking</h5>
</ol>
<p><font color="ffffff">So the locking is absolutely required in
- order to coordinate
- clearing of the bits with the grace-period numbers in
- <tt>->gpnum</tt> and <tt>->completed</tt>.
+ order to coordinate clearing of the bits with updating of the
+ grace-period sequence number in <tt>->gp_seq</tt>.
</font></td></tr>
<tr><td> </td></tr>
</table>
as follows:
<pre>
- 1 unsigned long completed;
- 2 unsigned long gpnum;
+ 1 unsigned long gp_seq;
+ 2 unsigned long gp_seq_needed;
3 bool cpu_no_qs;
4 bool core_needs_qs;
5 bool gpwrap;
6 unsigned long rcu_qs_ctr_snap;
</pre>
-<p>The <tt>completed</tt> and <tt>gpnum</tt>
+<p>The <tt>->gp_seq</tt> and <tt>->gp_seq_needed</tt>
fields are the counterparts of the fields of the same name
in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
They may each lag up to one behind their <tt>rcu_node</tt>
<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
will catch up upon exit from dyntick-idle mode).
-If a given <tt>rcu_data</tt> structure's <tt>->gpnum</tt> and
-<tt>->complete</tt> fields are equal, then this <tt>rcu_data</tt>
+If the lower two bits of a given <tt>rcu_data</tt> structure's
+<tt>->gp_seq</tt> are zero, then this <tt>rcu_data</tt>
structure believes that RCU is idle.
-Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
-structure,
-the <tt>->gpnum</tt> field will be one greater than the
-<tt>->complete</tt> fields, with <tt>->gpnum</tt>
-indicating which grace period this <tt>rcu_data</tt> believes
-is still being waited for.
<table>
<tr><th> </th></tr>
<tr><td>
All this replication of the grace period numbers can only cause
massive confusion.
- Why not just keep a global pair of counters and be done with it???
+ Why not just keep a global sequence number and be done with it???
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
- Because if there was only a single global pair of grace-period
+ Because if there was only a single global sequence
numbers, there would need to be a single global lock to allow
- safely accessing and updating them.
+ safely accessing and updating it.
And if we are not going to have a single global lock, we need
to carefully manage the numbers on a per-node basis.
Recall from the answer to a previous Quick Quiz that the consequences
while the <tt>->core_needs_qs</tt> flag indicates that the
RCU core needs a quiescent state from the corresponding CPU.
The <tt>->gpwrap</tt> field indicates that the corresponding
-CPU has remained idle for so long that the <tt>completed</tt>
-and <tt>gpnum</tt> counters are in danger of overflow, which
+CPU has remained idle for so long that the
+<tt>gp_seq</tt> counter is in danger of overflow, which
will cause the CPU to disregard the values of its counters on
its next exit from idle.
Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
whenever it notices that another RCU grace period has completed.
The CPU detects the completion of an RCU grace period by noticing
that the value of its <tt>rcu_data</tt> structure's
-<tt>->completed</tt> field differs from that of its leaf
+<tt>->gp_seq</tt> field differs from that of its leaf
<tt>rcu_node</tt> structure.
Recall that each <tt>rcu_node</tt> structure's
-<tt>->completed</tt> field is updated at the end of each
+<tt>->gp_seq</tt> field is updated at the beginnings and ends of each
grace period.
<p>
projects / openwrt / staging / blogic.git / commitdiff