diff options
| author | Vineet Gupta <vgupta@synopsys.com> | 2014-11-13 15:54:01 +0530 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2015-07-21 10:10:12 -0700 |
| commit | 83f36a272a4dcbf63360e7a3aac81b6f147961fd (patch) | |
| tree | da59fdfd4dff95ed90cf161194af171b8b7fae82 | |
| parent | 373981c510d6be2ab76a9a09bacf458011e9f02b (diff) | |
ARC: add compiler barrier to LLSC based cmpxchg
commit d57f727264f1425a94689bafc7e99e502cb135b5 upstream.
When auditing cmpxchg call sites, Chuck noted that gcc was optimizing
away some of the desired LDs.
| do {
| new = old = *ipi_data_ptr;
| new |= 1U << msg;
| } while (cmpxchg(ipi_data_ptr, old, new) != old);
was generating to below
| 8015cef8: ld r2,[r4,0] <-- First LD
| 8015cefc: bset r1,r2,r1
|
| 8015cf00: llock r3,[r4] <-- atomic op
| 8015cf04: brne r3,r2,8015cf10
| 8015cf08: scond r1,[r4]
| 8015cf0c: bnz 8015cf00
|
| 8015cf10: brne r3,r2,8015cf00 <-- Branch doesn't go to orig LD
Although this was fixed by adding a ACCESS_ONCE in this call site, it
seems safer (for now at least) to add compiler barrier to LLSC based
cmpxchg
Reported-by: Chuck Jordan <cjordan@synopsys.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
| -rw-r--r-- | arch/arc/include/asm/cmpxchg.h | 9 |
1 files changed, 5 insertions, 4 deletions
diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h index c9b1f461a587..44fd531f4d7b 100644 --- a/arch/arc/include/asm/cmpxchg.h +++ b/arch/arc/include/asm/cmpxchg.h @@ -33,10 +33,11 @@ __cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new) " scond %3, [%1] \n" " bnz 1b \n" "2: \n" - : "=&r"(prev) - : "r"(ptr), "ir"(expected), - "r"(new) /* can't be "ir". scond can't take limm for "b" */ - : "cc"); + : "=&r"(prev) /* Early clobber, to prevent reg reuse */ + : "r"(ptr), /* Not "m": llock only supports reg direct addr mode */ + "ir"(expected), + "r"(new) /* can't be "ir". scond can't take LIMM for "b" */ + : "cc", "memory"); /* so that gcc knows memory is being written here */ smp_mb(); |