linux-kernel - Re: [PATCH 08/12] add trace events for each syscall entry/exit

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Message-ID: <20090825170441.GD2656@redhat.com>
Date:	Tue, 25 Aug 2009 13:04:41 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org, jiayingz@...gle.com,
	mbligh@...gle.com, lizf@...fujitsu.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 08/12] add trace events for each syscall entry/exit

On Tue, Aug 25, 2009 at 12:20:04PM -0400, Mathieu Desnoyers wrote:
> 
> Uh ? kernel threads can invoke a system call. There are rare places
> where kernel code actually invoke system calls. I don't see why we
> should not deal with them.
> 
> Moreover, the problem you face is more general: if we set the
> TIF_SYSCALL_FTRACE flag of a standard thread right in the middle of its
> system call, x86_64 will cause the syscall exit to execute by re-reading
> the thread flags and run a syscall trace exit.
> 
> We could simply initialize the "saved system calls id" number to
> something like -1, so that if we happen to return from a syscall that
> did not get its id recorded at syscall entry, we know it because it's
> not initialized.
> 
> We would need to carefully put back the -1 value after clearing the
> thread flag when we stop tracing too (while still holding a mutex).
> 
> Mathieu
> 

why can't we have a syscall exit that is unmatched? we calculate
the exit syscall number for the the pt_regs structure at exit, so we
don't need to match it up with an entry to know which syscall it is.

thanks,

-Jason
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