From: Hans Peter Jepsen <hpj@lodam.com>
To: 'eCos Discussion' <ecos-discuss@sourceware.org>
Subject: [ECOS] FW: Very serious problem with Cyg_Mutex::trylock(), when eCos is built with CYGDBG_USE_ASSERTS
Date: Wed, 19 Nov 2014 15:08:00 -0000 [thread overview]
Message-ID: <8BA14503034EAE47B93AE568C5A9B47097CF330F@exch1srv.lodam.local> (raw)
Hi
After a few days of serious debugging, we think, that we have found, that eCos has a very serious problem with Cyg_Mutex::trylock(), when eCos is built with CYGDBG_USE_ASSERTS.
The problem has the impact, that when using cyg_mutex_trylock(), the system might die with an assert failed, although there is no reason for it.
Here is, what we have found:
When build with CYGDBG_USE_ASSERTS the Cyg_Mutex::check_this() method will check, that there is consistency between the instance variables "locked" and "owner". See code below (from file packages/kernel/.../src/sync/mutex.cxx). However, since this consistency check is not atomic, is must only be done, when thread switching is prohibited.
#ifdef CYGDBG_USE_ASSERTS
cyg_bool
Cyg_Mutex::check_this( cyg_assert_class_zeal zeal) const
{
// CYG_REPORT_FUNCTION();
// check that we have a non-NULL pointer first
if( this == NULL ) return false;
switch( zeal )
{
case cyg_system_test:
case cyg_extreme:
case cyg_thorough:
case cyg_quick:
case cyg_trivial:
if( locked && owner == NULL ) return false;
if( !locked && owner != NULL ) return false;
case cyg_none:
default:
break;
};
return true;
}
#endif
This checking is used in the macro call CYG_ASSERTCLASS( this, "Bad this pointer") .
In Cyg_Mutex::trylock() the checking is done, before launching Cyg_Scheduler::lock() :
cyg_bool
Cyg_Mutex::trylock(void)
{
CYG_REPORT_FUNCTYPE("returning %d");
cyg_bool result = true;
CYG_ASSERTCLASS( this, "Bad this pointer");
// Prevent preemption
Cyg_Scheduler::lock();
We ended up using Cyg_Mutex::lock() instead, where the order of Cyg_Scheduler::lock() and CYG_ASSERTCLASS( this, "Bad this pointer") in our eyes are correct:
cyg_bool
Cyg_Mutex::lock(void)
{
CYG_REPORT_FUNCTYPE("returning %d");
cyg_bool result = true;
Cyg_Thread *self = Cyg_Thread::self();
// Prevent preemption
Cyg_Scheduler::lock();
CYG_ASSERTCLASS( this, "Bad this pointer");
Kind regards
Hans Peter Jepsen
Lodam Electronics, Sønderborg, Denmark
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