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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 RCU flavors, an RCU read-side critical section is entered
using rcu_read_lock(), anything that disables bottom halves, using rcu_read_lock(), anything that disables bottom halves,
anything that disables interrupts, or anything that disables 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 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, on the one hand, or protected by (say) my_lock on the other,