• Peter Zijlstra's avatar
    arch: Introduce smp_load_acquire(), smp_store_release() · 47933ad4
    Peter Zijlstra authored
    A number of situations currently require the heavyweight smp_mb(),
    even though there is no need to order prior stores against later
    loads.  Many architectures have much cheaper ways to handle these
    situations, but the Linux kernel currently has no portable way
    to make use of them.
    
    This commit therefore supplies smp_load_acquire() and
    smp_store_release() to remedy this situation.  The new
    smp_load_acquire() primitive orders the specified load against
    any subsequent reads or writes, while the new smp_store_release()
    primitive orders the specifed store against any prior reads or
    writes.  These primitives allow array-based circular FIFOs to be
    implemented without an smp_mb(), and also allow a theoretical
    hole in rcu_assign_pointer() to be closed at no additional
    expense on most architectures.
    
    In addition, the RCU experience transitioning from explicit
    smp_read_barrier_depends() and smp_wmb() to rcu_dereference()
    and rcu_assign_pointer(), respectively resulted in substantial
    improvements in readability.  It therefore seems likely that
    replacing other explicit barriers with smp_load_acquire() and
    smp_store_release() will provide similar benefits.  It appears
    that roughly half of the explicit barriers in core kernel code
    might be so replaced.
    
    [Changelog by PaulMck]
    Reviewed-by: default avatar"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Acked-by: default avatarWill Deacon <will.deacon@arm.com>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    Cc: Michael Ellerman <michael@ellerman.id.au>
    Cc: Michael Neuling <mikey@neuling.org>
    Cc: Russell King <linux@arm.linux.org.uk>
    Cc: Geert Uytterhoeven <geert@linux-m68k.org>
    Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: Victor Kaplansky <VICTORK@il.ibm.com>
    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Link: http://lkml.kernel.org/r/20131213150640.908486364@infradead.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    47933ad4
barrier.h 2.89 KB