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: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Will 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.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Another whitespace clean-up, this removes tabs from between sentences in
some comments.
Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6103/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Having received another series of whitespace patches I decided to do this
once and for all rather than dealing with this kind of patches trickling
in forever.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
The current locking mechanism uses a ll/sc sequence to release a
spinlock. This is slower than a wmb() followed by a store to unlock.
The branching forward to .subsection 2 on sc failure slows down the
contended case. So we get rid of that part too.
Since we are now working on naturally aligned u16 values, we can get
rid of a masking operation as the LHU already does the right thing.
The ANDI are reversed for better scheduling on multi-issue CPUs
On a 12 CPU 750MHz Octeon cn5750 this patch improves ipv4 UDP packet
forwarding rates from 3.58*10^6 PPS to 3.99*10^6 PPS, or about 11%.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/937/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
In order to achieve correct synchronization semantics, the Octeon port
had defined CONFIG_WEAK_REORDERING_BEYOND_LLSC. This resulted in code
that looks like:
sync
ll ...
.
.
.
sc ...
.
.
sync
The second SYNC was redundant, but harmless.
Octeon has a SYNCW instruction that acts as a write-memory-barrier
(due to an erratum in some parts two SYNCW are used). It is much
faster than SYNC because it imposes ordering on the writes, but
doesn't otherwise stall the execution pipeline. On Octeon, SYNC
stalls execution until all preceeding writes are committed to the
coherent memory system.
Using:
syncw;syncw
ll
.
.
.
sc
.
.
Has identical semantics to the first sequence, but is much faster.
The SYNCW orders the writes, and the SC will not complete successfully
until the write is committed to the coherent memory system. So at the
end all preceeding writes have been committed. Since Octeon does not
do speculative reads, this functions as a full barrier.
The patch removes CONFIG_WEAK_REORDERING_BEYOND_LLSC, and substitutes
SYNCW for SYNC in write-memory-barriers.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/850/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Replace some instances of smp_llsc_mb() with a new macro
smp_mb__before_llsc(). It is used before ll/sc sequences that are
documented as needing write barrier semantics.
The default implementation of smp_mb__before_llsc() is just smp_llsc_mb(),
so there are no changes in semantics.
Also simplify definition of smp_mb(), smp_rmb(), and smp_wmb() to be just
barrier() in the non-SMP case.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/851/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
The smp_llsc_rmb() and smp_llsc_wmb() macros are not used in the tree,
remove them.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/848/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Peter Zijlstra
Maciej W. Rozycki
Ralf Baechle
David Howells
David Daney