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srcu: Update comment after the index flip
Because there is not guaranteed to be a full memory barrier between the ->srcu_unlock_count increment of an srcu_read_unlock() and the ->srcu_lock_count increment of the next srcu_read_lock(), this next srcu_read_lock() is not guaranteed to see the effect of the index flip just prior to this comment. However, this next srcu_read_lock() will execute a full memory barrier, so the srcu_read_lock() after that is guaranteed to see that index flip. This guarantee is illustrated by the following diagram of events and the litmus test following that. ------------------------------------------------------------------------ READER UPDATER ------------- ---------- // idx is initially 0. srcu_flip() { smp_mb(); // RSCS srcu_read_unlock() { smp_mb(); idx++; // P smp_mb(); // QQ } srcu_readers_unlock_idx(0) { ,--counted------------ count all unlock[0]; // Q | unlock[0]++; // X } smp_mb(); srcu_read_lock() { READ(idx) = 0; ,---- count all lock[0]; // contributes imbalance of 1. lock[0]++; ----counted | smp_mb(); // PP } | } | | // RSCS not going to effect above scan | srcu_read_unlock() { | smp_mb(); | unlock[0]++; | } | / / srcu_read_lock() { | READ(idx); // Y -----cannot be counted because of P (has to sample idx as 1) lock[1]++; ... } ------------------------------------------------------------------------ This makes it similar to the store buffer pattern. Using X, Y, P and Q annotated above, we get: ------------------------------------------------------------------------ READER UPDATER X (write) P (write) smp_mb(); //PP smp_mb(); //QQ Y (read) Q (read) ------------------------------------------------------------------------ ASCII art courtesy of Joel Fernandes. Reported-by: Joel Fernandes <joel@joelfernandes.org> Reported-by: Boqun Feng <boqun.feng@gmail.com> Reported-by: Frederic Weisbecker <frederic@kernel.org> Reported-by: Neeraj Upadhyay <quic_neeraju@quicinc.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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@ -1098,10 +1098,11 @@ static void srcu_flip(struct srcu_struct *ssp)
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/*
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* Ensure that if the updater misses an __srcu_read_unlock()
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* increment, that task's next __srcu_read_lock() will see the
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* above counter update. Note that both this memory barrier
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* and the one in srcu_readers_active_idx_check() provide the
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* guarantee for __srcu_read_lock().
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* increment, that task's __srcu_read_lock() following its next
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* __srcu_read_lock() or __srcu_read_unlock() will see the above
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* counter update. Note that both this memory barrier and the
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* one in srcu_readers_active_idx_check() provide the guarantee
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* for __srcu_read_lock().
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*/
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smp_mb(); /* D */ /* Pairs with C. */
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}
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