* Re: [bug report] x86/split_lock: Make life miserable for split lockers
2023-11-28 17:40 ` [bug report] x86/split_lock: Make life miserable for split lockers Tony Luck
@ 2023-11-28 18:33 ` Tom Lendacky
0 siblings, 0 replies; 2+ messages in thread
From: Tom Lendacky @ 2023-11-28 18:33 UTC (permalink / raw)
To: Tony Luck, Dan Carpenter, Joerg Roedel; +Cc: kernel-janitors, linux-kernel
+Joerg
On 11/28/23 11:40, Tony Luck wrote:
> On Tue, Nov 28, 2023 at 04:12:24PM +0300, Dan Carpenter wrote:
>> Hello Tony Luck,
>>
>> The patch b041b525dab9: "x86/split_lock: Make life miserable for
>> split lockers" from Mar 10, 2022 (linux-next), leads to the following
>> Smatch static checker warning:
>>
>> arch/x86/kernel/cpu/intel.c:1179 split_lock_warn()
>> warn: sleeping in atomic context
>>
>> arch/x86/kernel/cpu/intel.c
>> 1158 static void split_lock_warn(unsigned long ip)
>> 1159 {
>> 1160 struct delayed_work *work;
>> 1161 int cpu;
>> 1162
>> 1163 if (!current->reported_split_lock)
>> 1164 pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
>> 1165 current->comm, current->pid, ip);
>> 1166 current->reported_split_lock = 1;
>> 1167
>> 1168 if (sysctl_sld_mitigate) {
>> 1169 /*
>> 1170 * misery factor #1:
>> 1171 * sleep 10ms before trying to execute split lock.
>> 1172 */
>> 1173 if (msleep_interruptible(10) > 0)
>> 1174 return;
>> 1175 /*
>> 1176 * Misery factor #2:
>> 1177 * only allow one buslocked disabled core at a time.
>> 1178 */
>> --> 1179 if (down_interruptible(&buslock_sem) == -EINTR)
>> 1180 return;
>> 1181 work = &sl_reenable_unlock;
>> 1182 } else {
>> 1183 work = &sl_reenable;
>> 1184 }
>> 1185
>> 1186 cpu = get_cpu();
>> 1187 schedule_delayed_work_on(cpu, work, 2);
>> 1188
>> 1189 /* Disable split lock detection on this CPU to make progress */
>> 1190 sld_update_msr(false);
>> 1191 put_cpu();
>> 1192 }
>>
>> The call tree is:
>>
>> kernel_exc_vmm_communication() <- disables preempt
>> -> vc_raw_handle_exception()
>> -> vc_forward_exception()
>> -> exc_alignment_check()
>> -> __exc_alignment_check()
>> -> handle_user_split_lock()
>> -> split_lock_warn()
>>
>> I think maybe the mismatch is that kernel_exc_vmm_communication() calls
>> irqentry_nmi_enter(regs); which disable preemption but exc_alignment_check()
>> does local_irq_enable() which doesn't enable it.
>
> I think we need some arch/x86/kernel/sev.c expertise to explain the
> preemption requirements in that stack trace. Adding Tom Lendacky.
Adding Joerg as the original developer of this code.
I believe that irqentry_nmi_enter() is used so that we are ensured that
the kernel can't be interrupted while using the per-CPU GHCB when entered
from kernel-mode in order to avoid nested #VCs (except for an NMI). Joerg
might have further insights since there was a lot of discussion around
these changes.
I'm not sure if is possible, but I wonder if irqentry_nmi_exit() can be
issued before forwarding the exception - or even delay forwarding the
exception until after irqentry_nmi_exit().
Thanks,
Tom
>
>> Also why does arch/x86 not have a dedicated mailing list?
>
> Good question. X86 was once the default architecture. So everything went to
> linux-kernel@vger.kernel.org. I'll add that to Cc: for this. But maybe
> it's time for an x86 specific list?
>
>> regards,
>> dan carpenter
>
> -Tony
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