Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH 1/2] xenbus: req->body should be updated before req->state | From | Julien Grall <> | Date | Tue, 3 Mar 2020 09:40:12 +0000 |
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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. > > [ 22.356105] general protection fault: 0000 [#1] SMP PTI > [ 22.361185] CPU: 2 PID: 52 Comm: xenwatch Not tainted 5.5.0xen+ #6 > [ 22.366727] Hardware name: Xen HVM domU, BIOS ... > [ 22.372245] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 > ... ... > [ 22.392163] RSP: 0018:ffffb2d64023fdf0 EFLAGS: 00010246 > [ 22.395933] RAX: 0000000000000000 RBX: 75746e7562755f6d RCX: 0000000000000000 > [ 22.400871] RDX: 0000000000000000 RSI: ffffb2d64023fdfc RDI: 75746e7562755f6d > [ 22.405874] RBP: 0000000000000000 R08: 00000000000001e8 R09: 0000000000cdcdcd > [ 22.410945] R10: ffffb2d6402ffe00 R11: ffff9d95395eaeb0 R12: ffff9d9535935000 > [ 22.417613] R13: ffff9d9526d4a000 R14: ffff9d9526f4f340 R15: ffff9d9537654000 > [ 22.423726] FS: 0000000000000000(0000) GS:ffff9d953bc80000(0000) knlGS:0000000000000000 > [ 22.429898] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 22.434342] CR2: 000000c4206a9000 CR3: 00000001ea3fc002 CR4: 00000000001606e0 > [ 22.439645] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > [ 22.444941] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > [ 22.450342] Call Trace: > [ 22.452509] simple_strtoull+0x27/0x70 > [ 22.455572] xenbus_transaction_start+0x31/0x50 > [ 22.459104] netback_changed+0x76c/0xcc1 [xen_netfront] > [ 22.463279] ? find_watch+0x40/0x40 > [ 22.466156] xenwatch_thread+0xb4/0x150 > [ 22.469309] ? wait_woken+0x80/0x80 > [ 22.472198] kthread+0x10e/0x130 > [ 22.474925] ? kthread_park+0x80/0x80 > [ 22.477946] ret_from_fork+0x35/0x40 > [ 22.480968] Modules linked in: xen_kbdfront xen_fbfront(+) xen_netfront xen_blkfront > [ 22.486783] ---[ end trace a9222030a747c3f7 ]--- > [ 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.
> > Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com> > --- > drivers/xen/xenbus/xenbus_comms.c | 2 ++ > drivers/xen/xenbus/xenbus_xs.c | 4 ++-- > 2 files changed, 4 insertions(+), 2 deletions(-) > > diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c > index d239fc3c5e3d..852ed161fc2a 100644 > --- a/drivers/xen/xenbus/xenbus_comms.c > +++ b/drivers/xen/xenbus/xenbus_comms.c > @@ -313,6 +313,8 @@ static int process_msg(void) > req->msg.type = state.msg.type; > req->msg.len = state.msg.len; > req->body = state.body; > + /* write body, then update state */ > + virt_wmb(); > req->state = xb_req_state_got_reply; > req->cb(req); > } else > diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c > index ddc18da61834..f5b0a6a72ad3 100644 > --- a/drivers/xen/xenbus/xenbus_xs.c > +++ b/drivers/xen/xenbus/xenbus_xs.c > @@ -202,7 +202,7 @@ static bool test_reply(struct xb_req_data *req) > > static void *read_reply(struct xb_req_data *req) > { > - while (req->state != xb_req_state_got_reply) { > + do { > wait_event(req->wq, test_reply(req)); > > if (!xenbus_ok()) > @@ -216,7 +216,7 @@ static void *read_reply(struct xb_req_data *req) > if (req->err) > return ERR_PTR(req->err); > > - } > + } while (req->state != xb_req_state_got_reply); > > return req->body; > } >
Cheers,
-- Julien Grall
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