Re: [PATCH] Add FUTEX_CMP_REQUEUE futex op

[Andrew Morton]

Jakub Jelinek <jakub@xxxxxxxxxx> wrote:
>
> The patch below extends FUTEX_REQUEUE operation with something FUTEX_WAIT
> already uses:
> FUTEX_CMP_REQUEUE is passed an additional argument which is the expected
> value of *futex.  Kernel then while holding the futex locks checks if

                                      ^^^^^^^^^^^^^^^^^^^^^^^

But in your patch the check is happening _prior_ to taking the futex locks.

> *futex != expected and returns -EAGAIN in that case, while if it is equal,
> continues with a normal FUTEX_REQUEUE operation.
> If the syscall returns -EAGAIN, NPTL can fall back to FUTEX_WAKE INT_MAX
> operation which doesn't have this problem, but is less efficient, while
> in the likely case that nobody hit the (small) window the efficient
> FUTEX_REQUEUE operation is used.

You've added an smp_mb().  These things are becoming like lock_kernel() -
hard for the reader to discern what it's protecting against.

Please always include a comment when adding a barrier like this.
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