Re: [PATCH 3/4] irq_work: Slightly simplify IRQ_WORK_PENDING clearing

Frederic Weisbecker
2019-11-13T00:22:41Z

On Tue, Nov 12, 2019 at 08:27:05PM +0000, Leonard Crestez wrote:
> On 08.11.2019 18:09, Frederic Weisbecker wrote:
> > Instead of fetching the value of flags and perform an xchg() to clear
> > a bit, just use atomic_fetch_andnot() that is more suitable to do that
> > job in one operation while keeping the full ordering.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > ---
> > kernel/irq_work.c | 7 +++----
> > 1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> > index 255454a48346..49c53f80a13a 100644
> > --- a/kernel/irq_work.c
> > +++ b/kernel/irq_work.c
> > @@ -34,7 +34,7 @@ static bool irq_work_claim(struct irq_work *work)
> > oflags = atomic_fetch_or(IRQ_WORK_CLAIMED, &work->flags);
> > /*
> > * If the work is already pending, no need to raise the IPI.
> > - * The pairing atomic_xchg() in irq_work_run() makes sure
> > + * The pairing atomic_fetch_andnot() in irq_work_run() makes sure
> > * everything we did before is visible.
> > */
> > if (oflags & IRQ_WORK_PENDING)
> > @@ -135,7 +135,6 @@ static void irq_work_run_list(struct llist_head *list)
> > {
> > struct irq_work *work, *tmp;
> > struct llist_node *llnode;
> > - int flags;
> >
> > BUG_ON(!irqs_disabled());
> >
> > @@ -144,6 +143,7 @@ static void irq_work_run_list(struct llist_head *list)
> >
> > llnode = llist_del_all(list);
> > llist_for_each_entry_safe(work, tmp, llnode, llnode) {
> > + int flags;
> > /*
> > * Clear the PENDING bit, after this point the @work
> > * can be re-used.
> > @@ -151,8 +151,7 @@ static void irq_work_run_list(struct llist_head *list)
> > * to claim that work don't rely on us to handle their data
> > * while we are in the middle of the func.
> > */
> > - flags = atomic_read(&work->flags) & ~IRQ_WORK_PENDING;
> > - atomic_xchg(&work->flags, flags);
> > + flags = atomic_fetch_andnot(IRQ_WORK_PENDING, &work->flags);
> >
> > work->func(work);
> > /*
>
> This breaks switching between cpufreq governors in linux-next on arm64
> and various other stuff. The fix in this email doesn't compile:
>
> https://lkml.org/lkml/2019/11/12/622
>
> I assume you meant "&= ~" instead of "~=", this seems to work:

Indeed, duh again!

I still think that ~= would be nice to have though...

Thanks!

>
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 49c53f80a13a..828cc30774bc 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -156,10 +156,11 @@ static void irq_work_run_list(struct llist_head *list)
> work->func(work);
> /*
> * Clear the BUSY bit and return to the free state if
> * no-one else claimed it meanwhile.
> */
> + flags &= ~IRQ_WORK_PENDING;
> (void)atomic_cmpxchg(&work->flags, flags, flags &
> ~IRQ_WORK_BUSY);
> }
> }