From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58037) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gSiFm-0008I7-UN for qemu-devel@nongnu.org; Fri, 30 Nov 2018 07:50:45 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gSi25-00037B-B2 for qemu-devel@nongnu.org; Fri, 30 Nov 2018 07:36:34 -0500 References: <20181129101801.6421-1-vsementsov@virtuozzo.com> <20181129101801.6421-2-vsementsov@virtuozzo.com> From: Max Reitz Message-ID: <3329707d-d16b-716e-88ac-e40ecfdd7290@redhat.com> Date: Fri, 30 Nov 2018 13:36:24 +0100 MIME-Version: 1.0 In-Reply-To: <20181129101801.6421-2-vsementsov@virtuozzo.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="N5DUwsBA3V6bTFnOg9RrvNtPxzksiezlW" Subject: Re: [Qemu-devel] [PATCH v2 1/2] mirror: fix dead-lock List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org Cc: kwolf@redhat.com, jcody@redhat.com, pbonzini@redhat.com, dplotnikov@virtuozzo.com, den@openvz.org, qemu-stable@nongnu.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --N5DUwsBA3V6bTFnOg9RrvNtPxzksiezlW From: Max Reitz To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org Cc: kwolf@redhat.com, jcody@redhat.com, pbonzini@redhat.com, dplotnikov@virtuozzo.com, den@openvz.org, qemu-stable@nongnu.org Message-ID: <3329707d-d16b-716e-88ac-e40ecfdd7290@redhat.com> Subject: Re: [PATCH v2 1/2] mirror: fix dead-lock References: <20181129101801.6421-1-vsementsov@virtuozzo.com> <20181129101801.6421-2-vsementsov@virtuozzo.com> In-Reply-To: <20181129101801.6421-2-vsementsov@virtuozzo.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 29.11.18 11:18, Vladimir Sementsov-Ogievskiy wrote: > Let start from the beginning: >=20 > Commit b9e413dd375 (in 2.9) > "block: explicitly acquire aiocontext in aio callbacks that need it" > added pairs of aio_context_acquire/release to mirror_write_complete and= > mirror_read_complete, when they were aio callbacks for blk_aio_* calls.= >=20 > Then, commit 2e1990b26e5 (in 3.0) "block/mirror: Convert to coroutines"= > dropped these blk_aio_* calls, than mirror_write_complete and > mirror_read_complete are not callbacks more, and don't need additional > aiocontext acquiring. Yes, that alone makes enough sense to me. > Furthermore, mirror_read_complete calls > blk_co_pwritev inside these pair of aio_context_acquire/release, which > leads to the following dead-lock with mirror: >=20 > (gdb) info thr > Id Target Id Frame > 3 Thread (LWP 145412) "qemu-system-x86" syscall () > 2 Thread (LWP 145416) "qemu-system-x86" __lll_lock_wait () > * 1 Thread (LWP 145411) "qemu-system-x86" __lll_lock_wait () >=20 > (gdb) bt > #0 __lll_lock_wait () > #1 _L_lock_812 () > #2 __GI___pthread_mutex_lock > #3 qemu_mutex_lock_impl (mutex=3D0x561032dce420 , > file=3D0x5610327d8654 "util/main-loop.c", line=3D236) at > util/qemu-thread-posix.c:66 > #4 qemu_mutex_lock_iothread_impl > #5 os_host_main_loop_wait (timeout=3D480116000) at util/main-loop.c:2= 36 > #6 main_loop_wait (nonblocking=3D0) at util/main-loop.c:497 > #7 main_loop () at vl.c:1892 > #8 main >=20 > Printing contents of qemu_global_mutex, I see that "__owner =3D 145416"= , > so, thr1 is main loop, and now it wants BQL, which is owned by thr2. >=20 > (gdb) thr 2 > (gdb) bt > #0 __lll_lock_wait () > #1 _L_lock_870 () > #2 __GI___pthread_mutex_lock > #3 qemu_mutex_lock_impl (mutex=3D0x561034d25dc0, ... > #4 aio_context_acquire (ctx=3D0x561034d25d60) > #5 dma_blk_cb > #6 dma_blk_io > #7 dma_blk_read > #8 ide_dma_cb > #9 bmdma_cmd_writeb > #10 bmdma_write > #11 memory_region_write_accessor > #12 access_with_adjusted_size > #15 flatview_write > #16 address_space_write > #17 address_space_rw > #18 kvm_handle_io > #19 kvm_cpu_exec > #20 qemu_kvm_cpu_thread_fn > #21 qemu_thread_start > #22 start_thread > #23 clone () >=20 > Printing mutex in fr 2, I see "__owner =3D 145411", so thr2 wants aio > context mutex, which is owned by thr1. Classic dead-lock. >=20 > Then, let's check that aio context is hold by mirror coroutine: just > print coroutine stack of first tracked request in mirror job target: >=20 > (gdb) [...] > (gdb) qemu coroutine 0x561035dd0860 > #0 qemu_coroutine_switch > #1 qemu_coroutine_yield > #2 qemu_co_mutex_lock_slowpath > #3 qemu_co_mutex_lock > #4 qcow2_co_pwritev > #5 bdrv_driver_pwritev > #6 bdrv_aligned_pwritev > #7 bdrv_co_pwritev > #8 blk_co_pwritev > #9 mirror_read_complete () at block/mirror.c:232 > #10 mirror_co_read () at block/mirror.c:370 > #11 coroutine_trampoline > #12 __start_context >=20 > Yes it is mirror_read_complete calling blk_co_pwritev after acquiring > aio context. >=20 > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- > block/mirror.c | 13 +++++-------- > 1 file changed, 5 insertions(+), 8 deletions(-) Reviewed-by: Max Reitz --N5DUwsBA3V6bTFnOg9RrvNtPxzksiezlW Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAlwBLsgACgkQ9AfbAGHV z0D2MggAqN6L5sqsqT5LFTtTAwSP6bXUG9gCzmaIqhswWkQwv1jQxq7cu0vLyPob vlGPlK+Iemp4MLUiEpFCtXSPJza91cx+NHAp8Fzx10U9sLB4dZgjGCVi3lNpSKhg /HdJFrThFLN59KCToMuwfXhOZIg0dyEtkdeoHJZgPJm3oAEOpNTSHbx1eICeyaww NRkX+IyLrWT4KoRU/vf0AFq1DjonWE2+Rd3L2vlpG0uxU/D/vZ1FRorM0RwHWNX2 pNc5MNbbr2tte58gLormemWRX9jynSj9Ouj4f99+jhb2IgbZEj8iF9ZCs8GNGF07 q6r82Cvys9u+AHxC9k/K2BjLZ8Ow9g== =HreZ -----END PGP SIGNATURE----- --N5DUwsBA3V6bTFnOg9RrvNtPxzksiezlW--