linux-kernel - Re: [PATCH v2 5/8] rust: workqueue: add helper for defining work

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Date:   Fri,  2 Jun 2023 08:38:56 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     boqun.feng@...il.com
Cc:     alex.gaynor@...il.com, aliceryhl@...gle.com,
        benno.lossin@...ton.me, bjorn3_gh@...tonmail.com, gary@...yguo.net,
        jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        ojeda@...nel.org, patches@...ts.linux.dev,
        rust-for-linux@...r.kernel.org, tj@...nel.org, wedsonaf@...il.com
Subject: Re: [PATCH v2 5/8] rust: workqueue: add helper for defining
 work_struct fields

Boqun Feng <boqun.feng@...il.com> writes:
> On Thu, Jun 01, 2023 at 01:49:43PM +0000, Alice Ryhl wrote:
>> diff --git a/rust/helpers.c b/rust/helpers.c
>> index 81e80261d597..7f0c2fe2fbeb 100644
>> --- a/rust/helpers.c
>> +++ b/rust/helpers.c
>> @@ -26,6 +26,7 @@
>>  #include <linux/spinlock.h>
>>  #include <linux/sched/signal.h>
>>  #include <linux/wait.h>
>> +#include <linux/workqueue.h>
>>  
>>  __noreturn void rust_helper_BUG(void)
>>  {
>> @@ -128,6 +129,13 @@ void rust_helper_put_task_struct(struct task_struct *t)
>>  }
>>  EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
>>  
>> +void rust_helper___INIT_WORK(struct work_struct *work, work_func_t func,
>> +			     bool on_stack)
>> +{
>> +	__INIT_WORK(work, func, on_stack);
> 
> Note here all the work items in Rust will share the same lockdep class.
> That could be problematic: the lockdep classes for work are for
> detecting deadlocks in the following scenario:
> 
> step 1: queue a work "foo", whose work function is:
> 
> 	mutex_lock(&bar);
> 	do_something(...);
> 	mutex_unlock(&bar);
> 
> step 2: in another thread do:
> 
> 	mutex_lock(&bar);
> 	flush_work(foo);	// wait until work "foo" is finished.
> 
> if this case, if step 2 get the lock "bar" first, it's a deadlock.
> 
> With the current implementation, all the work items share the same
> lockdep class, so the following will be treated as deadlock:
> 
> 	<in work "work1">
> 	mutex_lock(&bar);
> 	do_something(...);
> 	mutex_unlock(&bar);
> 
> 	<in another thread>
> 	mutex_lock(&bar);
> 	flush_work(work2);	// flush work2 intead of work1.
> 
> which is a false positive. We at least need some changes in C side to
> make it work:
> 
> 	https://lore.kernel.org/rust-for-linux/20220802015052.10452-7-ojeda@kernel.org/
> 
> however, that still has the disadvantage that all Rust work items have
> the same name for the lockdep classes.. maybe we should extend that for
> an extra "name" parameter. And then it's not necessary to be a macro.

Yeah, I did know about this issue, but I didn't know what the best way
to fix it is. What solution would you like me to use?

Alice