Commit f05eda16 authored by Yan Zhao's avatar Yan Zhao Committed by Sean Christopherson
Browse files

srcu: Add an API for a memory barrier after SRCU read lock 



To avoid redundant memory barriers, add smp_mb__after_srcu_read_lock() to
pair with smp_mb__after_srcu_read_unlock() for use in paths that need to
emit a memory barrier, but already do srcu_read_lock(), which includes a
full memory barrier.  Provide an API, e.g. as opposed to having callers
document the behavior via a comment, as the full memory barrier provided
by srcu_read_lock() is an implementation detail that shouldn't bleed into
random subsystems.

KVM will use smp_mb__after_srcu_read_lock() in it's VM-Exit path to ensure
a memory barrier is emitted, which is necessary to ensure correctness of
mixed memory types on CPUs that support self-snoop.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: default avatarYan Zhao <yan.y.zhao@intel.com>
[sean: massage changelog]
Tested-by: default avatarXiangfei Ma <xiangfeix.ma@intel.com>
Tested-by: default avatarYongwei Ma <yongwei.ma@intel.com>
Reviewed-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240309010929.1403984-4-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent e1548088

include/linux/srcu.h

+14 −0
Original line number Diff line number Diff line
@@ -343,6 +343,20 @@ static inline void smp_mb__after_srcu_read_unlock(void) 
	/* __srcu_read_unlock has smp_mb() internally so nothing to do here. */

}



/**

 * smp_mb__after_srcu_read_lock - ensure full ordering after srcu_read_lock

 *

 * Converts the preceding srcu_read_lock into a two-way memory barrier.

 *

 * Call this after srcu_read_lock, to guarantee that all memory operations

 * that occur after smp_mb__after_srcu_read_lock will appear to happen after

 * the preceding srcu_read_lock.

 */

static inline void smp_mb__after_srcu_read_lock(void)

{

	/* __srcu_read_lock has smp_mb() internally so nothing to do here. */

}



DEFINE_LOCK_GUARD_1(srcu, struct srcu_struct,

		    _T->idx = srcu_read_lock(_T->lock),

		    srcu_read_unlock(_T->lock, _T->idx),