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91 lines
3.0 KiB
91 lines
3.0 KiB
1 year ago
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From 244e92fea388d2be9fe81a5c5912d92b8f599caa Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Thu, 23 Mar 2023 10:48:59 -0400
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Subject: [PATCH 1/2] aio-posix: fix race between epoll upgrade and
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aio_set_fd_handler()
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RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
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RH-MergeRequest: 292: aio-posix: fix race between epoll upgrade and aio_set_fd_handler()
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RH-Bugzilla: 2211923
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RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
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RH-Acked-by: Hanna Czenczek <hreitz@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Commit: [1/1] 182471bac79fa2b2ae8a34087eb6c4ab1af786e1
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If another thread calls aio_set_fd_handler() while the IOThread event
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loop is upgrading from ppoll(2) to epoll(7) then we might miss new
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AioHandlers. The epollfd will not monitor the new AioHandler's fd,
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resulting in hangs.
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Take the AioHandler list lock while upgrading to epoll. This prevents
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AioHandlers from changing while epoll is being set up. If we cannot lock
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because we're in a nested event loop, then don't upgrade to epoll (it
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will happen next time we're not in a nested call).
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The downside to taking the lock is that the aio_set_fd_handler() thread
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has to wait until the epoll upgrade is finished, which involves many
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epoll_ctl(2) system calls. However, this scenario is rare and I couldn't
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think of another solution that is still simple.
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Reported-by: Qing Wang <qinwang@redhat.com>
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Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2090998
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Cc: Paolo Bonzini <pbonzini@redhat.com>
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Cc: Fam Zheng <fam@euphon.net>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Message-Id: <20230323144859.1338495-1-stefanha@redhat.com>
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Reviewed-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit e62da98527fa35fe5f532cded01a33edf9fbe7b2)
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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---
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util/fdmon-epoll.c | 25 ++++++++++++++++++-------
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1 file changed, 18 insertions(+), 7 deletions(-)
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diff --git a/util/fdmon-epoll.c b/util/fdmon-epoll.c
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index e11a8a022e..1683aa1105 100644
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--- a/util/fdmon-epoll.c
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+++ b/util/fdmon-epoll.c
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@@ -127,6 +127,8 @@ static bool fdmon_epoll_try_enable(AioContext *ctx)
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bool fdmon_epoll_try_upgrade(AioContext *ctx, unsigned npfd)
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{
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+ bool ok;
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+
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if (ctx->epollfd < 0) {
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return false;
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}
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@@ -136,14 +138,23 @@ bool fdmon_epoll_try_upgrade(AioContext *ctx, unsigned npfd)
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return false;
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}
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- if (npfd >= EPOLL_ENABLE_THRESHOLD) {
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- if (fdmon_epoll_try_enable(ctx)) {
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- return true;
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- } else {
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- fdmon_epoll_disable(ctx);
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- }
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+ if (npfd < EPOLL_ENABLE_THRESHOLD) {
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+ return false;
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+ }
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+
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+ /* The list must not change while we add fds to epoll */
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+ if (!qemu_lockcnt_dec_if_lock(&ctx->list_lock)) {
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+ return false;
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+ }
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+
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+ ok = fdmon_epoll_try_enable(ctx);
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+
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+ qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
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+
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+ if (!ok) {
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+ fdmon_epoll_disable(ctx);
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}
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- return false;
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+ return ok;
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}
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void fdmon_epoll_setup(AioContext *ctx)
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--
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2.39.3
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