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Commit 3a8d1788 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

x86: atomic64: Improve atomic64_xchg() 



Remove the read-first logic from atomic64_xchg() and simplify
the loop.

This function was the last user of __atomic64_read() - remove it.

Also, change the 'real_val' assumption from the somewhat quirky
1ULL << 32 value to the (just as arbitrary, but simpler) value
of 0.

Reported-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
LKML-Reference: <tip-05118ab8859492ac9ddda0154cf90e37b0a4a0b0@git.kernel.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 1fde902d

arch/x86/include/asm/atomic_32.h

+0 −9
Original line number Diff line number Diff line
@@ -268,15 +268,6 @@ typedef struct { 


#define ATOMIC64_INIT(val)	{ (val) }



/**

 * atomic64_read - read atomic64 variable

 * @ptr: pointer of type atomic64_t

 *

 * Atomically reads the value of @v.

 * Doesn't imply a read memory barrier.

 */

#define __atomic64_read(ptr)		((ptr)->counter)



extern u64 atomic64_cmpxchg(atomic64_t *ptr, u64 old_val, u64 new_val);



/**

arch/x86/lib/atomic64_32.c

+15 −6
Original line number Diff line number Diff line
@@ -33,14 +33,23 @@ EXPORT_SYMBOL(atomic64_cmpxchg); 
 * Atomically xchgs the value of @ptr to @new_val and returns

 * the old value.

 */



u64 atomic64_xchg(atomic64_t *ptr, u64 new_val)

{

	u64 old_val;

	/*

	 * Try first with a (possibly incorrect) assumption about

	 * what we have there. We'll do two loops most likely,

	 * but we'll get an ownership MESI transaction straight away

	 * instead of a read transaction followed by a

	 * flush-for-ownership transaction:

	 */

	u64 old_val, real_val = 0;



	do {

		old_val = __atomic64_read(ptr);

	} while (atomic64_cmpxchg(ptr, old_val, new_val) != old_val);

		old_val = real_val;



		real_val = atomic64_cmpxchg(ptr, old_val, new_val);



	} while (real_val != old_val);



	return old_val;

}
@@ -91,13 +100,13 @@ EXPORT_SYMBOL(atomic64_read); 
noinline u64 atomic64_add_return(u64 delta, atomic64_t *ptr)

{

	/*

	 * Try first with a (probably incorrect) assumption about

	 * Try first with a (possibly incorrect) assumption about

	 * what we have there. We'll do two loops most likely,

	 * but we'll get an ownership MESI transaction straight away

	 * instead of a read transaction followed by a

	 * flush-for-ownership transaction:

	 */

	u64 old_val, new_val, real_val = 1ULL << 32;

	u64 old_val, new_val, real_val = 0;



	do {

		old_val = real_val;