Messages in this thread Patch in this message | | | From | David Howells <> | Subject | Re: x86 rwsem: up_read() does not check active count in fast path | Date | Tue, 04 May 2010 13:41:36 +0100 |
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Michel Lespinasse <walken@google.com> wrote:
> Looking at the x86 rwsem code, I have been wondering about the up_read() > path. __rwsem_do_wake() comment mentions that one should check the active > count on the way there; however I could not find that check when coming from > up_read().
Look in call_rwsem_wake(), which is implemented in assembly in two places in the x86 arch:
arch/x86/lib/rwsem_64.S arch/x86/lib/semaphore_32.S
Note that the x86_64 version contains a bug, a patch for which is attached.
David --- From: David Howells <dhowells@redhat.com> Subject: [PATCH] Fix the x86_64 implementation of call_rwsem_wait()
The x86_64 call_rwsem_wait() treats the active state counter part of the R/W semaphore state as being 16-bit when it's actually 32-bit (it's half of the 64-bit state). It should do "decl %edx" not "decw %dx".
Signed-off-by: David Howells <dhowells@redhat.com> ---
arch/x86/lib/rwsem_64.S | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/arch/x86/lib/rwsem_64.S b/arch/x86/lib/rwsem_64.S index 15acecf..41fcf00 100644 --- a/arch/x86/lib/rwsem_64.S +++ b/arch/x86/lib/rwsem_64.S @@ -60,7 +60,7 @@ ENTRY(call_rwsem_down_write_failed) ENDPROC(call_rwsem_down_write_failed) ENTRY(call_rwsem_wake) - decw %dx /* do nothing if still outstanding active readers */ + decl %edx /* do nothing if still outstanding active readers */ jnz 1f save_common_regs movq %rax,%rdi
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