linux-kernel - Re: [RFC PATCH 1/4] locking/percpu-rwsem: add freezable alternative to down

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Message-ID: <f6bdfa23b9f54055f8a539ce396f1134b0921417.camel@HansenPartnership.com>
Date: Mon, 31 Mar 2025 21:13:20 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Christian Brauner <brauner@...nel.org>, jack@...e.cz
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 mcgrof@...nel.org, hch@...radead.org, david@...morbit.com,
 rafael@...nel.org,  djwong@...nel.org, pavel@...nel.org,
 peterz@...radead.org, mingo@...hat.com,  will@...nel.org,
 boqun.feng@...il.com
Subject: Re: [RFC PATCH 1/4] locking/percpu-rwsem: add freezable alternative
 to down_read

On Tue, 2025-04-01 at 01:32 +0200, Christian Brauner wrote:
> On Mon, Mar 31, 2025 at 03:51:43PM -0400, James Bottomley wrote:
> > On Thu, 2025-03-27 at 10:06 -0400, James Bottomley wrote:
> > [...]
> > > -static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem,
> > > bool
> > > reader)
> > > +static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem,
> > > bool
> > > reader,
> > > +			      bool freeze)
> > >  {
> > >  	DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
> > >  	bool wait;
> > > @@ -156,7 +157,8 @@ static void percpu_rwsem_wait(struct
> > > percpu_rw_semaphore *sem, bool reader)
> > >  	spin_unlock_irq(&sem->waiters.lock);
> > >  
> > >  	while (wait) {
> > > -		set_current_state(TASK_UNINTERRUPTIBLE);
> > > +		set_current_state(TASK_UNINTERRUPTIBLE |
> > > +				  freeze ? TASK_FREEZABLE : 0);
> > 
> > This is a bit embarrassing, the bug I've been chasing is here: the
> > ?
> > operator is lower in precedence than | meaning this expression
> > always
> > evaluates to TASK_FREEZABLE and nothing else (which is why the
> > process
> > goes into R state and never wakes up).
> > 
> > Let me fix that and redo all the testing.
> 
> I don't think that's it. I think you're missing making pagefault
> writers such
> as systemd-journald freezable:
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b379a46b5576..528e73f192ac 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1782,7 +1782,8 @@ static inline void __sb_end_write(struct
> super_block *sb, int level)
>  static inline void __sb_start_write(struct super_block *sb, int
> level)
>  {
>         percpu_down_read_freezable(sb->s_writers.rw_sem + level - 1,
> -                                  level == SB_FREEZE_WRITE);
> +                                  (level == SB_FREEZE_WRITE ||
> +                                   level == SB_FREEZE_PAGEFAULT));
>  }

Yes, I was about to tell Jan that the condition here simply needs to be
true.  All our rwsem levels need to be freezable to avoid a hibernation
failure.

Regards,

James