From e0bf95443ee9326d44031373420cf9f3513ee255 Mon Sep 17 00:00:00 2001 From: Sergey Dyasli Date: Fri, 12 Jul 2024 09:26:59 +0000 Subject: [PATCH] Revert "qemu-char: do not operate on sources from finalize callbacks" This reverts commit 2b316774f60291f57ca9ecb6a9f0712c532cae34. After 038b4217884c ("Revert "chardev: use a child source for qio input source"") we've been observing the "iwp->src == NULL" assertion triggering periodically during the initial capabilities querying by libvirtd. One of possible backtraces: Thread 1 (Thread 0x7f16cd4f0700 (LWP 43858)): 0 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50 1 0x00007f16c6c21e65 in __GI_abort () at abort.c:79 2 0x00007f16c6c21d39 in __assert_fail_base at assert.c:92 3 0x00007f16c6c46e86 in __GI___assert_fail (assertion=assertion@entry=0x562e9bcdaadd "iwp->src == NULL", file=file@entry=0x562e9bcdaac8 "../chardev/char-io.c", line=line@entry=99, function=function@entry=0x562e9bcdab10 <__PRETTY_FUNCTION__.20549> "io_watch_poll_finalize") at assert.c:101 4 0x0000562e9ba20c2c in io_watch_poll_finalize (source=) at ../chardev/char-io.c:99 5 io_watch_poll_finalize (source=) at ../chardev/char-io.c:88 6 0x00007f16c904aae0 in g_source_unref_internal () from /lib64/libglib-2.0.so.0 7 0x00007f16c904baf9 in g_source_destroy_internal () from /lib64/libglib-2.0.so.0 8 0x0000562e9ba20db0 in io_remove_watch_poll (source=0x562e9d6720b0) at ../chardev/char-io.c:147 9 remove_fd_in_watch (chr=chr@entry=0x562e9d5f3800) at ../chardev/char-io.c:153 10 0x0000562e9ba23ffb in update_ioc_handlers (s=0x562e9d5f3800) at ../chardev/char-socket.c:592 11 0x0000562e9ba2072f in qemu_chr_fe_set_handlers_full at ../chardev/char-fe.c:279 12 0x0000562e9ba207a9 in qemu_chr_fe_set_handlers at ../chardev/char-fe.c:304 13 0x0000562e9ba2ca75 in monitor_qmp_setup_handlers_bh (opaque=0x562e9d4c2c60) at ../monitor/qmp.c:509 14 0x0000562e9bb6222e in aio_bh_poll (ctx=ctx@entry=0x562e9d4c2f20) at ../util/async.c:216 15 0x0000562e9bb4de0a in aio_poll (ctx=0x562e9d4c2f20, blocking=blocking@entry=true) at ../util/aio-posix.c:722 16 0x0000562e9b99dfaa in iothread_run (opaque=0x562e9d4c26f0) at ../iothread.c:63 17 0x0000562e9bb505a4 in qemu_thread_start (args=0x562e9d4c7ea0) at ../util/qemu-thread-posix.c:543 18 0x00007f16c70081ca in start_thread (arg=) at pthread_create.c:479 19 0x00007f16c6c398d3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95 io_remove_watch_poll(), which makes sure that iwp->src is NULL, calls g_source_destroy() which finds that iwp->src is not NULL in the finalize callback. This can only happen if another thread has managed to trigger io_watch_poll_prepare() callback in the meantime. Move iwp->src destruction back to the finalize callback to prevent the described race, and also remove the stale comment. The deadlock glib bug was fixed back in 2010 by b35820285668 ("gmain: move finalization of GSource outside of context lock"). Suggested-by: Paolo Bonzini Signed-off-by: Sergey Dyasli Link: https://lore.kernel.org/r/20240712092659.216206-1-sergey.dyasli@nutanix.com Signed-off-by: Paolo Bonzini --- chardev/char-io.c | 19 +++++-------------- 1 file changed, 5 insertions(+), 14 deletions(-) diff --git a/chardev/char-io.c b/chardev/char-io.c index dab77b112e..3be17b51ca 100644 --- a/chardev/char-io.c +++ b/chardev/char-io.c @@ -87,16 +87,12 @@ static gboolean io_watch_poll_dispatch(GSource *source, GSourceFunc callback, static void io_watch_poll_finalize(GSource *source) { - /* - * Due to a glib bug, removing the last reference to a source - * inside a finalize callback causes recursive locking (and a - * deadlock). This is not a problem inside other callbacks, - * including dispatch callbacks, so we call io_remove_watch_poll - * to remove this source. At this point, iwp->src must - * be NULL, or we would leak it. - */ IOWatchPoll *iwp = io_watch_poll_from_source(source); - assert(iwp->src == NULL); + if (iwp->src) { + g_source_destroy(iwp->src); + g_source_unref(iwp->src); + iwp->src = NULL; + } } static GSourceFuncs io_watch_poll_funcs = { @@ -139,11 +135,6 @@ static void io_remove_watch_poll(GSource *source) IOWatchPoll *iwp; iwp = io_watch_poll_from_source(source); - if (iwp->src) { - g_source_destroy(iwp->src); - g_source_unref(iwp->src); - iwp->src = NULL; - } g_source_destroy(&iwp->parent); }