We were testing logback's new <configuration scan="true"> feature and when we changed the logging level while our application was under load, all threads became blocked inside logback (full stack trace is very long, here are two examples):

"http-0.0.0.0-8080-148" daemon prio=10 tid=0x00002aab58679800 nid=0x26ed waiting for monitor entry [0x000000004e617000..0x000000004e618c10]
java.lang.Thread.State: BLOCKED (on object monitor)
at ch.qos.logback.classic.turbo.ReconfigureOnChangeFilter.decide(ReconfigureOnChangeFilter.java:99)

• waiting to lock <0x00002aaabd743d58> (a ch.qos.logback.classic.LoggerContext)
  at ch.qos.logback.classic.spi.TurboFilterList.getTurboFilterChainDecision(TurboFilterList.java:47)
  at ch.qos.logback.classic.LoggerContext.getTurboFilterChainDecision_0_3OrMore(LoggerContext.java:251)
  at ch.qos.logback.classic.Logger.callTurboFilters(Logger.java:770)
  at ch.qos.logback.classic.Logger.isDebugEnabled(Logger.java:488)
  at ch.qos.logback.classic.Logger.isDebugEnabled(Logger.java:484)
  at org.slf4j.ext.LoggerWrapper.debug(LoggerWrapper.java:238)
  at com.diginsite.product.ofx.presentation.CustomRequestWrapper.addCNtoOFXRequest(OFXFilter.java:191)
  at com.diginsite.product.ofx.presentation.CustomRequestWrapper.<init>(OFXFilter.java:153)
  at com.diginsite.product.ofx.presentation.OFXFilter.doFilter(OFXFilter.java:56)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
  at org.hyperic.hq.product.servlet.filter.JMXFilter.doFilter(JMXFilter.java:324)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
  at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
  at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
  at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
  at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:182)
  at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:84)
  at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
  at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
  at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:157)
  at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
  at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:262)
  at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
  at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
  at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:446)
  at java.lang.Thread.run(Thread.java:619)

"http-0.0.0.0-8080-15" daemon prio=10 tid=0x00002aab600b0400 nid=0x5669 waiting on condition [0x0000000046799000..0x000000004679ac10]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)

• parking to wait for <0x00002aaabd755128> (a java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:747)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireShared(AbstractQueuedSynchronizer.java:877)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireShared(AbstractQueuedSynchronizer.java:1197)
  at java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock.lock(ReentrantReadWriteLock.java:594)
  at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:57)
  at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:271)
  at ch.qos.logback.classic.Logger.callAppenders(Logger.java:258)
  at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:440)
  at ch.qos.logback.classic.Logger.filterAndLog_0_Or3Plus(Logger.java:394)
  at ch.qos.logback.classic.Logger.log(Logger.java:813)
  at org.slf4j.ext.EventLogger.logEvent(EventLogger.java:36)
  at com.diginsite.services.logging.LoggerUtil.logEvent(LoggerUtil.java:29)