Re: [PATCH] lockdep: Fix wait context check on softirq for PREEMPT_RT

[Peter Zijlstra]

On Mon, Dec 02, 2024 at 10:20:17AM +0900, Ryo Takakura wrote:
> Commit 0c1d7a2c2d32 ("lockdep: Remove softirq accounting on
> PREEMPT_RT.") stopped updating @softirq_context on PREEMPT_RT
> to ignore "inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage"
> as the report accounts softirq context which PREEMPT_RT doesn't
> have to.
> 
> However, wait context check still needs to report mutex usage
> within softirq, even when its threaded on PREEMPT_RT. The check
> is failing to report the usage as task_wait_context() checks if
> its in softirq by referencing @softirq_context, ending up not 
> assigning the correct wait type of LD_WAIT_CONFIG for PREEMPT_RT's
> softirq.
> 
> [    0.184549]   | wait context tests |
> [    0.184549]   --------------------------------------------------------------------------
> [    0.184549]                                  | rcu  | raw  | spin |mutex |
> [    0.184549]   --------------------------------------------------------------------------
> [    0.184550]                in hardirq context:  ok  |  ok  |  ok  |  ok  |
> [    0.185083] in hardirq context (not threaded):  ok  |  ok  |  ok  |  ok  |
> [    0.185606]                in softirq context:  ok  |  ok  |  ok  |FAILED|
> 
> Account softirq context but only when !PREEMPT_RT so that
> task_wait_context() returns LD_WAIT_CONFIG as intended.
> 
> Signed-off-by: Ryo Takakura <ryotkkr98@xxxxxxxxx>
> 
> 
> ---
> 
> Hi! 
> 
> I wasn't able come up with a way to fix the wait context test while 
> keeping the commit 0c1d7a2c2d32 ("lockdep: Remove softirq accounting 
> on PREEMPT_RT.") without referencing @softirq_context...
> Hoping to get a feedback on it!
> 
> Also I wonder if the test can be skipped as I believe its taken care 
> by spinlock wait context test since the PREEMPT_RT's softirq context is 
> protected by local_lock which is mapped to rt_spinlock.

Right,.. so I remember talking about this with Boqun, and I think we
were going to 'fix' the test, but I can't quite remember.

Perhaps adding the local_lock to SOFTIRQ_ENTER?