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