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doc: Spinlocks are implied RCU readers

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In kernels built with CONFIG_PREEMPT_RT=n, spinlock critical sections
are RCU readers because they disable preemption.  However, they are also
RCU readers in CONFIG_PREEMPT_RT=y because the -rt locking primitives
contain rcu_read_lock() and rcu_read_unlock().  Therefore, upgrade
rcu_dereference.rst to document this non-obvious case.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/lkml/CAHk-=whGKvjHCtJ6W4pQ0_h_k9fiFQ8V2GpM=BqYnB2X=SJ+XQ@mail.gmail.com/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
This commit is contained in:
Paul E. McKenney 2023-11-24 14:55:37 -08:00 committed by Boqun Feng
parent 41bccc98fb
commit 120311acb0
1 changed files with 4 additions and 1 deletions
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5
Documentation/RCU/rcu_dereference.rst
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@ -408,7 +408,10 @@ member of the rcu_dereference() to use in various situations:
RCU flavors, an RCU read-side critical section is entered
using rcu_read_lock(), anything that disables bottom halves,
anything that disables interrupts, or anything that disables
preemption.
preemption. Please note that spinlock critical sections
are also implied RCU read-side critical sections, even when
they are preemptible, as they are in kernels built with
CONFIG_PREEMPT_RT=y.
2. If the access might be within an RCU read-side critical section
on the one hand, or protected by (say) my_lock on the other,