Rust, i386, accelerator changes for 2025-10-14

[PULL 13/28] async: access bottom half flags with qatomic_read

Paolo Bonzini posted 28 patches 1 month ago

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Maintainers: Paolo Bonzini <pbonzini@redhat.com>, "Dr. David Alan Gilbert" <dave@treblig.org>, Eduardo Habkost <eduardo@habkost.net>, Marcel Apfelbaum <marcel.apfelbaum@gmail.com>, "Philippe Mathieu-Daudé" <philmd@linaro.org>, Yanan Wang <wangyanan55@huawei.com>, Zhao Liu <zhao1.liu@intel.com>, "Michael S. Tsirkin" <mst@redhat.com>, Eric Blake <eblake@redhat.com>, Markus Armbruster <armbru@redhat.com>, Richard Henderson <richard.henderson@linaro.org>, Manos Pitsidianakis <manos.pitsidianakis@linaro.org>, Peter Maydell <peter.maydell@linaro.org>, Marcelo Tosatti <mtosatti@redhat.com>, Song Gao <gaosong@loongson.cn>, Huacai Chen <chenhuacai@kernel.org>, Aurelien Jarno <aurelien@aurel32.net>, Jiaxun Yang <jiaxun.yang@flygoat.com>, Aleksandar Rikalo <arikalo@gmail.com>, Nicholas Piggin <npiggin@gmail.com>, Harsh Prateek Bora <harshpb@linux.ibm.com>, Chinmay Rath <rathc@linux.ibm.com>, Palmer Dabbelt <palmer@dabbelt.com>, Alistair Francis <alistair.francis@wdc.com>, Weiwei Li <liwei1518@gmail.com>, Daniel Henrique Barboza <dbarboza@ventanamicro.com>, Liu Zhiwei <zhiwei_liu@linux.alibaba.com>, Halil Pasic <pasic@linux.ibm.com>, Christian Borntraeger <borntraeger@linux.ibm.com>, Eric Farman <farman@linux.ibm.com>, Matthew Rosato <mjrosato@linux.ibm.com>, Thomas Huth <thuth@redhat.com>, David Hildenbrand <david@redhat.com>, Ilya Leoshkevich <iii@linux.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>, Fam Zheng <fam@euphon.net>

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[PULL 13/28] async: access bottom half flags with qatomic_read

Posted by Paolo Bonzini 1 month ago

Running test-aio-multithread under TSAN reveals data races on bh->flags.
Because bottom halves may be scheduled or canceled asynchronously,
without taking a lock, adjust aio_compute_bh_timeout() and aio_ctx_check()
to use a relaxed read to access the flags.

Use an acquire load to ensure that anything that was written prior to
qemu_bh_schedule() is visible.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/2749
Closes: https://gitlab.com/qemu-project/qemu/-/issues/851
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 util/async.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/util/async.c b/util/async.c
index 2719c629ae9..a736d2cd0d0 100644
--- a/util/async.c
+++ b/util/async.c
@@ -256,8 +256,9 @@ static int64_t aio_compute_bh_timeout(BHList *head, int timeout)
     QEMUBH *bh;
 
     QSLIST_FOREACH_RCU(bh, head, next) {
-        if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
-            if (bh->flags & BH_IDLE) {
+        int flags = qatomic_load_acquire(&bh->flags);
+        if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
+            if (flags & BH_IDLE) {
                 /* idle bottom halves will be polled at least
                  * every 10ms */
                 timeout = 10000000;
@@ -335,14 +336,16 @@ aio_ctx_check(GSource *source)
     aio_notify_accept(ctx);
 
     QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {
-        if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
+        int flags = qatomic_load_acquire(&bh->flags);
+        if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
             return true;
         }
     }
 
     QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
         QSLIST_FOREACH_RCU(bh, &s->bh_list, next) {
-            if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
+            int flags = qatomic_load_acquire(&bh->flags);
+            if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
                 return true;
             }
         }
-- 
2.51.0