doc: Update data-structure documentation for ->gp_seq
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Paul E. McKenney
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1 changed files with 63 additions and 55 deletions
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@ -380,31 +380,26 @@ and therefore need no protection.
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as follows:
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<pre>
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1 unsigned long gpnum;
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2 unsigned long completed;
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1 unsigned long gp_seq;
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</pre>
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<p>RCU grace periods are numbered, and
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the <tt>->gpnum</tt> field contains the number of the grace
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period that started most recently.
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The <tt>->completed</tt> field contains the number of the
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grace period that completed most recently.
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If the two fields are equal, the RCU grace period that most recently
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started has already completed, and therefore the corresponding
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flavor of RCU is idle.
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If <tt>->gpnum</tt> is one greater than <tt>->completed</tt>,
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then <tt>->gpnum</tt> gives the number of the current RCU
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grace period, which has not yet completed.
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Any other combination of values indicates that something is broken.
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These two fields are protected by the root <tt>rcu_node</tt>'s
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the <tt>->gp_seq</tt> field contains the current grace-period
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sequence number.
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The bottom two bits are the state of the current grace period,
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which can be zero for not yet started or one for in progress.
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In other words, if the bottom two bits of <tt>->gp_seq</tt> are
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zero, the corresponding flavor of RCU is idle.
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Any other value in the bottom two bits indicates that something is broken.
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This field is protected by the root <tt>rcu_node</tt> structure's
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<tt>->lock</tt> field.
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</p><p>There are <tt>->gpnum</tt> and <tt>->completed</tt> fields
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</p><p>There are <tt>->gp_seq</tt> fields
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in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
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as well.
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The fields in the <tt>rcu_state</tt> structure represent the
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most current values, and those of the other structures are compared
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in order to detect the start of a new grace period in a distributed
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most current value, and those of the other structures are compared
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in order to detect the beginnings and ends of grace periods in a distributed
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fashion.
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The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
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(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
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@ -512,27 +507,47 @@ than to be heisenbugged out of existence.
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as follows:
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<pre>
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1 unsigned long gpnum;
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2 unsigned long completed;
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1 unsigned long gp_seq;
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2 unsigned long gp_seq_needed;
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</pre>
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<p>These fields are the counterparts of the fields of the same name in
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the <tt>rcu_state</tt> structure.
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They each may lag up to one behind their <tt>rcu_state</tt>
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counterparts.
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If a given <tt>rcu_node</tt> structure's <tt>->gpnum</tt> and
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<tt>->complete</tt> fields are equal, then this <tt>rcu_node</tt>
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<p>The <tt>rcu_node</tt> structures' <tt>->gp_seq</tt> fields are
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the counterparts of the field of the same name in the <tt>rcu_state</tt>
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structure.
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They each may lag up to one step behind their <tt>rcu_state</tt>
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counterpart.
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If the bottom two bits of a given <tt>rcu_node</tt> structure's
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<tt>->gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
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structure believes that RCU is idle.
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Otherwise, as with the <tt>rcu_state</tt> structure,
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the <tt>->gpnum</tt> field will be one greater than the
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<tt>->complete</tt> fields, with <tt>->gpnum</tt>
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indicating which grace period this <tt>rcu_node</tt> believes
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is still being waited for.
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</p><p>The <tt>>gp_seq</tt> field of each <tt>rcu_node</tt>
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structure is updated at the beginning and the end
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of each grace period.
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</p><p>The <tt>>gpnum</tt> field of each <tt>rcu_node</tt>
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structure is updated at the beginning
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of each grace period, and the <tt>->completed</tt> fields are
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updated at the end of each grace period.
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<p>The <tt>->gp_seq_needed</tt> fields record the
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furthest-in-the-future grace period request seen by the corresponding
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<tt>rcu_node</tt> structure. The request is considered fulfilled when
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the value of the <tt>->gp_seq</tt> field equals or exceeds that of
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the <tt>->gp_seq_needed</tt> field.
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<table>
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<tr><th> </th></tr>
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<tr><th align="left">Quick Quiz:</th></tr>
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<tr><td>
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Suppose that this <tt>rcu_node</tt> structure doesn't see
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a request for a very long time.
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Won't wrapping of the <tt>->gp_seq</tt> field cause
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problems?
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</td></tr>
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<tr><th align="left">Answer:</th></tr>
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<tr><td bgcolor="#ffffff"><font color="ffffff">
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No, because if the <tt>->gp_seq_needed</tt> field lags behind the
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<tt>->gp_seq</tt> field, the <tt>->gp_seq_needed</tt> field
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will be updated at the end of the grace period.
