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Message-ID: <20241007145037.GE4879@noisy.programming.kicks-ass.net>
Date: Mon, 7 Oct 2024 16:50:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Waiman Long <longman@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Luis Goncalves <lgoncalv@...hat.com>, Chunyu Hu <chuhu@...hat.com>
Subject: Re: [PATCH] locking/rtmutex: Always use trylock in rt_mutex_trylock()
On Wed, Oct 02, 2024 at 01:54:16PM -0400, Waiman Long wrote:
>
> On 10/2/24 05:37, Peter Zijlstra wrote:
> > On Thu, Sep 26, 2024 at 11:13:15AM -0400, Waiman Long wrote:
> > > One reason to use a trylock is to avoid a ABBA deadlock in case we need
> > > to use a locking sequence that is not in the expected locking order. That
> > > requires the use of trylock all the ways in the abnormal locking
> > > sequence. Unfortunately, this is not the case for rt_mutex_trylock() as
> > > it uses a raw_spin_lock_irqsave() to acquire the lock->wait_lock. That
> > > will cause a lockdep splat like the following in a PREEMPT_RT kernel:
> > >
> > > [ 63.695668] -> #0 (&lock->wait_lock){-...}-{2:2}:
> > > [ 63.695674] check_prev_add+0x1bd/0x1310
> > > [ 63.695678] validate_chain+0x6cf/0x7c0
> > > [ 63.695682] __lock_acquire+0x879/0xf40
> > > [ 63.695686] lock_acquire.part.0+0xfa/0x2d0
> > > [ 63.695690] _raw_spin_lock_irqsave+0x46/0x90
> > > [ 63.695695] rt_mutex_slowtrylock+0x3f/0xb0
> > > [ 63.695699] rt_spin_trylock+0x13/0xc0
> > > [ 63.695702] rmqueue_pcplist+0x5b/0x180
> > > [ 63.695705] rmqueue+0xb01/0x1040
> > > [ 63.695708] get_page_from_freelist+0x1d0/0xa00
> > > [ 63.695712] __alloc_pages_noprof+0x19a/0x450
> > > [ 63.695716] alloc_pages_mpol_noprof+0xaf/0x1e0
> > > [ 63.695721] stack_depot_save_flags+0x4db/0x520
> > > [ 63.695727] kasan_save_stack+0x3f/0x50
> > > [ 63.695731] __kasan_record_aux_stack+0x8e/0xa0
> > > [ 63.695736] task_work_add+0x1ad/0x240
> > > [ 63.695741] sched_tick+0x1c7/0x500
> > > [ 63.695744] update_process_times+0xf1/0x130
> > > [ 63.695750] tick_nohz_handler+0xf7/0x230
> > > [ 63.695754] __hrtimer_run_queues+0x13b/0x7b0
> > > [ 63.695758] hrtimer_interrupt+0x1c2/0x350
> > > [ 63.695763] __sysvec_apic_timer_interrupt+0xdb/0x340
> > > [ 63.695770] sysvec_apic_timer_interrupt+0x9c/0xd0
> > > [ 63.695774] asm_sysvec_apic_timer_interrupt+0x1a/0x20
> > > [ 63.695780] __asan_load8+0x8/0xa0
> > > [ 63.695784] mas_wr_end_piv+0x28/0x2c0
> > > [ 63.695789] mas_preallocate+0x3a8/0x680
> > > [ 63.695793] vma_shrink+0x180/0x3f0
> > > [ 63.695799] shift_arg_pages+0x219/0x2c0
> > > [ 63.695804] setup_arg_pages+0x343/0x5e0
> > > [ 63.695807] load_elf_binary+0x5ac/0x15d0
> > > [ 63.695813] search_binary_handler+0x125/0x370
> > > [ 63.695818] exec_binprm+0xc9/0x3d0
> > > [ 63.695821] bprm_execve+0x103/0x230
> > > [ 63.695824] kernel_execve+0x187/0x1c0
> > > [ 63.695828] call_usermodehelper_exec_async+0x145/0x1e0
> > > [ 63.695832] ret_from_fork+0x31/0x60
> > > [ 63.695836] ret_from_fork_asm+0x1a/0x30
> > > [ 63.695840]
> > > [ 63.695840] other info that might help us debug this:
> > > [ 63.695840]
> > > [ 63.695842] Chain exists of:
> > > [ 63.695842] &lock->wait_lock --> &p->pi_lock --> &rq->__lock
> > > [ 63.695842]
> > > [ 63.695850] Possible unsafe locking scenario:
> > > [ 63.695850]
> > > [ 63.695851] CPU0 CPU1
> > > [ 63.695852] ---- ----
> > > [ 63.695854] lock(&rq->__lock);
> > > [ 63.695857] lock(&p->pi_lock);
> > > [ 63.695861] lock(&rq->__lock);
> > > [ 63.695864] lock(&lock->wait_lock);
> > > [ 63.695868]
> > > [ 63.695868] *** DEADLOCK ***
> > >
> > > Fix it by using raw_spin_trylock_irqsave() instead.
> > That truncated lockdep report doesn't really explain anything. Please
> > just transcribe the full lockdep report into something small and
> > coherent.
>
> I was trying to show where the offending call is coming from. I will send a
> v2 with a condensed lockdep splat.
No no no... explain the actual problem.
Is the problem that:
sched_tick()
task_tick_mm_cid()
task_work_add()
kasan_save_stack()
idiotic crap while holding rq->__lock ?
Because afaict that is completely insane. And has nothing to do with
rtmutex.
We are not going to change rtmutex because instrumentation shit is shit.
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