From: Mark Charlebois <[email protected]>
The call to __builtin_log2 presumes there is a
double log2(double x) function defined in the kernel.
The call to hash_log is a call to hash_64 which is
defined in include/linux/hash.h
static __always_inline u64 hash_64(u64 val, unsigned int bits)
That means that __builtin_log2(NR_BG_LOCKS) is converting
NR_BG_LOCKS to a double and returning a double and then that
is converted to an unsigned int.
Using ilog2 is much more appropriate and efficient.
Another side effect of using __builtin_log2 is that is uses
__aeabi_* functions for ARM that require linking with libgcc.a.
Author: Mark Charlebois <[email protected]>
Signed-off-by: Mark Charlebois <[email protected]>
Signed-off-by: Behan Webster <[email protected]>
---
fs/mbcache.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/mbcache.c b/fs/mbcache.c
index bf166e3..2c0752b 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -93,7 +93,7 @@
#define MB_CACHE_WRITER ((unsigned short)~0U >> 1)
-#define MB_CACHE_ENTRY_LOCK_BITS __builtin_log2(NR_BG_LOCKS)
+#define MB_CACHE_ENTRY_LOCK_BITS ilog2(NR_BG_LOCKS)
#define MB_CACHE_ENTRY_LOCK_INDEX(ce) \
(hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))
--
1.8.3.2
I suspect this will generate FP instructions on x86 which will corrupt user space state. This is thus a critical bug!!
On April 29, 2014 6:10:39 PM PDT, [email protected] wrote:
>From: Mark Charlebois <[email protected]>
>
>The call to __builtin_log2 presumes there is a
>double log2(double x) function defined in the kernel.
>
>The call to hash_log is a call to hash_64 which is
>defined in include/linux/hash.h
>
>static __always_inline u64 hash_64(u64 val, unsigned int bits)
>
>That means that __builtin_log2(NR_BG_LOCKS) is converting
>NR_BG_LOCKS to a double and returning a double and then that
>is converted to an unsigned int.
>
>Using ilog2 is much more appropriate and efficient.
>
>Another side effect of using __builtin_log2 is that is uses
>__aeabi_* functions for ARM that require linking with libgcc.a.
>
>Author: Mark Charlebois <[email protected]>
>Signed-off-by: Mark Charlebois <[email protected]>
>Signed-off-by: Behan Webster <[email protected]>
>---
> fs/mbcache.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/fs/mbcache.c b/fs/mbcache.c
>index bf166e3..2c0752b 100644
>--- a/fs/mbcache.c
>+++ b/fs/mbcache.c
>@@ -93,7 +93,7 @@
>
> #define MB_CACHE_WRITER ((unsigned short)~0U >> 1)
>
>-#define MB_CACHE_ENTRY_LOCK_BITS __builtin_log2(NR_BG_LOCKS)
>+#define MB_CACHE_ENTRY_LOCK_BITS ilog2(NR_BG_LOCKS)
> #define MB_CACHE_ENTRY_LOCK_INDEX(ce) \
> (hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))
>
--
Sent from my mobile phone. Please pardon brevity and lack of formatting.
On 04/29/2014 06:13 PM, H. Peter Anvin wrote:
> I suspect this will generate FP instructions on x86 which will corrupt user space state. This is thus a critical bug!!
It looks like at least x86-64 gcc does the evaluation at compile time
and there aren't any fp instructions left in the code. However, it
should still be fixed.
-hpa