> -----Original Message-----
> From: Stephen Hemminger <[email protected]>
> Sent: Friday, September 18, 2026 4:10 AM
> To: [email protected]
> Cc: Stephen Hemminger <[email protected]>; Wei Hu
> <[email protected]>
> Subject: [EXTERNAL] [PATCH v8 16/25] bus/vmbus: convert from rte_atomic
> to stdatomic
>
> The tx ring producer uses two cursors:
> - tbr->windex which is driver private. It lets producers reserve space
> concurrently without a lock
> - vbr->windex is the host visible commit cursor and has to advance in
> reservation order so the host never sees it point past unwritten data.
>
> Convert the reservation CAS on tbr->windex to a weak compare exchange, and
> replace the smp_wmb plus CAS spin that published the commit with
> rte_wait_until_equal_32 on the previous producer followed by a release store.
> The release ordering on that store covers what the barrier did.
>
> The wait on the previous producer uses acquire ordering, not relaxed.
> A release store only carries the writes of the storing thread and anything it
> has
> a happens-before edge to. Producers publish in reservation order, so when
> this thread stores its own windex the host may then consume the data of
> every earlier producer as well. The acquire on the wait pairs with the earlier
> producer's release store, putting that producer's data writes into this
> thread's
> happens-before set so they are carried by this store too.
>
> vbr->windex stays volatile uint32_t in the packed bufring struct.
> The host is not a C11 thread.
>
> The atomic qualifier is applied by cast at the single publishing store; the
> (uintptr_t) launder there avoids a bogus misaligned atomic warning from the
> packed attribute, since windex sits at offset 0 of a page aligned struct.
>
> Signed-off-by: Stephen Hemminger <[email protected]>
Reviewed-by: Wei Hu <[email protected]>
> ---
> drivers/bus/vmbus/private.h | 2 +-
> drivers/bus/vmbus/vmbus_bufring.c | 40 +++++++++++++++++--------------
> 2 files changed, 23 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/bus/vmbus/private.h b/drivers/bus/vmbus/private.h index
> 6efac86b77..6b7782724f 100644
> --- a/drivers/bus/vmbus/private.h
> +++ b/drivers/bus/vmbus/private.h
> @@ -25,7 +25,7 @@ extern int vmbus_logtype_bus; struct vmbus_br {
> struct vmbus_bufring *vbr;
> uint32_t dsize;
> - uint32_t windex; /* next available location */
> + RTE_ATOMIC(uint32_t) windex; /* next available location */
> };
>
> #define UIO_NAME_MAX 64
> diff --git a/drivers/bus/vmbus/vmbus_bufring.c
> b/drivers/bus/vmbus/vmbus_bufring.c
> index fcb97287dc..dcd66390a3 100644
> --- a/drivers/bus/vmbus/vmbus_bufring.c
> +++ b/drivers/bus/vmbus/vmbus_bufring.c
> @@ -15,7 +15,7 @@
> #include <rte_tailq.h>
> #include <rte_log.h>
> #include <rte_malloc.h>
> -#include <rte_atomic.h>
> +#include <rte_stdatomic.h>
> #include <rte_memory.h>
> #include <rte_pause.h>
> #include <rte_bus_vmbus.h>
> @@ -121,17 +121,13 @@ vmbus_txbr_write(struct vmbus_br *tbr, const
> struct iovec iov[], int iovlen,
> total += iov[i].iov_len;
> total += sizeof(save_windex);
>
> + /* Get current free location */
> + old_windex = rte_atomic_load_explicit(&tbr->windex,
> + rte_memory_order_relaxed);
> +
> /* Reserve space in ring */
> do {
> - uint32_t avail;
> -
> - /* Get current free location */
> - old_windex = tbr->windex;
> -
> - /* Prevent compiler reordering this with calculation */
> - rte_compiler_barrier();
> -
> - avail = vmbus_br_availwrite(tbr, old_windex);
> + uint32_t avail = vmbus_br_availwrite(tbr, old_windex);
>
> /* If not enough space in ring, then tell caller. */
> if (avail <= total)
> @@ -139,8 +135,12 @@ vmbus_txbr_write(struct vmbus_br *tbr, const
> struct iovec iov[], int iovlen,
>
> next_windex = vmbus_br_idxinc(old_windex, total, ring_size);
>
> - /* Atomic update of next write_index for other threads */
> - } while (!rte_atomic32_cmpset(&tbr->windex, old_windex,
> next_windex));
> + /* Atomic update of next write_index for other threads
> + * Can use weak since easy to recompute and retry.
> + */
> + } while (!rte_atomic_compare_exchange_weak_explicit(
> + &tbr->windex, &old_windex, next_windex,
> + rte_memory_order_acquire,
> rte_memory_order_relaxed));
>
> /* Space from old..new is now reserved */
> windex = old_windex;
> @@ -157,12 +157,15 @@ vmbus_txbr_write(struct vmbus_br *tbr, const
> struct iovec iov[], int iovlen,
> /* The region reserved should match region used */
> RTE_ASSERT(windex == next_windex);
>
> - /* Ensure that data is available before updating host index */
> - rte_smp_wmb();
> + /* Wait for previous producer to publish their windex update */
> + rte_wait_until_equal_32(&vbr->windex, old_windex,
> +rte_memory_order_acquire);
>
> - /* Checkin for our reservation. wait for our turn to update host */
> - while (!rte_atomic32_cmpset(&vbr->windex, old_windex,
> next_windex))
> - rte_pause();
> + /* Publish our windex update; prior data writes ordered via release.
> + * windex is 4-byte aligned in practice (struct is page-aligned, windex
> + * at offset 0); cast launders the packed-struct alignment-1 attribute.
> + */
> + rte_atomic_store_explicit((volatile __rte_atomic uint32_t
> *)(uintptr_t)&vbr->windex,
> + next_windex, rte_memory_order_release);
>
> /* If host had read all data before this, then need to signal */
> *need_sig |= vmbus_txbr_need_signal(vbr, old_windex); @@ -224,7
> +227,8 @@ vmbus_rxbr_read(struct vmbus_br *rbr, void *data, size_t dlen,
> size_t skip)
> return -EAGAIN;
>
> /* Record where host was when we started read (for debug) */
> - rbr->windex = rbr->vbr->windex;
> + rte_atomic_store_explicit(&rbr->windex, rbr->vbr->windex,
> + rte_memory_order_relaxed);
>
> /*
> * Copy channel packet from RX bufring.
> --
> 2.53.0
