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Date:   Wed, 17 Aug 2022 09:23:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Yang Jihong <yangjihong1@huawei.com>, rostedt@goodmis.org,
        mingo@redhat.com, acme@kernel.org,
        alexander.shishkin@linux.intel.com, jolsa@kernel.org,
        namhyung@kernel.org, linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org
Subject: Re: [PATCH v3] perf/core: Fix reentry problem in
 perf_output_read_group
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On Tue, Aug 16, 2022 at 05:45:31PM +0100, Mark Rutland wrote:

> > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > > index ee8b9ecdc03b..4d9cf508c510 100644
> > > --- a/include/linux/perf_event.h
> > > +++ b/include/linux/perf_event.h
> > > @@ -631,7 +631,21 @@ struct pmu_event_list {
> > >  	struct list_head	list;
> > >  };
> > >  
> > > +#ifdef CONFIG_LOCKDEP
> > > +#define LOCKDEP_ASSERT_EVENT_CTX(event)				\
> > > +	WARN_ON_ONCE(__lockdep_enabled &&			\
> > > +		     (this_cpu_read(hardirqs_enabled) ||	\
> > > +		      lockdep_is_held(&(event)->ctx->mutex) != LOCK_STATE_HELD))
> > 
> > Uh, should that `||` be `&&`, or am I missing the plot?

Nah, definitely me being daft again.

> > This'll warn if IRQs are enabled regardless of whether the mutex is held.
> 
> With `&&` I see:
> 
> [   25.194796] ------------[ cut here ]------------
> [   25.196017] WARNING: CPU: 0 PID: 177 at kernel/events/core.c:2231 pmu_filter_match+0x12c/0x154
> [   25.198140] Modules linked in:
> [   25.198933] CPU: 0 PID: 177 Comm: perf_fuzzer Not tainted 6.0.0-rc1-00002-g2dee3ea2f881 #9
> [   25.200945] Hardware name: linux,dummy-virt (DT)
> [   25.202083] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [   25.203799] pc : pmu_filter_match+0x12c/0x154
> [   25.204881] lr : pmu_filter_match+0x124/0x154
> [   25.205964] sp : ffff80000b753a90
> [   25.206807] x29: ffff80000b753a90 x28: 0000000001200000 x27: 0000000000000000
> [   25.208557] x26: ffff00000523ca40 x25: ffff80000a90bbd0 x24: 0000000000000000
> [   25.210316] x23: ffff800009d92008 x22: ffff8000082669a0 x21: ffff80000b753b58
> [   25.212065] x20: ffff000005890560 x19: 0000000000000001 x18: 0000000000000001
> [   25.213819] x17: 00000000c3d5f54b x16: 000000006eb75123 x15: 00000000000000dc
> [   25.215574] x14: 0000000001200011 x13: 0000000060000000 x12: 0000000000000015
> [   25.217325] x11: 0000000000000015 x10: 000007ffffffffff x9 : 00000000ffffffff
> [   25.219091] x8 : ffff80000a910718 x7 : 0000000000000000 x6 : 0000000000000001
> [   25.220847] x5 : 00000000c3affd79 x4 : 0000000000000000 x3 : ffff000003b15800
> [   25.222605] x2 : 0000000000000001 x1 : 00000000000000c0 x0 : 0000000000000000
> [   25.224361] Call trace:
> [   25.224987]  pmu_filter_match+0x12c/0x154
> [   25.225987]  perf_iterate_ctx+0x110/0x11c
> [   25.226997]  perf_iterate_sb+0x258/0x2f0
> [   25.227971]  perf_event_fork+0x74/0xc0
> [   25.228910]  copy_process+0x1484/0x1964
> [   25.229869]  kernel_clone+0x9c/0x4a4
> [   25.230780]  __do_sys_clone+0x70/0xac
> [   25.231699]  __arm64_sys_clone+0x24/0x30
> [   25.232679]  invoke_syscall+0x48/0x114
> [   25.233615]  el0_svc_common.constprop.0+0x60/0x11c
> [   25.234817]  do_el0_svc+0x30/0xc0
> [   25.235659]  el0_svc+0x48/0xc0
> [   25.236438]  el0t_64_sync_handler+0x120/0x15c
> [   25.237523]  el0t_64_sync+0x18c/0x190
> [   25.238448] irq event stamp: 13502
> [   25.239301] hardirqs last  enabled at (13501): [<ffff8000091f0d68>] _raw_spin_unlock_irqrestore+0x88/0x94
> [   25.241627] hardirqs last disabled at (13502): [<ffff8000091e42d4>] el1_dbg+0x24/0x90
> [   25.243550] softirqs last  enabled at (13340): [<ffff800008016e98>] put_cpu_fpsimd_context+0x28/0x70
> [   25.245773] softirqs last disabled at (13338): [<ffff800008016e14>] get_cpu_fpsimd_context+0x0/0x5c
> [   25.247989] ---[ end trace 0000000000000000 ]---

Seems a valid complaint to me... *sigh*. Not sure what to do about it; I
mean we can slap on more local_irq_{save,restore}() ofcouse, but that
seems unfortunate.