From 29bcf843d796ffc2a0906dea947e4cdfe9f7ec60 Mon Sep 17 00:00:00 2001

From: Emanuele Giuseppe Esposito <eesposit@redhat.com>

Date: Thu, 9 Mar 2023 08:24:36 -0500

Subject: [PATCH 11/12] async: update documentation of the memory barriers

RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>

RH-MergeRequest: 158: qatomic: add smp_mb__before/after_rmw()

RH-Bugzilla: 2175660

RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

RH-Acked-by: Cornelia Huck <cohuck@redhat.com>

RH-Acked-by: David Hildenbrand <david@redhat.com>

RH-Acked-by: Eric Auger <eric.auger@redhat.com>

RH-Commit: [8/9] 5ca20e4c8983e0bc1ecee66bead3472777abe4d1 (eesposit/qemu-kvm)

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175660

commit 8dd48650b43dfde4ebea34191ac267e474bcc29e

Author: Paolo Bonzini <pbonzini@redhat.com>

Date:   Mon Mar 6 10:15:06 2023 +0100

    async: update documentation of the memory barriers

    Ever since commit 8c6b0356b539 ("util/async: make bh_aio_poll() O(1)",

    2020-02-22), synchronization between qemu_bh_schedule() and aio_bh_poll()

    is happening when the bottom half is enqueued in the bh_list; not

    when the flags are set.  Update the documentation to match.

    Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

---

 util/async.c | 33 +++++++++++++++++++--------------

 1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/util/async.c b/util/async.c

index 63434ddae4..37d3e6036d 100644

--- a/util/async.c

+++ b/util/async.c

@@ -73,14 +73,21 @@ static void aio_bh_enqueue(QEMUBH *bh, unsigned new_flags)

     unsigned old_flags;

     /*

-     * The memory barrier implicit in qatomic_fetch_or makes sure that:

-     * 1. idle & any writes needed by the callback are done before the

-     *    locations are read in the aio_bh_poll.

-     * 2. ctx is loaded before the callback has a chance to execute and bh

-     *    could be freed.

+     * Synchronizes with atomic_fetch_and() in aio_bh_dequeue(), ensuring that

+     * insertion starts after BH_PENDING is set.

      */

     old_flags = qatomic_fetch_or(&bh->flags, BH_PENDING | new_flags);

+

     if (!(old_flags & BH_PENDING)) {

+        /*

+         * At this point the bottom half becomes visible to aio_bh_poll().

+         * This insertion thus synchronizes with QSLIST_MOVE_ATOMIC in

+         * aio_bh_poll(), ensuring that:

+         * 1. any writes needed by the callback are visible from the callback

+         *    after aio_bh_dequeue() returns bh.

+         * 2. ctx is loaded before the callback has a chance to execute and bh

+         *    could be freed.

+         */

         QSLIST_INSERT_HEAD_ATOMIC(&ctx->bh_list, bh, next);

     }

@@ -106,11 +113,8 @@ static QEMUBH *aio_bh_dequeue(BHList *head, unsigned *flags)

     QSLIST_REMOVE_HEAD(head, next);

     /*

-     * The qatomic_and is paired with aio_bh_enqueue().  The implicit memory

-     * barrier ensures that the callback sees all writes done by the scheduling

-     * thread.  It also ensures that the scheduling thread sees the cleared

-     * flag before bh->cb has run, and thus will call aio_notify again if

-     * necessary.

+     * Synchronizes with qatomic_fetch_or() in aio_bh_enqueue(), ensuring that

+     * the removal finishes before BH_PENDING is reset.

      */

     *flags = qatomic_fetch_and(&bh->flags,

                               ~(BH_PENDING | BH_SCHEDULED | BH_IDLE));

@@ -157,6 +161,7 @@ int aio_bh_poll(AioContext *ctx)

     BHListSlice *s;

     int ret = 0;

+    /* Synchronizes with QSLIST_INSERT_HEAD_ATOMIC in aio_bh_enqueue().  */

     QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);

     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);

@@ -446,15 +451,15 @@ LuringState *aio_get_linux_io_uring(AioContext *ctx)

 void aio_notify(AioContext *ctx)

 {

     /*

-     * Write e.g. bh->flags before writing ctx->notified.  Pairs with smp_mb in

-     * aio_notify_accept.

+     * Write e.g. ctx->bh_list before writing ctx->notified.  Pairs with

+     * smp_mb() in aio_notify_accept().

      */

     smp_wmb();

     qatomic_set(&ctx->notified, true);

     /*

-     * Write ctx->notified before reading ctx->notify_me.  Pairs

-     * with smp_mb in aio_ctx_prepare or aio_poll.

+     * Write ctx->notified (and also ctx->bh_list) before reading ctx->notify_me.

+     * Pairs with smp_mb() in aio_ctx_prepare or aio_poll.

      */

     smp_mb();

     if (qatomic_read(&ctx->notify_me)) {

-- 

2.39.1