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locking/atomic, xen: Use sync_try_cmpxchg() instead of sync_cmpxchg()

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Use sync_try_cmpxchg() instead of sync_cmpxchg(*ptr, old, new) == old
in clear_masked_cond(), clear_linked() and
gnttab_end_foreign_access_ref_v1(). x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg), improving the
cmpxchg loop in gnttab_end_foreign_access_ref_v1() from:

     174:	eb 0e                	jmp    184 <...>
     176:	89 d0                	mov    %edx,%eax
     178:	f0 66 0f b1 31       	lock cmpxchg %si,(%rcx)
     17d:	66 39 c2             	cmp    %ax,%dx
     180:	74 11                	je     193 <...>
     182:	89 c2                	mov    %eax,%edx
     184:	89 d6                	mov    %edx,%esi
     186:	66 83 e6 18          	and    $0x18,%si
     18a:	74 ea                	je     176 <...>

to:

     614:	89 c1                	mov    %eax,%ecx
     616:	66 83 e1 18          	and    $0x18,%cx
     61a:	75 11                	jne    62d <...>
     61c:	f0 66 0f b1 0a       	lock cmpxchg %cx,(%rdx)
     621:	75 f1                	jne    614 <...>

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
This commit is contained in:
Uros Bizjak 2023-07-10 21:01:27 +02:00 committed by Ingo Molnar
parent 636d6a8b85
commit ad0a2e4c2f
2 changed files with 16 additions and 20 deletions
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26
drivers/xen/events/events_fifo.c
View File

@ -226,21 +226,20 @@ static bool evtchn_fifo_is_masked(evtchn_port_t port)
*/
static bool clear_masked_cond(volatile event_word_t *word)
{
event_word_t new, old, w;
event_word_t new, old;
w = *word;
old = *word;
do {
if (!(w & (1 << EVTCHN_FIFO_MASKED)))
if (!(old & (1 << EVTCHN_FIFO_MASKED)))
return true;
if (w & (1 << EVTCHN_FIFO_PENDING))
if (old & (1 << EVTCHN_FIFO_PENDING))
return false;
old = w & ~(1 << EVTCHN_FIFO_BUSY);
old = old & ~(1 << EVTCHN_FIFO_BUSY);
new = old & ~(1 << EVTCHN_FIFO_MASKED);
w = sync_cmpxchg(word, old, new);
} while (w != old);
} while (!sync_try_cmpxchg(word, &old, new));
return true;
}
@ -259,17 +258,16 @@ static void evtchn_fifo_unmask(evtchn_port_t port)
static uint32_t clear_linked(volatile event_word_t *word)
{
event_word_t new, old, w;
event_word_t new, old;
w = *word;
old = *word;
do {
old = w;
new = (w & ~((1 << EVTCHN_FIFO_LINKED)
| EVTCHN_FIFO_LINK_MASK));
} while ((w = sync_cmpxchg(word, old, new)) != old);
new = (old & ~((1 << EVTCHN_FIFO_LINKED)
| EVTCHN_FIFO_LINK_MASK));
} while (!sync_try_cmpxchg(word, &old, new));
return w & EVTCHN_FIFO_LINK_MASK;
return old & EVTCHN_FIFO_LINK_MASK;
}
static void consume_one_event(unsigned cpu, struct evtchn_loop_ctrl *ctrl,

10
drivers/xen/grant-table.c
View File

@ -427,16 +427,14 @@ EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access);
static int gnttab_end_foreign_access_ref_v1(grant_ref_t ref)
{
u16 flags, nflags;
u16 *pflags;
u16 *pflags = &gnttab_shared.v1[ref].flags;
u16 flags;
pflags = &gnttab_shared.v1[ref].flags;
nflags = *pflags;
flags = *pflags;
do {
flags = nflags;
if (flags & (GTF_reading|GTF_writing))
return 0;
} while ((nflags = sync_cmpxchg(pflags, flags, 0)) != flags);
} while (!sync_try_cmpxchg(pflags, &flags, 0));
return 1;
}