Index: src/printf.c
==================================================================
--- src/printf.c
+++ src/printf.c
@@ -934,30 +934,44 @@
   sqlite3VXPrintf(&acc, 0, zFormat, ap);
   va_end(ap);
   z = sqlite3StrAccumFinish(&acc);
   return z;
 }
+
+/*
+** This is the routine that actually formats the sqlite3_log() message.
+** We house it in a separate routine from sqlite3_log() to avoid using
+** stack space on small-stack systems when logging is disabled.
+**
+** sqlite3_log() must render into a static buffer.  It cannot dynamically
+** allocate memory because it might be called while the memory allocator
+** mutex is held.
+*/
+static void renderLogMsg(int iErrCode, const char *zFormat, va_list ap){
+  StrAccum acc;                           /* String accumulator */
+#ifdef SQLITE_SMALL_STACK
+  char zMsg[150];                         /* Complete log message */
+#else
+  char zMsg[400];                         /* Complete log message */
+#endif
+
+  sqlite3StrAccumInit(&acc, zMsg, sizeof(zMsg), 0);
+  acc.useMalloc = 0;
+  sqlite3VXPrintf(&acc, 0, zFormat, ap);
+  sqlite3GlobalConfig.xLog(sqlite3GlobalConfig.pLogArg, iErrCode,
+                           sqlite3StrAccumFinish(&acc));
+}
 
 /*
 ** Format and write a message to the log if logging is enabled.
 */
 void sqlite3_log(int iErrCode, const char *zFormat, ...){
-  void (*xLog)(void*, int, const char*);  /* The global logger function */
-  void *pLogArg;                          /* First argument to the logger */
   va_list ap;                             /* Vararg list */
-  char *zMsg;                             /* Complete log message */
-  
-  xLog = sqlite3GlobalConfig.xLog;
-  if( xLog && zFormat ){
+  if( sqlite3GlobalConfig.xLog ){
     va_start(ap, zFormat);
-    sqlite3BeginBenignMalloc();
-    zMsg = sqlite3_vmprintf(zFormat, ap);
-    sqlite3EndBenignMalloc();
+    renderLogMsg(iErrCode, zFormat, ap);
     va_end(ap);
-    pLogArg = sqlite3GlobalConfig.pLogArg;
-    xLog(pLogArg, iErrCode, zMsg ? zMsg : zFormat);
-    sqlite3_free(zMsg);
   }
 }
 
 #if defined(SQLITE_DEBUG)
 /*

Index: src/sqlite.h.in
==================================================================
--- src/sqlite.h.in
+++ src/sqlite.h.in
@@ -5651,10 +5651,16 @@
 **
 ** The sqlite3_log() interface is intended for use by extensions such as
 ** virtual tables, collating functions, and SQL functions.  While there is
 ** nothing to prevent an application from calling sqlite3_log(), doing so
 ** is considered bad form.
+**
+** To avoid deadlocks and other threading problems, the sqlite3_log() routine
+** will not use dynamically allocated memory.  The log message is stored in
+** a fixed-length buffer on the stack.  If the log message is longer than
+** a few hundred characters, it will be truncated to the length of the
+** buffer.
 */
 void sqlite3_log(int iErrCode, const char *zFormat, ...);
 
 /*
 ** Undo the hack that converts floating point types to integer for