Re: [PATCH v3 1/7] srcutiny: Do not call synchronize_rcu() from synchronize_srcu_atomic()

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Nov 12, 2026 at 08:37:25AM -0500, Mathieu Desnoyers a écrit :
> synchronize_srcu_atomic() never blocks, so that it can be called from
> atomic context.  This includes interrupt handlers, and code within
> rcu_read_lock().
> 
> With CONFIG_PREEMPTION=y, the Tiny SRCU version first calls
> synchronize_rcu().  Tiny synchronize_rcu() does not wait: it only
> advances rcu_ctrlblk.gp_seq.  This is valid where the caller is in a
> quiescent state, in task context and outside of any RCU reader.
> 
> The callers of synchronize_srcu_atomic() need not be in a quiescent
> state.  An interrupt handler may have interrupted an RCU reader.  A
> caller within rcu_read_lock() is itself an RCU reader.  In both cases,
> synchronize_rcu() ends an RCU grace period while an RCU reader is still
> in its critical section.  poll_state_synchronize_rcu() would then
> report a grace period that did not happen.  The caller within
> rcu_read_lock() also gets "Illegal synchronize_rcu() in RCU read-side
> critical section" from lockdep.
> 
> srcu_drive_gp() has the same call.  There, it makes an RCU Tasks
> Trace grace period imply an RCU grace period.  RCU Tasks Trace uses
> call_srcu() and synchronize_srcu(), which are both forbidden on a
> srcu_struct that uses synchronize_srcu_atomic().  And the Tree SRCU
> version of synchronize_srcu_atomic() does not wait for an RCU grace
> period either.
> 
> Fix this by removing the call.
> 
> This was found by reading the code.  An rcutorture test added later in
> this series fails without this change.
> 
> Fixes: a48669f0c49c ("srcutiny: Add an atomic Tiny SRCU")
> 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