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

[Christian Brauner]

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));
 }

 static inline bool __sb_start_write_trylock(struct super_block *sb, int level)