linux-kernel - Re: [POC 0/6] Allow SpinLockIrq to use a normal Guard interface

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Message-ID: <d43fd0406cff2fb07a7dc2e96443c466f934cb31.camel@redhat.com>
Date: Thu, 31 Oct 2024 16:56:51 -0400
From: Lyude Paul <lyude@...hat.com>
To: Boqun Feng <boqun.feng@...il.com>, Thomas Gleixner <tglx@...utronix.de>
Cc: Dirk Behme <dirk.behme@...il.com>, rust-for-linux@...r.kernel.org, 
 Danilo Krummrich <dakr@...hat.com>, airlied@...hat.com, Ingo Molnar
 <mingo@...hat.com>,  will@...nel.org, Waiman Long <longman@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, Miguel
 Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 wedsonaf@...il.com,  Gary Guo <gary@...yguo.net>, Björn
 Roy Baron <bjorn3_gh@...tonmail.com>,  Benno Lossin
 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>,
 aliceryhl@...gle.com,  Trevor Gross <tmgross@...ch.edu>
Subject: Re: [POC 0/6] Allow SpinLockIrq to use a normal Guard interface

So besides the Co-developed-by corrections and the few issues I pointed out, I
definitely like the design that we have here - and it's nice to see that we
can reasonably reuse SpinLockBackend with SpinLockIrqBackend now!

Reviewed-by: Lyude Paul <lyude@...hat.com>

On Thu, 2024-10-17 at 22:51 -0700, Boqun Feng wrote:
> Hi Thomas,
> 
> So this series is what I proposed, previously, because the nested
> interrupt API in C is local_irq_save() and local_irq_restore(), the
> following Rust code has the problem of enabling interrupt earlier:
> 
> 	// l1 and l2 are interrupt disabling locks, their guards (i.e.
> 	// return of lock()) can be used to track interrupt state.
> 
> 	// interrupts are enabled in the beginning.
> 	
> 	let g1 = l1.lock(); // previous interrupt state is enabled.
> 	let g2 = l2.lock(); // previous interrupt state is disabled.
> 
> 	drop(g1); // release l1, if we use g1's state, interrupt will be
> 		  // enabled. But this is obviously wrong. Because g2
> 		  // can only exist with interrupt disabled.
> 
> With the new interrupt disable and enable API, instead of a "unsigned
> long", a percpu variable is used to track the outermost interrupt state
> and the nested level, so that "drop(g1);" above won't enable interrupts.
> 
> Although this requires extra cost, but I think it might be worth paying,
> because this could make Rust's SpinLockIrq simply use a guard interface
> as SpinLock.
> 
> Of course, looking for any comments and suggestions.
> 
> Boqun Feng (3):
>   irq & spin_lock: Add counted interrupt disabling/enabling
>   rust: helper: Add spin_{un,}lock_irq_{enable,disable}() helpers
>   rust: sync: lock: Add `Backend::BackendInContext`
> 
> Lyude Paul (3):
>   rust: Introduce interrupt module
>   rust: sync: Add SpinLockIrq
>   rust: sync: Introduce lock::Backend::Context
> 
>  include/linux/irqflags.h          |  32 +++++++++-
>  include/linux/irqflags_types.h    |   6 ++
>  include/linux/spinlock.h          |  13 ++++
>  include/linux/spinlock_api_smp.h  |  29 +++++++++
>  include/linux/spinlock_rt.h       |  10 +++
>  kernel/locking/spinlock.c         |  16 +++++
>  kernel/softirq.c                  |   3 +
>  rust/helpers/helpers.c            |   1 +
>  rust/helpers/interrupt.c          |  18 ++++++
>  rust/helpers/spinlock.c           |  10 +++
>  rust/kernel/interrupt.rs          |  64 +++++++++++++++++++
>  rust/kernel/lib.rs                |   1 +
>  rust/kernel/sync.rs               |   2 +-
>  rust/kernel/sync/lock.rs          |  33 +++++++++-
>  rust/kernel/sync/lock/mutex.rs    |   2 +
>  rust/kernel/sync/lock/spinlock.rs | 103 ++++++++++++++++++++++++++++++
>  16 files changed, 340 insertions(+), 3 deletions(-)
>  create mode 100644 rust/helpers/interrupt.c
>  create mode 100644 rust/kernel/interrupt.rs
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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