From: Brian Behlendorf <behlendorf1@llnl.gov>
Date: Tue, 20 Apr 2010 22:16:27 +0000 (-0700)
Subject: Disable rw_tryupgrade() for newer kernels
X-Git-Tag: spl-0.4.9~21
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=ef6c1368848f364ac08ccba60bb88010548d9930;p=spl

Disable rw_tryupgrade() for newer kernels

For kernels using the CONFIG_RWSEM_GENERIC_SPINLOCK implementation
nothing has changed.  But if your kernel is building with arch
specific rwsems rw_tryupgrade() has been disabled until it can
be implemented correctly.  In particular, the x86 implementation
now leverages atomic primatives for serialization rather than
spinlocks.  So to get this working again it will need to be
implemented as a cmpxchg for x86 and likely something similiar
for other arches we are interested in.  For now it's safest
to simply disable it.
---

diff --git a/include/sys/rwlock.h b/include/sys/rwlock.h
index 89fdfa5..0fc8d24 100644
--- a/include/sys/rwlock.h
+++ b/include/sys/rwlock.h
@@ -70,7 +70,7 @@ extern int __down_write_trylock_locked(struct rw_semaphore *);
  */
 # if defined(_I386_RWSEM_H) || defined(_ASM_X86_RWSEM_H)
 #  define RW_COUNT(rwp)                 ((SEM(rwp)->count < 0) ? (-1) : \
-					 (SEM(rwp)->count & RWSEM_ACTIVE_MASK))
+                                        (SEM(rwp)->count & RWSEM_ACTIVE_MASK))
 # else
 #  define RW_COUNT(rwp)                 (SEM(rwp)->count & RWSEM_ACTIVE_MASK)
 # endif
@@ -225,6 +225,7 @@ RW_LOCK_HELD(krwlock_t *rwp)
         downgrade_write(SEM(rwp));                                      \
 })
 
+#if defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
 #define rw_tryupgrade(rwp)                                              \
 ({                                                                      \
         unsigned long _flags_;                                          \
@@ -239,6 +240,14 @@ RW_LOCK_HELD(krwlock_t *rwp)
         spin_unlock_irqrestore(&SEM(rwp)->wait_lock, _flags_);          \
         _rc_;                                                           \
 })
+#else
+/*
+ * This can be done correctly but for each supported arch we will need
+ * a custom cmpxchg() to atomically check and promote the rwsem.  That's
+ * not worth the trouble for now so rw_tryupgrade() will always fail.
+ */
+#define rw_tryupgrade(rwp)      ({ 0; })
+#endif
 
 int spl_rw_init(void);
 void spl_rw_fini(void);