eventfd: document lockless access in eventfd_poll
Since commit e22553e2 ("eventfd: don't take the spinlock in eventfd_poll", 2015-02-17), eventfd is reading ctx->count outside ctx->wqh.lock. However, things aren't as simple as the read barrier in eventfd_poll would suggest. In fact, the read barrier, besides lacking a comment, is not paired in any obvious manner with another read barrier, and it is pointless because it is sitting between a write (deep in poll_wait) and the read of ctx->count. The read barrier is acting just as a compiler barrier, for which we can use READ_ONCE instead. This is what the code change in this patch does. The documentation change is just as important, however. The question, posed by Andrea Arcangeli, is then why the thing is safe on architectures where spin_unlock does not imply a store-load memory barrier. The answer is that it's safe because writes of ctx->count use the same lock as poll_wait, and hence an acquire barrier implicit in poll_wait provides the necessary synchronization between eventfd_poll and callers of wake_up_locked_poll. This is sort of mentioned in the commit message with respect to eventfd_ctx_read ("eventfd_read is similar, it will do a single decrement with the lock held") but it applies to all other callers too. It's tricky enough that it should be documented in the code. Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com> Reviewed-by: NAndrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Davide Libenzi <davidel@xmailserver.org> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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