112 lines
4.4 KiB
Diff
112 lines
4.4 KiB
Diff
From ea3856bb545d19499602830cdc3076d83a981e7a Mon Sep 17 00:00:00 2001
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From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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Date: Thu, 9 Mar 2023 08:15:36 -0500
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Subject: [PATCH 09/13] async: update documentation of the memory barriers
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RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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RH-MergeRequest: 263: qatomic: add smp_mb__before/after_rmw()
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RH-Bugzilla: 2168472
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RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
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RH-Acked-by: Eric Auger <eric.auger@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: David Hildenbrand <david@redhat.com>
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RH-Commit: [9/10] d471da2acf7a107cf75f3327c5e8d7456307160e
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Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472
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commit 8dd48650b43dfde4ebea34191ac267e474bcc29e
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Author: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon Mar 6 10:15:06 2023 +0100
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async: update documentation of the memory barriers
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Ever since commit 8c6b0356b539 ("util/async: make bh_aio_poll() O(1)",
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2020-02-22), synchronization between qemu_bh_schedule() and aio_bh_poll()
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is happening when the bottom half is enqueued in the bh_list; not
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when the flags are set. Update the documentation to match.
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Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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---
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util/async.c | 33 +++++++++++++++++++--------------
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1 file changed, 19 insertions(+), 14 deletions(-)
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diff --git a/util/async.c b/util/async.c
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index 6f6717a34b..795fe699b6 100644
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--- a/util/async.c
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+++ b/util/async.c
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@@ -71,14 +71,21 @@ static void aio_bh_enqueue(QEMUBH *bh, unsigned new_flags)
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unsigned old_flags;
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/*
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- * The memory barrier implicit in qatomic_fetch_or makes sure that:
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- * 1. idle & any writes needed by the callback are done before the
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- * locations are read in the aio_bh_poll.
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- * 2. ctx is loaded before the callback has a chance to execute and bh
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- * could be freed.
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+ * Synchronizes with atomic_fetch_and() in aio_bh_dequeue(), ensuring that
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+ * insertion starts after BH_PENDING is set.
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*/
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old_flags = qatomic_fetch_or(&bh->flags, BH_PENDING | new_flags);
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+
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if (!(old_flags & BH_PENDING)) {
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+ /*
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+ * At this point the bottom half becomes visible to aio_bh_poll().
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+ * This insertion thus synchronizes with QSLIST_MOVE_ATOMIC in
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+ * aio_bh_poll(), ensuring that:
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+ * 1. any writes needed by the callback are visible from the callback
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+ * after aio_bh_dequeue() returns bh.
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+ * 2. ctx is loaded before the callback has a chance to execute and bh
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+ * could be freed.
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+ */
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QSLIST_INSERT_HEAD_ATOMIC(&ctx->bh_list, bh, next);
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}
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@@ -97,11 +104,8 @@ static QEMUBH *aio_bh_dequeue(BHList *head, unsigned *flags)
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QSLIST_REMOVE_HEAD(head, next);
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/*
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- * The qatomic_and is paired with aio_bh_enqueue(). The implicit memory
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- * barrier ensures that the callback sees all writes done by the scheduling
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- * thread. It also ensures that the scheduling thread sees the cleared
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- * flag before bh->cb has run, and thus will call aio_notify again if
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- * necessary.
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+ * Synchronizes with qatomic_fetch_or() in aio_bh_enqueue(), ensuring that
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+ * the removal finishes before BH_PENDING is reset.
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*/
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*flags = qatomic_fetch_and(&bh->flags,
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~(BH_PENDING | BH_SCHEDULED | BH_IDLE));
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@@ -148,6 +152,7 @@ int aio_bh_poll(AioContext *ctx)
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BHListSlice *s;
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int ret = 0;
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+ /* Synchronizes with QSLIST_INSERT_HEAD_ATOMIC in aio_bh_enqueue(). */
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QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
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QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
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@@ -437,15 +442,15 @@ LuringState *aio_get_linux_io_uring(AioContext *ctx)
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void aio_notify(AioContext *ctx)
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{
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/*
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- * Write e.g. bh->flags before writing ctx->notified. Pairs with smp_mb in
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- * aio_notify_accept.
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+ * Write e.g. ctx->bh_list before writing ctx->notified. Pairs with
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+ * smp_mb() in aio_notify_accept().
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*/
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smp_wmb();
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qatomic_set(&ctx->notified, true);
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/*
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- * Write ctx->notified before reading ctx->notify_me. Pairs
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- * with smp_mb in aio_ctx_prepare or aio_poll.
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+ * Write ctx->notified (and also ctx->bh_list) before reading ctx->notify_me.
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+ * Pairs with smp_mb() in aio_ctx_prepare or aio_poll.
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*/
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smp_mb();
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if (qatomic_read(&ctx->notify_me)) {
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--
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2.37.3
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