x86: Fix write lock scalability 64-bit issue
authorJan Beulich <JBeulich@novell.com>
Tue, 19 Jul 2011 12:00:45 +0000 (13:00 +0100)
committerIngo Molnar <mingo@elte.hu>
Thu, 21 Jul 2011 07:03:36 +0000 (09:03 +0200)
commita750036f35cda160ef77408ec92c3dc41f8feebb
tree1198013e1289dfb9b5a299388ee09515642c4030
parenta738669464a1e0d8e7b20f631120192f9cf7cfbd
x86: Fix write lock scalability 64-bit issue

With the write lock path simply subtracting RW_LOCK_BIAS there
is, on large systems, the theoretical possibility of overflowing
the 32-bit value that was used so far (namely if 128 or more
CPUs manage to do the subtraction, but don't get to do the
inverse addition in the failure path quickly enough).

A first measure is to modify RW_LOCK_BIAS itself - with the new
value chosen, it is good for up to 2048 CPUs each allowed to
nest over 2048 times on the read path without causing an issue.
Quite possibly it would even be sufficient to adjust the bias a
little further, assuming that allowing for significantly less
nesting would suffice.

However, as the original value chosen allowed for even more
nesting levels, to support more than 2048 CPUs (possible
currently only for 64-bit kernels) the lock itself gets widened
to 64 bits.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/4E258E0D020000780004E3F0@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
arch/x86/include/asm/asm.h
arch/x86/include/asm/rwlock.h
arch/x86/include/asm/spinlock.h
arch/x86/include/asm/spinlock_types.h
arch/x86/lib/rwlock.S
arch/x86/lib/thunk_64.S