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

[Leonard Crestez]

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@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>   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:

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);
         }
  }