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rcu: Update lockdep while in RCU read-side critical section

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With Ankur's lazy-/auto-preemption patches applied and with a
lazy-preemptible kernel in combination with a non-preemptible RCU,
lockdep sometimes complains about context switches within RCU read-side
critical sections.  This is a false positive due to rcu_read_unlock()
updating lockdep state too late:

	__release(RCU);
	__rcu_read_unlock();
	// Context switch here results in lockdep false positive!!!
	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */

Although this complaint could also happen with preemptible RCU
in a preemptible kernel, the odds of that happening aer quite low.
In constrast, with non-preemptible RCU, a long critical section has a
high probability of performing a context switch from the preempt_enable()
in __rcu_read_unlock().

The fix is straightforward, just move the rcu_lock_release()
within rcu_read_unlock() to obtain the reverse order from that of
rcu_read_lock():

	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
	__release(RCU);
	__rcu_read_unlock();

This commit makes this change.

Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Uladzislau Rezki (Sony) 2024-02-14 14:45:53 -08:00
parent 3183059ad8
commit 058e87782c
1 changed files with 1 additions and 1 deletions
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2
include/linux/rcupdate.h
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@ -809,9 +809,9 @@ static inline void rcu_read_unlock(void)
{
RCU_LOCKDEP_WARN(!rcu_is_watching(),
"rcu_read_unlock() used illegally while idle");
rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
__release(RCU);
__rcu_read_unlock();
rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
}
/**