Messages in this thread | | | Subject | Re: [PATCH] net/9p/trans_virtio.c: replace mutex_lock with spin_lock to protect 'virtio_chan_list' | From | piaojun <> | Date | Thu, 19 Jul 2018 15:44:28 +0800 |
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On 2018/7/19 11:36, Dominique Martinet wrote: > piaojun wrote on Thu, Jul 19, 2018: >>> piaojun wrote on Wed, Jul 18, 2018: >>> That's not a fast path operation, I don't mind changing things but I'd >>> like to understand why - these functions are only ever called at unmount >>> time or when something happens on the virtio bus (probe will happen on >>> probing on the pci bus and I'm not too sure on remove but probably pci >>> removal i.e. basically never?) >>> >>> I don't see why this wouldn't work, but I won't take this without a >>> (good?) reason. >>> >> virtio_9p_lock is responsable for protecting virtio_chan_list which has 3 >> operation: >> >> 1. Add a virtio chan to virtio_chan_list. This will happen when we insmod >> 9pnet_virtio.ko: >> p9_virtio_probe >> --list_add_tail(&chan->chan_list, &virtio_chan_list); >> >> 2. Remove a virtio chan. This will happen when remnod 9pnet_virtio.ko: >> p9_virtio_remove >> --list_del(&chan->chan_list); >> >> 3. Find a unused virtio chan when mount 9p: >> mount >> --p9_virtio_create >> --list_for_each_entry(chan, &virtio_chan_list, chan_list) >> >> Multi mount process will compete for virtio_9p_lock when finding unused >> virtio chan, in which case mutex lock will cause process sleep and wake >> up. I think this a waste of CPU time. So we could use spin lock to avoid >> this. > > Well, sure, that's theory; but how is that in practice? > I actually took the time to run some tests, setting up 20 virtio mount > points in qemu, and running this command with and without your patch: > # time sh -c 'for i in {1..20}; do > sh -c "for j in {1..100}; do > mount -t 9p d$i d.$i; > umount d.$i; > done" & > done; > wait' > > This is quick & dirty but basically, mounts and unmounts 100 times in a > loop all 20 mount points in parallel to stress that lock. > I get these times 5 times (one run per column), > without patch: > real 0m19.357s 0m19.626s 0m19.904s 0m19.926s 0m21.321s > user 0m6.795s 0m6.874s 0m6.807s 0m6.768s 0m6.892s > sys 0m29.936s 0m31.196s 0m31.702s 0m31.914s 0m30.791s > > With patch: > real 0m19.439s 0m19.849s 0m19.683s 0m19.600s 0m20.689s > user 0m6.948s 0m6.582s 0m6.706s 0m6.598s 0m6.876s > sys 0m29.364s 0m30.898s 0m30.695s 0m31.311s 0m33.391s > > I honestly can't say I'm convinced with a difference either way, the > variations look more like noise than anything to me. > > > More to the point, while these tests ran my dmesg buffer was filled with > errors like: > FS-Cache: Duplicate cookie detected > FS-Cache: O-cookie c=0000000000368cdb [p=00000000548b03c2 fl=222 nc=0 na=1] > FS-Cache: O-cookie d=000000004cebd15f n=00000000029a0b83 > FS-Cache: O-key=[10] '34323935303838343536' > FS-Cache: N-cookie c=00000000d4089478 [p=00000000548b03c2 fl=2 nc=0 na=1] > FS-Cache: N-cookie d=000000004cebd15f n=00000000959d4d37 > FS-Cache: N-key=[10] '34323935303838343536' > > or > (output mangled a bit) > > ================================================================== > BUG: KASAN: use-after-free in p9_client_cb+0x14d/0x160 [9pnet] > Read of size 8 at addr ffff88003522a088 by task systemd-udevd/492 > > CPU: 1 PID: 492 Comm: systemd-udevd Tainted: G O 4.18.0-rc5+ #9 > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 0> > Call Trace: > <IRQ> > dump_stack+0x7b/0xad > print_address_description+0x6a/0x209 > ? p9_client_cb+0x14d/0x160 [9pnet] > kasan_report.cold.7+0x242/0x2fe > __asan_report_load8_noabort+0x19/0x20 > p9_client_cb+0x14d/0x160 [9pnet] > req_done+0x22f/0x280 [9pnet_virtio] > ? p9_mount_tag_show+0x120/0x120 [9pnet_virtio] > vring_interrupt+0x108/0x1b0 [virtio_ring] > ? vring_map_single.constprop.23+0x350/0x350 [virtio_ring] > __handle_irq_event_percpu+0xec/0x460 > handle_irq_event_percpu+0x71/0x140 > ? __handle_irq_event_percpu+0x460/0x460 > ? apic_ack_irq+0xa3/0xe0 > handle_irq_event+0xb9/0x14a > handle_edge_irq+0x1ea/0x7a0 > ? kasan_check_read+0x11/0x20 > handle_irq+0x48/0x60 > do_IRQ+0x67/0x140 > common_interrupt+0xf/0xf > </IRQ> > RIP: 0010:finish_task_switch+0x10e/0x630 > Code: e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 6d 04 00 00 41 c7 45 38 00 00 00 00 4c 89 e7 ff 14 25 28 f5 66 8e fb 66 0f > > RSP: 0018:ffff8800633e7a60 