Remove the nanosleep calls from the spin loops in the locking code.

They provided little benefit (if any) and they caused some problems
in OpenOffice, at least in post-KSE -current and perhaps in other
environments too.  The nanosleep calls prevented the profiling timer
from advancing during the spinloops, thereby preventing the thread
scheduler from ever pre-empting the spinning thread.  Alexander
Kabaev diagnosed this problem, Martin Blapp helped with testing,
and Matt Dillon provided some helpful suggestions.

This is a short-term fix for a larger problem.  The use of spinlocking
isn't guaranteed to work in all cases.  For example, if the spinning
thread has higher priority than all other threads, it may never be
pre-empted, and the thread holding the lock may never progress far
enough to release the lock.  On the other hand, spinlocking is the
only locking that can work with an arbitrary unknown threads package.

I have some ideas for a much better fix in the longer term.  It
would eliminate all locking inside the dynamic linker by making it
safe for symbol lookups and lazy binding to proceed in parallel
with a call to dlopen or dlclose.  This means that the only mutual
exclusion needed would be to prevent multiple simultaneous calls
to dlopen and/or dlclose.  That mutual exclusion could be put into
the native pthreads library.  Applications using foreign threads
packages would have to make their own arrangements to ensure that
they did not have multiple threads in dlopen and/or dlclose -- a
reasonable requirement in my opinion.

MFC after:	3 days

1 files changed, 2 insertions, 9 deletions

diff --git a/libexec/rtld-elf/i386/lockdflt.c b/libexec/rtld-elf/i386/lockdflt.c
index 88dd569..46f8922 100644
--- a/libexec/rtld-elf/i386/lockdflt.c
+++ b/libexec/rtld-elf/i386/lockdflt.c
@@ -46,11 +46,6 @@
  * 80386 we must use simple test-and-set exclusive locks instead.  We
  * determine which kind of lock to use by trying to execute a "cmpxchg"
  * instruction and catching the SIGILL which results on the 80386.
- *
- * These are spinlocks.  When spinning we call nanosleep() for 1
- * microsecond each time around the loop.  This will most likely yield
- * the CPU to other threads (including, we hope, the lockholder) allowing
- * them to make some progress.
  */
 
 #include <setjmp.h>
@@ -71,7 +66,6 @@ typedef struct Struct_Lock {
 	void *base;
 } Lock;
 
-static const struct timespec usec = { 0, 1000 };	/* 1 usec. */
 static sigset_t fullsigmask, oldsigmask;
 
 static inline int
@@ -153,7 +147,7 @@ lock80386_acquire(void *lock)
 	    break;
 	sigprocmask(SIG_SETMASK, &tmp_oldsigmask, NULL);
 	while (l->lock != 0)
-	    nanosleep(&usec, NULL);
+	    ;	/* Spin */
     }
     oldsigmask = tmp_oldsigmask;
 }
@@ -177,7 +171,7 @@ rlock_acquire(void *lock)
 
     atomic_add_int(&l->lock, RC_INCR);
     while (l->lock & WAFLAG)
-	    nanosleep(&usec, NULL);
+	    ;	/* Spin */
 }
 
 static void
@@ -191,7 +185,6 @@ wlock_acquire(void *lock)
 	if (cmpxchgl(0, WAFLAG, &l->lock) == 0)
 	    break;
 	sigprocmask(SIG_SETMASK, &tmp_oldsigmask, NULL);
-	nanosleep(&usec, NULL);
     }
     oldsigmask = tmp_oldsigmask;
 }