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Modulo-arithmetic comparisons therefore will always get the
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correct answer, even with wrapping.
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</font></td></tr>
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<tr><td> </td></tr>
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</table>
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<h5>Quiescent-State Tracking</h5>
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@ -626,9 +641,8 @@ normal and expedited grace periods, respectively.
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</ol>
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<p><font color="ffffff">So the locking is absolutely required in
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order to coordinate
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clearing of the bits with the grace-period numbers in
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<tt>->gpnum</tt> and <tt>->completed</tt>.
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order to coordinate clearing of the bits with updating of the
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grace-period sequence number in <tt>->gp_seq</tt>.
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</font></td></tr>
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<tr><td> </td></tr>
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</table>
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@ -1038,15 +1052,15 @@ out any <tt>rcu_data</tt> structure for which this flag is not set.
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as follows:
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<pre>
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1 unsigned long completed;
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2 unsigned long gpnum;
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1 unsigned long gp_seq;
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2 unsigned long gp_seq_needed;
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3 bool cpu_no_qs;
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4 bool core_needs_qs;
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5 bool gpwrap;
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6 unsigned long rcu_qs_ctr_snap;
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</pre>
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<p>The <tt>completed</tt> and <tt>gpnum</tt>
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<p>The <tt>->gp_seq</tt> and <tt>->gp_seq_needed</tt>
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fields are the counterparts of the fields of the same name
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in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
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They may each lag up to one behind their <tt>rcu_node</tt>
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@ -1054,15 +1068,9 @@ counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
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<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
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arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
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will catch up upon exit from dyntick-idle mode).
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If a given <tt>rcu_data</tt> structure's <tt>->gpnum</tt> and
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<tt>->complete</tt> fields are equal, then this <tt>rcu_data</tt>
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If the lower two bits of a given <tt>rcu_data</tt> structure's
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<tt>->gp_seq</tt> are zero, then this <tt>rcu_data</tt>
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structure believes that RCU is idle.
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Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
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structure,
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the <tt>->gpnum</tt> field will be one greater than the
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<tt>->complete</tt> fields, with <tt>->gpnum</tt>
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indicating which grace period this <tt>rcu_data</tt> believes
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is still being waited for.
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<table>
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<tr><th> </th></tr>
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@ -1070,13 +1078,13 @@ is still being waited for.
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<tr><td>
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All this replication of the grace period numbers can only cause
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massive confusion.
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Why not just keep a global pair of counters and be done with it???
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Why not just keep a global sequence number and be done with it???
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</td></tr>
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<tr><th align="left">Answer:</th></tr>
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<tr><td bgcolor="#ffffff"><font color="ffffff">
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Because if there was only a single global pair of grace-period
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Because if there was only a single global sequence
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numbers, there would need to be a single global lock to allow
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safely accessing and updating them.
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safely accessing and updating it.
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And if we are not going to have a single global lock, we need
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to carefully manage the numbers on a per-node basis.
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Recall from the answer to a previous Quick Quiz that the consequences
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@ -1091,8 +1099,8 @@ CPU has not yet passed through a quiescent state,
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while the <tt>->core_needs_qs</tt> flag indicates that the
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RCU core needs a quiescent state from the corresponding CPU.
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The <tt>->gpwrap</tt> field indicates that the corresponding
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CPU has remained idle for so long that the <tt>completed</tt>
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and <tt>gpnum</tt> counters are in danger of overflow, which
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CPU has remained idle for so long that the
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<tt>gp_seq</tt> counter is in danger of overflow, which
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will cause the CPU to disregard the values of its counters on
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its next exit from idle.
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Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
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@ -1130,10 +1138,10 @@ The CPU advances the callbacks in its <tt>rcu_data</tt> structure
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whenever it notices that another RCU grace period has completed.
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The CPU detects the completion of an RCU grace period by noticing
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that the value of its <tt>rcu_data</tt> structure's
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<tt>->completed</tt> field differs from that of its leaf
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<tt>->gp_seq</tt> field differs from that of its leaf
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<tt>rcu_node</tt> structure.
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Recall that each <tt>rcu_node</tt> structure's
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<tt>->completed</tt> field is updated at the end of each
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<tt>->gp_seq</tt> field is updated at the beginnings and ends of each
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grace period.
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<p>
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