linux-kernel - [PATCH 3.16 074/133] xen/events: events_fifo: Don't use {get,put}_cpu() in xen_evtchn_fifo

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Message-ID: <lsq.1511315893.429203491@decadent.org.uk>
Date:   Wed, 22 Nov 2017 01:58:13 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Julien Grall" <julien.grall@....com>,
        "Andre Przywara" <andre.przywara@....com>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Subject: [PATCH 3.16 074/133] xen/events: events_fifo: Don't use
 {get,put}_cpu() in xen_evtchn_fifo_init()

3.16.51-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Julien Grall <julien.grall@....com>

commit 22f12f0df802cea865672d8f39fbebdc03981050 upstream.

When booting Linux as Xen guest with CONFIG_DEBUG_ATOMIC, the following
splat appears:

[    0.002323] Mountpoint-cache hash table entries: 1024 (order: 1, 8192 bytes)
[    0.019717] ASID allocator initialised with 65536 entries
[    0.020019] xen:grant_table: Grant tables using version 1 layout
[    0.020051] Grant table initialized
[    0.020069] BUG: sleeping function called from invalid context at /data/src/linux/mm/page_alloc.c:4046
[    0.020100] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: swapper/0
[    0.020123] no locks held by swapper/0/1.
[    0.020143] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.13.0-rc5 #598
[    0.020166] Hardware name: FVP Base (DT)
[    0.020182] Call trace:
[    0.020199] [<ffff00000808a5c0>] dump_backtrace+0x0/0x270
[    0.020222] [<ffff00000808a95c>] show_stack+0x24/0x30
[    0.020244] [<ffff000008c1ef20>] dump_stack+0xb8/0xf0
[    0.020267] [<ffff0000081128c0>] ___might_sleep+0x1c8/0x1f8
[    0.020291] [<ffff000008112948>] __might_sleep+0x58/0x90
[    0.020313] [<ffff0000082171b8>] __alloc_pages_nodemask+0x1c0/0x12e8
[    0.020338] [<ffff00000827a110>] alloc_page_interleave+0x38/0x88
[    0.020363] [<ffff00000827a904>] alloc_pages_current+0xdc/0xf0
[    0.020387] [<ffff000008211f38>] __get_free_pages+0x28/0x50
[    0.020411] [<ffff0000086566a4>] evtchn_fifo_alloc_control_block+0x2c/0xa0
[    0.020437] [<ffff0000091747b0>] xen_evtchn_fifo_init+0x38/0xb4
[    0.020461] [<ffff0000091746c0>] xen_init_IRQ+0x44/0xc8
[    0.020484] [<ffff000009128adc>] xen_guest_init+0x250/0x300
[    0.020507] [<ffff000008083974>] do_one_initcall+0x44/0x130
[    0.020531] [<ffff000009120df8>] kernel_init_freeable+0x120/0x288
[    0.020556] [<ffff000008c31ca8>] kernel_init+0x18/0x110
[    0.020578] [<ffff000008083710>] ret_from_fork+0x10/0x40
[    0.020606] xen:events: Using FIFO-based ABI
[    0.020658] Xen: initializing cpu0
[    0.027727] Hierarchical SRCU implementation.
[    0.036235] EFI services will not be available.
[    0.043810] smp: Bringing up secondary CPUs ...

This is because get_cpu() in xen_evtchn_fifo_init() will disable
preemption, but __get_free_page() might sleep (GFP_ATOMIC is not set).

xen_evtchn_fifo_init() will always be called before SMP is initialized,
so {get,put}_cpu() could be replaced by a simple smp_processor_id().

This also avoid to modify evtchn_fifo_alloc_control_block that will be
called in other context.

Signed-off-by: Julien Grall <julien.grall@....com>
Reported-by: Andre Przywara <andre.przywara@....com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Fixes: 1fe565517b57 ("xen/events: use the FIFO-based ABI if available")
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/xen/events/events_fifo.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/drivers/xen/events/events_fifo.c
+++ b/drivers/xen/events/events_fifo.c
@@ -445,19 +445,18 @@ static struct notifier_block evtchn_fifo
 
 int __init xen_evtchn_fifo_init(void)
 {
-	int cpu = get_cpu();
+	int cpu = smp_processor_id();
 	int ret;
 
 	ret = evtchn_fifo_alloc_control_block(cpu);
 	if (ret < 0)
-		goto out;
+		return ret;
 
 	pr_info("Using FIFO-based ABI\n");
 
 	evtchn_ops = &evtchn_ops_fifo;
 
 	register_cpu_notifier(&evtchn_fifo_cpu_notifier);
-out:
-	put_cpu();
+
 	return ret;
 }