Implement simple accessors to probe percpu-rwsem's locked state:
percpu_is_write_locked(), percpu_is_read_locked().
Signed-off-by: Marco Elver <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>
---
v2:
* New patch.
---
include/linux/percpu-rwsem.h | 6 ++++++
kernel/locking/percpu-rwsem.c | 6 ++++++
2 files changed, 12 insertions(+)
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 5fda40f97fe9..36b942b67b7d 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -121,9 +121,15 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
preempt_enable();
}
+extern bool percpu_is_read_locked(struct percpu_rw_semaphore *);
extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);
+static inline bool percpu_is_write_locked(struct percpu_rw_semaphore *sem)
+{
+ return atomic_read(&sem->block);
+}
+
extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
const char *, struct lock_class_key *);
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 5fe4c5495ba3..213d114fb025 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -192,6 +192,12 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
__sum; \
})
+bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
+{
+ return per_cpu_sum(*sem->read_count) != 0;
+}
+EXPORT_SYMBOL_GPL(percpu_is_read_locked);
+
/*
* Return true if the modular sum of the sem->read_count per-CPU variable is
* zero. If this sum is zero, then it is stable due to the fact that if any
--
2.37.0.rc0.161.g10f37bed90-goog
On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <[email protected]> wrote:
>
> Implement simple accessors to probe percpu-rwsem's locked state:
> percpu_is_write_locked(), percpu_is_read_locked().
>
> Signed-off-by: Marco Elver <[email protected]>
> Reviewed-by: Dmitry Vyukov <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Thanks,
Ian
> ---
> v2:
> * New patch.
> ---
> include/linux/percpu-rwsem.h | 6 ++++++
> kernel/locking/percpu-rwsem.c | 6 ++++++
> 2 files changed, 12 insertions(+)
>
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index 5fda40f97fe9..36b942b67b7d 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -121,9 +121,15 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
> preempt_enable();
> }
>
> +extern bool percpu_is_read_locked(struct percpu_rw_semaphore *);
> extern void percpu_down_write(struct percpu_rw_semaphore *);
> extern void percpu_up_write(struct percpu_rw_semaphore *);
>
> +static inline bool percpu_is_write_locked(struct percpu_rw_semaphore *sem)
> +{
> + return atomic_read(&sem->block);
> +}
> +
> extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
> const char *, struct lock_class_key *);
>
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index 5fe4c5495ba3..213d114fb025 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -192,6 +192,12 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
> __sum; \
> })
>
> +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> +{
> + return per_cpu_sum(*sem->read_count) != 0;
> +}
> +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
> +
> /*
> * Return true if the modular sum of the sem->read_count per-CPU variable is
> * zero. If this sum is zero, then it is stable due to the fact that if any
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>
On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:
> +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> +{
> + return per_cpu_sum(*sem->read_count) != 0;
> +}
> +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
I don't think this is correct; read_count can have spurious increments.
If we look at __percpu_down_read_trylock(), it does roughly something
like this:
this_cpu_inc(*sem->read_count);
smp_mb();
if (!sem->block)
return true;
this_cpu_dec(*sem->read_count);
return false;
So percpu_is_read_locked() needs to ensure the read_count is non-zero
*and* that block is not set.
That said; I really dislike the whole _is_locked family with a passion.
Let me try and figure out what you need this for.
On Wed, 17 Aug 2022 at 14:48, Peter Zijlstra <[email protected]> wrote:
> On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:
> > +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> > +{
> > + return per_cpu_sum(*sem->read_count) != 0;
> > +}
> > +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
>
> I don't think this is correct; read_count can have spurious increments.
>
> If we look at __percpu_down_read_trylock(), it does roughly something
> like this:
>
> this_cpu_inc(*sem->read_count);
> smp_mb();
> if (!sem->block)
> return true;
> this_cpu_dec(*sem->read_count);
> return false;
>
> So percpu_is_read_locked() needs to ensure the read_count is non-zero
> *and* that block is not set.
I shall go and fix. v4 incoming (if more comments before that, please shout).
> That said; I really dislike the whole _is_locked family with a passion.
> Let me try and figure out what you need this for.
As in the other email, it's for the dbg_*() functions for kgdb's
benefit (avoiding deadlock if kgdb wants a breakpoint, while we're in
the process of handing out a breakpoint elsewhere and have the locks
taken).
Thanks,
-- Marco