diff options
| -rw-r--r-- | kernel/rcu/srcutree.c | 33 |
1 files changed, 24 insertions, 9 deletions
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 157654fa436a..fceca84df6b0 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -275,15 +275,20 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx) * not mean that there are no more readers, as one could have read * the current index but not have incremented the lock counter yet. * - * Possible bug: There is no guarantee that there haven't been - * ULONG_MAX increments of ->srcu_lock_count[] since the unlocks were - * counted, meaning that this could return true even if there are - * still active readers. Since there are no memory barriers around - * srcu_flip(), the CPU is not required to increment ->srcu_idx - * before running srcu_readers_unlock_idx(), which means that there - * could be an arbitrarily large number of critical sections that - * execute after srcu_readers_unlock_idx() but use the old value - * of ->srcu_idx. + * So suppose that the updater is preempted here for so long + * that more than ULONG_MAX non-nested readers come and go in + * the meantime. It turns out that this cannot result in overflow + * because if a reader modifies its unlock count after we read it + * above, then that reader's next load of ->srcu_idx is guaranteed + * to get the new value, which will cause it to operate on the + * other bank of counters, where it cannot contribute to the + * overflow of these counters. This means that there is a maximum + * of 2*NR_CPUS increments, which cannot overflow given current + * systems, especially not on 64-bit systems. + * + * OK, how about nesting? This does impose a limit on nesting + * of floor(ULONG_MAX/NR_CPUS/2), which should be sufficient, + * especially on 64-bit systems. */ return srcu_readers_lock_idx(sp, idx) == unlocks; } @@ -671,6 +676,16 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount) */ static void srcu_flip(struct srcu_struct *sp) { + /* + * Ensure that if this updater saw a given reader's increment + * from __srcu_read_lock(), that reader was using an old value + * of ->srcu_idx. Also ensure that if a given reader sees the + * new value of ->srcu_idx, this updater's earlier scans cannot + * have seen that reader's increments (which is OK, because this + * grace period need not wait on that reader). + */ + smp_mb(); /* E */ /* Pairs with B and C. */ + WRITE_ONCE(sp->srcu_idx, sp->srcu_idx + 1); /* |