Re: [Xen-devel] [PATCH 1/2] xenbus: req->body should be updated before req->state

[Julien Grall]

Hi,

On 03/03/2020 17:36, dongli.zhang@xxxxxxxxxx wrote:
> On 3/3/20 1:40 AM, Julien Grall wrote:
> > Hi,
> >
> > On 03/03/2020 01:58, Dongli Zhang wrote:
> > > The req->body should be updated before req->state is updated and the
> > > order should be guaranteed by a barrier.
> > >
> > > Otherwise, read_reply() might return req->body = NULL.
> > >
> > > Below is sample callstack when the issue is reproduced on purpose by
> > > reordering the updates of req->body and req->state and adding delay in
> > > code between updates of req->state and req->body.
> > >
> > > [ÂÂ 22.356105] general protection fault: 0000 [#1] SMP PTI
> > > [ÂÂ 22.361185] CPU: 2 PID: 52 Comm: xenwatch Not tainted 5.5.0xen+ #6
> > > [ÂÂ 22.366727] Hardware name: Xen HVM domU, BIOS ...
> > > [ÂÂ 22.372245] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60
> > > ... ...
> > > [ÂÂ 22.392163] RSP: 0018:ffffb2d64023fdf0 EFLAGS: 00010246
> > > [ÂÂ 22.395933] RAX: 0000000000000000 RBX: 75746e7562755f6d RCX: 0000000000000000
> > > [ÂÂ 22.400871] RDX: 0000000000000000 RSI: ffffb2d64023fdfc RDI: 75746e7562755f6d
> > > [ÂÂ 22.405874] RBP: 0000000000000000 R08: 00000000000001e8 R09: 0000000000cdcdcd
> > > [ÂÂ 22.410945] R10: ffffb2d6402ffe00 R11: ffff9d95395eaeb0 R12: ffff9d9535935000
> > > [ÂÂ 22.417613] R13: ffff9d9526d4a000 R14: ffff9d9526f4f340 R15: ffff9d9537654000
> > > [ÂÂ 22.423726] FS:Â 0000000000000000(0000) GS:ffff9d953bc80000(0000)
> > > knlGS:0000000000000000
> > > [ÂÂ 22.429898] CS:Â 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > [ÂÂ 22.434342] CR2: 000000c4206a9000 CR3: 00000001ea3fc002 CR4: 00000000001606e0
> > > [ÂÂ 22.439645] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > [ÂÂ 22.444941] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > [ÂÂ 22.450342] Call Trace:
> > > [ÂÂ 22.452509]Â simple_strtoull+0x27/0x70
> > > [ÂÂ 22.455572]Â xenbus_transaction_start+0x31/0x50
> > > [ÂÂ 22.459104]Â netback_changed+0x76c/0xcc1 [xen_netfront]
> > > [ÂÂ 22.463279]Â ? find_watch+0x40/0x40
> > > [ÂÂ 22.466156]Â xenwatch_thread+0xb4/0x150
> > > [ÂÂ 22.469309]Â ? wait_woken+0x80/0x80
> > > [ÂÂ 22.472198]Â kthread+0x10e/0x130
> > > [ÂÂ 22.474925]Â ? kthread_park+0x80/0x80
> > > [ÂÂ 22.477946]Â ret_from_fork+0x35/0x40
> > > [ÂÂ 22.480968] Modules linked in: xen_kbdfront xen_fbfront(+) xen_netfront
> > > xen_blkfront
> > > [ÂÂ 22.486783] ---[ end trace a9222030a747c3f7 ]---
> > > [ÂÂ 22.490424] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60
> > >
> > > The "while" is changed to "do while" so that wait_event() is used as a
> > > barrier.
> >
> > The correct barrier for read_reply() should be virt_rmb(). While on x86, this is
> > equivalent to barrier(), on Arm this will be a dmb(ish) to prevent the processor
> > re-ordering memory access.
> >
> > Therefore the barrier in test_reply() (called by wait_event()) is not going to
> > be sufficient for Arm.
>
> Sorry that I just erroneously thought wait_event() would be used as read barrier.

I was also kind of expecting wait_event() to contain a memory barrier. 
But it does not at least if condition is valid before waiting.

Cheers,


--
Julien Grall