Re: [PATCH v3 2/7] srcu: Rule for using synchronize_srcu_atomic() from interrupt handlers

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Nov 12, 2026 at 08:37:26AM -0500, Mathieu Desnoyers a écrit :
> This is about the update side.  A user that calls
> synchronize_srcu_atomic() from an interrupt handler must disable
> interrupts around all its calls on that srcu_struct, task context
> included.  Document this rule, and warn when it is broken.  The rules
> for readers are unchanged.
> 
> synchronize_srcu_atomic() spins rather than sleeps, so it may be called
> from atomic context, interrupt handlers included.  Its callers on one
> srcu_struct exclude each other with ->srcu_atomic_gp_flag.  One of them
> drives the grace period.  The others spin until a grace period ends.
> 
> The flag owner disables only preemption.  So an interrupt can arrive on
> its CPU while it holds the flag.  If that interrupt handler calls
> synchronize_srcu_atomic() on the same srcu_struct, it spins forever:
> 
>   CPU 0, task                        CPU 0, hardirq
>   synchronize_srcu_atomic()
>     takes ->srcu_atomic_gp_flag
>                                      synchronize_srcu_atomic()
>                                        spins until a grace period ends
>                                        it never ends: its driver is the
>                                        task this interrupt interrupted
> 
> Nothing reports it.  lockdep cannot, because lock_sync() records no
> interrupt state.
> 
> synchronize_srcu_atomic() cannot disable interrupts itself.  A reader
> that disables only preemption may wait for an IPI to the waiter's CPU.
> That IPI never runs while the waiter spins with interrupts disabled.
> The kernel-doc already lets the caller choose the waiter's context.
> It then asks the readers to run in that context, or a stricter one.
> 
> So the caller disables interrupts.  The existing reader rule then
> requires the readers of that srcu_struct to disable interrupts too.
> 
> Also store the owner's CPU number plus one in the flag, instead of 1.
> A waiter that finds its own CPU as the owner warns once.  It still
> hangs, but no longer silently.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Tested-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Kunwu Chan <chentao@kylinos.cn>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

-- 
Frederic Weisbecker
SUSE Labs