Date: Thu, 25 Sep 2008 11:45:20 +0800 From: David Xu <davidxu@freebsd.org> To: Amol Dixit <dixit@netapp.com> Cc: freebsd-threads@freebsd.org Subject: Re: SIGTRAP during thr_new syscall Message-ID: <48DB0950.3060405@freebsd.org> In-Reply-To: <48DAABAF.9030709@netapp.com> References: <48DAABAF.9030709@netapp.com>
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Amol Dixit wrote: > Hi, > I am seeing an unexpected SIGTRAP being reported to gdbserver when the > debugged process creates a new thread via the _pthread_create() call of > libthr library. [libthr/thread/thr_create.c,v 1.22.4.1, Freebsd 6.0] > Gdbserver has internally set a breakpoint on address of > _thread_bp_create() so that it gets notified on thread creation and is > expecting a SIGTRAP at address (stop pc) of _thread_bp_create(). But > instead SIGTRAP happens as a side-effect of thr_new() system call and > the stop pc at that point is that of routine thread_start() which is the > starting function of new thread. So gdbserver cannot match expected > breakpoint (ie. _thread_bp_create) and is confused. > For testing purpose, if I call _thread_bp_create() before thr_new() in > _pthread_create(), I get the _expected_ SIGTRAP with address of > _thread_bp_create. But that is not the fix. > Does anyone have any idea about this SIGTRAP being reported to tracing > process gdbserver as part of thr_new? Where is it originating from and why? > Thanks, > Amol > I found kernel clears trap flag for new process but not for new thread in cpu_fork(), you may try following patch: Index: i386/i386/vm_machdep.c =================================================================== --- i386/i386/vm_machdep.c (revision 183337) +++ i386/i386/vm_machdep.c (working copy) @@ -413,6 +413,15 @@ bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe)); /* + * If the current thread has the trap bit set (i.e. a debugger had + * single stepped the process to the system call), we need to clear + * the trap flag from the new frame. Otherwise, the new thread will + * receive a (likely unexpected) SIGTRAP when it executes the first + * instruction after returning to userland. + */ + td->td_frame->tf_eflags &= ~PSL_T; + + /* * Set registers for trampoline to user mode. Leave space for the * return address on stack. These are the kernel mode register values. */
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