[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>

3 files changed, 50 insertions, 2 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
diff --git a/include/asm-sparc/spinlock.h b/include/asm-sparc/spinlock.h
index 557d08959d2f..de2249b267c6 100644
--- a/include/asm-sparc/spinlock.h
+++ b/include/asm-sparc/spinlock.h
@@ -129,6 +129,7 @@ static inline void __raw_write_lock(raw_rwlock_t *rw)
         : /* no outputs */
         : "r" (lp)
         : "g2", "g4", "memory", "cc");
+        *(volatile __u32 *)&lp->lock = ~0U;
 }
 
 static inline int __raw_write_trylock(raw_rwlock_t *rw)
@@ -144,15 +145,40 @@ static inline int __raw_write_trylock(raw_rwlock_t *rw)
                 val = rw->lock & ~0xff;
                 if (val)
                         ((volatile u8*)&rw->lock)[3] = 0;
+                else
+                        *(volatile u32*)&rw->lock = ~0U;
         }
 
         return (val == 0);
 }
 
+static inline int __read_trylock(raw_rwlock_t *rw)
+{
+        register raw_rwlock_t *lp asm("g1");
+        register int res asm("o0");
+        lp = rw;
+        __asm__ __volatile__(
+        "mov    %%o7, %%g4\n\t"
+        "call   ___rw_read_try\n\t"
+        " ldstub        [%%g1 + 3], %%g2\n"
+        : "=r" (res)
+        : "r" (lp)
+        : "g2", "g4", "memory", "cc");
+        return res;
+}
+
+#define __raw_read_trylock(lock) \
+({      unsigned long flags; \
+        int res; \
+        local_irq_save(flags); \
+        res = __read_trylock(lock); \
+        local_irq_restore(flags); \
+        res; \
+})
+
 #define __raw_write_unlock(rw)  do { (rw)->lock = 0; } while(0)
 
 #define __raw_spin_lock_flags(lock, flags) __raw_spin_lock(lock)
-#define __raw_read_trylock(lock) generic__raw_read_trylock(lock)
 
 #define _raw_spin_relax(lock)   cpu_relax()
 #define _raw_read_relax(lock)   cpu_relax()