Avoid a mutex in-and-out in unixShmBarrier() on this branch. Use

__sync_synchronize() instead.

FossilOrigin-Name: 280d1a72dc269b4402853159949d7a905d414786074f5d80e951c9341ebbd1b5

manifest

@@ -1,5 +1,5 @@
-C Add\sthe\smutexfree-shmlock\sexperiment\sto\sthe\sbegin-concurrent-pnu\sbranch.
-D 2018-12-06T20:26:13.389
+C Avoid\sa\smutex\sin-and-out\sin\sunixShmBarrier()\son\sthis\sbranch.\sUse\n__sync_synchronize()\sinstead.
+D 2018-12-10T09:36:34.858
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F Makefile.in 68d0ba0f0b533d5bc84c78c13a6ce84ee81183a67014caa47a969e67f028fa1c
@@ -495,7 +495,7 @@ F src/os.c 8aeb0b0f40f8f5b0da03fe49706695adaf42d2f516ab95abc72e86c245e119de
 F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c 1ef4b733679ea62fa4b787dc33af95c28dca459c0a0e5cf815698104f63ceffa
+F src/os_unix.c 4cd8758cbd16ce1acc3559ff9cd5c78d7570e88076ef98b0d2601a75fee0146c
 F src/os_win.c 85d9e532d0444ab6c16d7431490c2e279e282aa0917b0e988996b1ae0de5c5a0
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c 79b8ced46a3c32be19ab4b00d8836292e460411d283a333cb008923c69fbca6a
@@ -1796,10 +1796,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P f2083ee410b7504e8478f7373e76ded913e3a6a92cb7345b8c1ac27554f6edf8 8f4cb9dd3396bcaaf85dcdb4e3ae3c96f28a4d71d72665c4abf7c221370be626
-R f4f10a25e69698e14c2c4db650b11fdc
-T *branch * begin-concurrent-pnu-mutexfree-shmlock
-T *sym-begin-concurrent-pnu-mutexfree-shmlock *
-T -sym-begin-concurrent-pnu *
+P 3aead209b4040c6c682fb892ee593219e52f766cd8da49bd7ad2994645e58433
+R bcc800104bf27a3aabac69060664e6d8
 U dan
-Z c57d12eda52ad2bd268eef9bce5b903a
+Z eacacd23271d083db31c16e8c29f76f3

manifest.uuid

@@ -1 +1 @@
-3aead209b4040c6c682fb892ee593219e52f766cd8da49bd7ad2994645e58433
+280d1a72dc269b4402853159949d7a905d414786074f5d80e951c9341ebbd1b5

src/os_unix.c

@@ -4996,12 +4996,16 @@ static void unixShmBarrier(
   sqlite3_file *fd                /* Database file holding the shared memory */
 ){
   UNUSED_PARAMETER(fd);
+#ifdef SQLITE_MUTEXFREE_SHMLOCK
+  __sync_synchronize();
+#else
   sqlite3MemoryBarrier();         /* compiler-defined memory barrier */
   assert( fd->pMethods->xLock==nolockLock 
        || unixFileMutexNotheld((unixFile*)fd) 
   );
   unixEnterMutex();               /* Also mutex, for redundancy */
   unixLeaveMutex();
+#endif
 }
 
 /*