[PATCH] sparc32 rwlock fix

read_trylock() is broken on sparc32 (doesn't build and didn't work
right, actually).  Proposed fix:

 - make "writer holds lock" distinguishable from "reader tries to grab
   lock"

 - have __raw_read_trylock() try to acquire the mutex (in LSB of lock),
   terminating spin if we see that there's writer holding it.  Then do
   the rest as we do in read_lock().

Thanks to Ingo for discussion...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 23 insertions, 1 deletions

diff --git a/arch/sparc/kernel/sparc_ksyms.c b/arch/sparc/kernel/sparc_ksyms.c
index 4d441a554d35..33dadd9f2871 100644
--- a/arch/sparc/kernel/sparc_ksyms.c
+++ b/arch/sparc/kernel/sparc_ksyms.c
@@ -87,6 +87,7 @@ extern void ___set_bit(void);
 extern void ___clear_bit(void);
 extern void ___change_bit(void);
 extern void ___rw_read_enter(void);
+extern void ___rw_read_try(void);
 extern void ___rw_read_exit(void);
 extern void ___rw_write_enter(void);
 
@@ -104,8 +105,9 @@ extern unsigned _Urem(unsigned, unsigned);
 EXPORT_SYMBOL(sparc_cpu_model);
 EXPORT_SYMBOL(kernel_thread);
 #ifdef CONFIG_SMP
-// XXX find what uses (or used) these.
+// XXX find what uses (or used) these.   AV: see asm/spinlock.h
 EXPORT_SYMBOL(___rw_read_enter);
+EXPORT_SYMBOL(___rw_read_try);
 EXPORT_SYMBOL(___rw_read_exit);
 EXPORT_SYMBOL(___rw_write_enter);
 #endif
diff --git a/arch/sparc/lib/locks.S b/arch/sparc/lib/locks.S
index 95fa48424967..b1df55cb2215 100644
--- a/arch/sparc/lib/locks.S
+++ b/arch/sparc/lib/locks.S
@@ -25,6 +25,15 @@ ___rw_read_enter_spin_on_wlock:
          ldstub [%g1 + 3], %g2
         b       ___rw_read_enter_spin_on_wlock
          ldub   [%g1 + 3], %g2
+___rw_read_try_spin_on_wlock:
+        andcc   %g2, 0xff, %g0
+        be,a    ___rw_read_try
+         ldstub [%g1 + 3], %g2
+        xnorcc  %g2, 0x0, %o0   /* if g2 is ~0, set o0 to 0 and bugger off */
+        bne,a   ___rw_read_enter_spin_on_wlock
+         ld     [%g1], %g2
+        retl
+         mov    %g4, %o7
 ___rw_read_exit_spin_on_wlock:
         orcc    %g2, 0x0, %g0
         be,a    ___rw_read_exit
@@ -60,6 +69,17 @@ ___rw_read_exit:
         retl
          mov    %g4, %o7
 
+        .globl  ___rw_read_try
+___rw_read_try:
+        orcc    %g2, 0x0, %g0
+        bne     ___rw_read_try_spin_on_wlock
+         ld     [%g1], %g2
+        add     %g2, 1, %g2
+        st      %g2, [%g1]
+        set     1, %o1
+        retl
+         mov    %g4, %o7
+
         .globl  ___rw_write_enter
 ___rw_write_enter:
         orcc    %g2, 0x0, %g0