From 8dd48650b43dfde4ebea34191ac267e474bcc29e Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Mon, 6 Mar 2023 10:15:06 +0100 Subject: [PATCH] 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 Signed-off-by: Paolo Bonzini --- util/async.c | 33 +++++++++++++++++++-------------- 1 file changed, 19 insertions(+), 14 deletions(-) diff --git a/util/async.c b/util/async.c index 0657b75397..e4b494150e 100644 --- a/util/async.c +++ b/util/async.c @@ -74,14 +74,21 @@ 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); } @@ -107,11 +114,8 @@ 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)); @@ -158,6 +162,7 @@ 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); @@ -448,15 +453,15 @@ 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)) {