linux-kernel - Re: [PATCH v2] rt_spin_lock: To list the correct owner of rt_spin

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Message-ID: <20231221080229.GA38@DESKTOP-IM4PCEA.localdomain>
Date: Thu, 21 Dec 2023 13:32:29 +0530
From: "<Mintu Patel>" <mintupatel89@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: badolevishal1116@...il.com, chinmoyghosh2001@...il.com,
	linux-kernel@...r.kernel.org, mingo@...hat.com, rostedt@...dmis.org,
	vimal.kumar32@...il.com, will@...nel.org
Subject: Re: [PATCH v2] rt_spin_lock: To list the correct owner of
 rt_spin_lock

On Thu, Dec 07, 2023 at 10:31:30PM +0530, <Mintu Patel> wrote:
> On Wed, Dec 06, 2023 at 07:58:37PM +0100, Peter Zijlstra wrote:
> > On Mon, Jun 27, 2022 at 09:41:38PM +0530, Mintu Patel wrote:
> > >    rt_spin_lock is actually mutex on RT Kernel so it goes for contention
> > >    for lock. Currently owners of rt_spin_lock are decided before actual
> > >    acquiring of lock. This patch would depict the correct owner of
> > >    rt_spin_lock. The patch would help in solving crashes and deadlock
> > >    due to race condition of lock
> > > 
> > > acquiring rt_spin_lock        acquired the lock       released the lock
> > >                     <-------->                <------->
> > >                     contention period         Held period
> > > 
> > > Thread1                             Thread2
> > > _try_to_take_rt_mutex+0x95c+0x74    enqueue_task_dl+0x8cc/0x8dc
> > > rt_spin_lock_slowlock_locked+0xac+2 rt_mutex_setprio+0x28c/0x574
> > > rt_spin_lock_slowlock+0x5c/0x90     task_blocks_rt_mutex+0x240/0x310
> > > rt_spin_lock+0x58/0x5c              rt_spin_lock_slowlock_locked+0xac/0x2
> > > driverA_acquire_lock+0x28/0x56      rt_spin_lock_slowlock+0x5c/0x90
> > > 				    rt_spin_lock+0x58/0x5c
> > >                                     driverB_acquire_lock+0x48/0x6c
> > > 
> > > As per above call traces sample, Thread1 acquired the rt_spin_lock and
> > > went to critical section on the other hand Thread2 kept trying to acquire
> > > the same rt_spin_lock held by Thread1 ie contention period is too high.
> > > Finally Thread2 entered to dl queue due to high held time of the lock by
> > > Thread1. The below patch would help us to know the correct owner of
> > > rt_spin_lock and point us the driver's critical section. Respective
> > > driver need to be debugged for longer held period of lock.
> > > 
> > >    ex: cat /sys/kernel/debug/tracing/trace
> > > 
> > >    kworker/u13:0-150   [003] .....11   202.761025: rt_spinlock_acquire:
> > > Process: kworker/u13:0 is acquiring lock: &kbdev->hwaccess_lock
> > >    kworker/u13:0-150   [003] .....11   202.761039: rt_spinlock_acquired:
> > > Process: kworker/u13:0 has acquired lock: &kbdev->hwaccess_lock
> > >    kworker/u13:0-150   [003] .....11   202.761042: rt_spinlock_released:
> > > Process: kworker/u13:0 has released lock: &kbdev->hwaccess_lock
> > > 
> > 
> > The above is word salad and makes no sense. No other lock has special
> > tracing like this, so rt_lock doesn't need it either.
> > 
> Hi Peter,
> 
> As per current implementation of rt_spin_lock tracing mechanism on RTLinux, 
> if more than one threads are trying to acquire a rt_spin_lock,
> then multiple threads are assigned as owners of the same lock, more over
> only one thread is actual owner of lock and others are still
> contending for the same lock. Such trace logs can mislead the developers
> during debugging of critical issues like deadlock, crashes etc
> 
> The above patch would generate rt_spin_lock locking traces which would
> depict correct owner of the lock and other thread details which 
> are trying to acquire the lock.
> 
> Regards,
> Mintu Patel 

Hi Peter,

Hope you got a chance to check the reply.

Regards,
Mintu Patel