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Date:   Thu, 30 Mar 2023 11:56:07 -0700
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Wedson Almeida Filho <wedsonaf@gmail.com>,
        rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Gary Guo <gary@garyguo.net>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <walmeida@microsoft.com>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH 03/13] rust: lock: introduce `Mutex`
Message-ID: <ZCXbR+Pnff6jrstu@boqun-archlinux>
References: <20230330043954.562237-1-wedsonaf@gmail.com>
 <20230330043954.562237-3-wedsonaf@gmail.com>
 <20230330130108.GE124812@hirez.programming.kicks-ass.net>
 <ZCXZMNj7aOKbC7Ev@boqun-archlinux>
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On Thu, Mar 30, 2023 at 11:47:12AM -0700, Boqun Feng wrote:
> On Thu, Mar 30, 2023 at 03:01:08PM +0200, Peter Zijlstra wrote:
> > On Thu, Mar 30, 2023 at 01:39:44AM -0300, Wedson Almeida Filho wrote:
> > > From: Wedson Almeida Filho <walmeida@microsoft.com>
> > > 
> > > This is the `struct mutex` lock backend and allows Rust code to use the
> > > kernel mutex idiomatically.
> > 
> > What, if anything, are the plans to support the various lockdep
> > annotations? Idem for the spinlock thing in the other patch I suppose.
> 
> FWIW:
> 
> *	At the init stage, SpinLock and Mutex in Rust use initializers
> 	that are aware of the lockdep, so everything (lockdep_map and
> 	lock_class) is all set up.
> 
> *	At acquire or release time, Rust locks just use ffi to call C
> 	functions that have lockdep annotations in them, so lockdep
> 	should just work.
> 
> In fact, I shared some same worry as you, so I already work on adding
> lockdep selftests for Rust lock APIs, will send them shortly, although
> they are just draft.
> 

Needless to say, the test shows that lockdep works for deadlock
detection (although currently they are only simple cases):

	[...] locking selftest: Selftests for Rust locking APIs
	[...] rust_locking_selftest::SpinLockAATest: 
	[...] 
	[...] ============================================
	[...] WARNING: possible recursive locking detected
	[...] 6.3.0-rc1-00049-gee35790bd43e-dirty #99 Not tainted
	[...] --------------------------------------------
	[...] swapper/0/0 is trying to acquire lock:
	[...] ffffffff8c603e30 (A1){+.+.}-{2:2}, at: _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...] 
	[...] but task is already holding lock:
	[...] ffffffff8c603de0 (A1){+.+.}-{2:2}, at: _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...] 
	[...] other info that might help us debug this:
	[...]  Possible unsafe locking scenario:
	[...] 
	[...]        CPU0
	[...]        ----
	[...]   lock(A1);
	[...]   lock(A1);
	[...] 
	[...]  *** DEADLOCK ***
	[...] 
	[...]  May be due to missing lock nesting notation
	[...] 
	[...] 1 lock held by swapper/0/0:
	[...]  #0: ffffffff8c603de0 (A1){+.+.}-{2:2}, at: _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...] 
	[...] stack backtrace:
	[...] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.3.0-rc1-00049-gee35790bd43e-dirty #99
	[...] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.1-1-1 04/01/2014
	[...] Call Trace:
	[...]  <TASK>
	[...]  dump_stack_lvl+0x6d/0xa0
	[...]  __lock_acquire+0x825/0x2e20
	[...]  ? __lock_acquire+0x626/0x2e20
	[...]  ? prb_read_valid+0x24/0x50
	[...]  ? printk_get_next_message+0xf6/0x380
	[...]  ? _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...]  lock_acquire+0x109/0x2c0
	[...]  ? _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...]  _raw_spin_lock+0x2e/0x40
	[...]  ? _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...]  _RNvXNtNtNtCs1t6xtuX2C8s_6kernel4sync4lock8spinlockNtB2_15SpinLockBackendNtB4_7Backend4lock+0x6/0x10
	[...]  _RNvXCsaDWbe1gW6fC_21rust_locking_selftestNtB2_14SpinLockAATestNtB2_8LockTest4test+0xa5/0xe0
	[...]  ? prb_read_valid+0x24/0x50
	[...]  dotest+0x5a/0x8d0
	[...]  rust_locking_test+0xf8/0x210
	[...]  ? _printk+0x58/0x80
	[...]  ? local_lock_release+0x60/0x60
	[...]  locking_selftest+0x328f/0x32b0
	[...]  start_kernel+0x285/0x420
	[...]  secondary_startup_64_no_verify+0xe1/0xeb
	[...]  </TASK>
	[...]   ok  | lockclass mask: 100, debug_locks: 0, expected: 0

Regards,
Boqun

> Regards,
> Boqun