Commit 59f4a9f6 authored by Gabor Melis's avatar Gabor Melis
Browse files

1.0.24.26: fix release spinlock 

Both in the runtime and in Lisp releasing a spinlock was a simple
assignment. That doesn't work because the new value may not make it to
main memory by the time another CPU wants to acquire it making it
needlessly slow. Worse, it also allows the CPU to reorder instructions
from the critical section after the release.

There is a spinlock implementation for MIPS in the runtime, but it's
not used as we don't have threads on that platform. I don't know if
it's broken too.
parent 4d633465

src/code/target-thread.lisp

+9 −2
Original line number Diff line number Diff line
@@ -218,8 +218,15 @@ in future versions." 


(defun release-spinlock (spinlock)

  (declare (optimize (speed 3) (safety 0)))

  (setf (spinlock-value spinlock) nil)

  nil)

  ;; Simply setting SPINLOCK-VALUE to NIL is not enough as it does not

  ;; propagate to other processors, plus without a memory barrier the

  ;; CPU might reorder instructions allowing code from the critical

  ;; section to leak out. Use COMPARE-AND-SWAP for the memory barrier

  ;; effect and do some sanity checking while we are at it.

  (unless (eq *current-thread*

              (sb!ext:compare-and-swap (spinlock-value spinlock)

                                       *current-thread* nil))

    (error "Only the owner can release the spinlock ~S." spinlock)))



;;;; mutexes

src/runtime/x86-64-arch.h

+8 −6
Original line number Diff line number Diff line
@@ -49,12 +49,6 @@ get_spinlock(volatile lispobj *word,long value) 
#endif

}



static inline void

release_spinlock(volatile lispobj *word)

{

    *word=0;

}



static inline lispobj

swap_lispobjs(volatile lispobj *dest, lispobj value)

{
@@ -67,4 +61,12 @@ swap_lispobjs(volatile lispobj *dest, lispobj value) 
    return old_value;

}



static inline void

release_spinlock(volatile lispobj *word)

{

    /* A memory barrier is needed, use swap_lispobjs. See comment in

     * RELEASE-SPINLOCK in target-thread.lisp. */

    swap_lispobjs(word,0);

}



#endif /* _X86_64_ARCH_H */

src/runtime/x86-arch.h

+8 −6
Original line number Diff line number Diff line
@@ -48,12 +48,6 @@ get_spinlock(volatile lispobj *word, unsigned long value) 
#endif

}



static inline void

release_spinlock(volatile lispobj *word)

{

    *word=0;

}



#include <stdio.h>



static inline lispobj
@@ -74,6 +68,14 @@ swap_lispobjs(volatile lispobj *dest, lispobj value) 
    return old_value;

}



static inline void

release_spinlock(volatile lispobj *word)

{

    /* A memory barrier is needed, use swap_lispobjs. See comment in

     * RELEASE-SPINLOCK in target-thread.lisp. */

    swap_lispobjs(word,0);

}



extern void fast_bzero_detect(void *, size_t);

extern void (*fast_bzero_pointer)(void *, size_t);

version.lisp-expr

+1 −1
Original line number Diff line number Diff line
@@ -17,4 +17,4 @@ 
;;; checkins which aren't released. (And occasionally for internal

;;; versions, especially for internal versions off the main CVS

;;; branch, it gets hairier, e.g. "0.pre7.14.flaky4.13".)

"1.0.24.25"
 
"1.0.24.26"