Re: [PATCH] srcu: Avoid local_irq_save() before acquiring spinlock_t

[Paul E. McKenney]

On Tue, May 26, 2020 at 03:41:34PM +0200, Sebastian Andrzej Siewior wrote:
> SRCU disables interrupts to get a stable per-CPU pointer and then
> acquires the spinlock which is in the per-CPU data structure. The
> release uses spin_unlock_irqrestore(). While this is correct on a non-RT
> kernel, this conflicts with the RT semantics because the spinlock is
> converted to a 'sleeping' spinlock. Sleeping locks can obviously not be
> acquired with interrupts disabled.
> 
> Acquire the per-CPU pointer `ssp->sda' without disabling preemption and
> then acquire the spinlock_t of the per-CPU data structure. The lock
> will ensure that the data is consistent.
> The added check_init_srcu_struct() is now needed because a statically 
> defined srcu_struct may remain uninitialized until this point and the
> newly introduced locking operation requires an initialized spinlock_t.
> 
> This change was tested for four hours with 8*SRCU-N and 8*SRCU-P without
> causing any warnings.

Queued, thank you!!!

							Thanx, Paul

> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: rcu@xxxxxxxxxxxxxxx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>  kernel/rcu/srcutree.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 0c71505f0e19c..9459bca58c380 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -764,14 +764,15 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
>  	unsigned long t;
>  	unsigned long tlast;
>  
> +	check_init_srcu_struct(ssp);
>  	/* If the local srcu_data structure has callbacks, not idle.  */
> -	local_irq_save(flags);
> -	sdp = this_cpu_ptr(ssp->sda);
> +	sdp = raw_cpu_ptr(ssp->sda);
> +	spin_lock_irqsave_rcu_node(sdp, flags);
>  	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
> -		local_irq_restore(flags);
> +		spin_unlock_irqrestore_rcu_node(sdp, flags);
>  		return false; /* Callbacks already present, so not idle. */
>  	}
> -	local_irq_restore(flags);
> +	spin_unlock_irqrestore_rcu_node(sdp, flags);
>  
>  	/*
>  	 * No local callbacks, so probabalistically probe global state.
> @@ -851,9 +852,8 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
>  	}
>  	rhp->func = func;
>  	idx = srcu_read_lock(ssp);
> -	local_irq_save(flags);
> -	sdp = this_cpu_ptr(ssp->sda);
> -	spin_lock_rcu_node(sdp);
> +	sdp = raw_cpu_ptr(ssp->sda);
> +	spin_lock_irqsave_rcu_node(sdp, flags);
>  	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
>  	rcu_segcblist_advance(&sdp->srcu_cblist,
>  			      rcu_seq_current(&ssp->srcu_gp_seq));
> -- 
> 2.27.0.rc0
>