linux-kernel - Re: [PATCH] lockdep: Let lock_is_held_type() detect recursive read as read

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Message-ID: <9af2b074-9fcf-5aea-f37d-9b2482146489@redhat.com>
Date:   Wed, 1 Sep 2021 14:32:54 -0400
From:   Waiman Long <llong@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] lockdep: Let lock_is_held_type() detect recursive read as
 read

On 9/1/21 12:22 PM, Sebastian Andrzej Siewior wrote:
> lock_is_held_type(, 1) detects acquired read locks. It only recognized
> locks acquired with lock_acquire_shared(). Read locks acquired with
> lock_acquire_shared_recursive() are not recognized because a `2' is
> stored as the read value.
>
> Rework the check to additionally recognise lock's read value one and two
> as a read held lock.
>
> Fixes: e918188611f07 ("locking: More accurate annotations for read_lock()")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>
> On a related note: What exactly means read_lock_is_recursive() in terms
> of recursive locking? The second items mentions QRW locks. Does this
> mean that a pending WRITER blocks further READER from acquiring the
> lock?
>
>   kernel/locking/lockdep.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index f15df3fd7c5a6..39f98454a8827 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -5366,7 +5366,9 @@ int __lock_is_held(const struct lockdep_map *lock, int read)
>   		struct held_lock *hlock = curr->held_locks + i;
>   
>   		if (match_held_lock(hlock, lock)) {
> -			if (read == -1 || hlock->read == read)
> +			if (read == -1 ||
> +			    (read == 0 && hlock->read == 0) ||
> +			    (read == 1 && hlock->read > 0))
>   				return LOCK_STATE_HELD;
>   
>   			return LOCK_STATE_NOT_HELD;

I think the check can be simplified as

     if (read == -1 || read == !!hlock->read)

Cheers,
Longman