EFLAGS: 00000246 ORIG_RAX: ffffffffffffffd4 > RAX: 0000000000000001 RBX: ffff880036632000 RCX: 0000000000000000 > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88006caaac00 > RBP: ffff8800633e7aa0 R08: ffffed000cea15cd R09: ffffed000cea15cc > R10: ffffed000cea15cc R11: ffff88006750ae63 R12: ffff88006caaac00 > R13: ffff88006558b000 R14: 0000000000000000 R15: ffff880036632000 > ? __switch_to_asm+0x34/0x70 > ? __switch_to_asm+0x40/0x70 > __schedule+0x733/0x1c10 > ? __bpf_prog_run64+0xd0/0xd0 > ? firmware_map_remove+0x174/0x174 > schedule+0x7a/0x1a0 > schedule_hrtimeout_range_clock+0x306/0x3b0 > ? kasan_check_write+0x14/0x20 > ? hrtimer_nanosleep_restart+0x290/0x290 > ? ep_busy_loop_end+0x110/0x110 > schedule_hrtimeout_range+0x13/0x20 > ep_poll+0x7a7/0xb50 > ? __ia32_sys_epoll_ctl+0x1170/0x1170 > ? __fget_light+0x59/0x1f0 > ? __audit_syscall_entry+0x347/0x980 > ? __audit_free+0x8a0/0x8a0 > 34 > ? wake_up_q+0x100/0x100 > 39 > ? kasan_check_read+0x11/0x20 > 3230373130' > FS-Cache: O-key=[10] '34323934393230373131' > FS-Cache: N-cookie c=00000000fa69c1f9 [p=00000000887326c4 fl=2 nc=0 na=1] > FS-Cache: N-cookie d=00000000a8f143d1 n=00000000446f741a > FS-Cache: N-key=[10] '34323934393230373131' > ? __fget_light+0x59/0x1f0 > do_epoll_wait+0x129/0x160 > __x64_sys_epoll_wait+0x97/0xf0 > do_syscall_64+0xa5/0x260 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > RIP: 0033:0x7f9099a22317 > Code: 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 8d 05 d1 46 2c 00 41 89 ca 8b 00 85 c0 75 10 b8 e8 00 > > RSP: 002b:00007ffff67e1f28 EFLAGS: 00000246 ORIG_RAX: 00000000000000e8 > RAX: ffffffffffffffda RBX: 0000558182d9e390 RCX: 00007f9099a22317 > RDX: 000000000000000b RSI: 00007ffff67e1f30 RDI: 000000000000000b > RBP: 00007ffff67e20b0 R08: 0000000006c65ded R09: 00007ffff67e1f30 > R10: 00000000ffffffff R11: 0000000000000246 R12: 0000000000000001 > R13: 00007ffff67e1f30 R14: ffffffffffffffff R15: 0000558182d7a4c0 > > Allocated by task 6390: > save_stack+0x43/0xd0 > kasan_kmalloc+0xc4/0xe0 > kasan_slab_alloc+0x12/0x20 > kmem_cache_alloc+0xe2/0x5e0 > p9_client_prepare_req+0xa4/0x670 [9pnet] > p9_client_rpc+0x133/0xd20 [9pnet] > p9_client_getattr_dotl+0x102/0x910 [9pnet] > v9fs_mount+0x5a6/0x7c0 [9p] > mount_fs+0x89/0x2ad > vfs_kern_mount.part.32+0x5d/0x390 > do_mount+0x379/0x2bb0 > ksys_mount+0xbf/0xe0 > __x64_sys_mount+0xbe/0x150 > do_syscall_64+0xa5/0x260 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > Freed by task 6390: > save_stack+0x43/0xd0 > __kasan_slab_free+0x118/0x170 > kasan_slab_free+0xe/0x10 > kmem_cache_free+0x49/0x160 > p9_free_req+0x106/0x140 [9pnet] > p9_client_getattr_dotl+0x590/0x910 [9pnet] > v9fs_mount+0x5a6/0x7c0 [9p] > mount_fs+0x89/0x2ad > vfs_kern_mount.part.32+0x5d/0x390 > do_mount+0x379/0x2bb0 > ksys_mount+0xbf/0xe0 > __x64_sys_mount+0xbe/0x150 > do_syscall_64+0xa5/0x260 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > The buggy address belongs to the object at ffff88003522a068 > which belongs to the cache p9_req_t of size 72 > The buggy address is located 32 bytes inside of > 72-byte region [ffff88003522a068, ffff88003522a0b0) > The buggy address belongs to the page: > page:ffffea0000d48a80 count:1 mapcount:0 mapping:ffff880064562580 index:0x0 > flags: 0xffffc000000100(slab) > raw: 00ffffc000000100 ffff880035e36618 ffffea00019fa888 ffff880064562580 > raw: 0000000000000000 ffff88003522a000 0000000100000027 0000000000000000 > page dumped because: kasan: bad access detected > > Memory state around the buggy address: > ffff880035229f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 > ffff88003522a000: fb fb fb fb fb fb fb fb fb fc fc fc fc fb fb fb >> ffff88003522a080: fb fb fb fb fb fb fc fc fc fc fb fb fb fb fb fb > ^ > ffff88003522a100: fb fb fb fc fc fc fc fb fb fb fb fb fb fb fb fb > ffff88003522a180: fc fc fc fc fb fb fb fb fb fb fb fb fb fc fc fc > ================================================================== > > so if you're concerned about parallel mountings, I think there are > others, more important, bugs to fix rather than replacing a hardly-used > mutex by a spin-lock... > It makes sense, and bug fix comes first. I will look into the bug you tested.
Thanks, Jun
> > > You've done the work now so it's not like I can't take the patch, but it > really feels pointless to me unless you can show me there is actual > improvement. >
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