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Date: Wed, 19 Jan 2011 21:05:40 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Prasad <prasad@...ux.vnet.ibm.com>,
Roland McGrath <roland@...hat.com>,
linux-kernel@...r.kernel.org
Subject: Re: Q: perf_event && task->ptrace_bps[]
On Wed, Jan 19, 2011 at 04:37:46PM +0100, Oleg Nesterov wrote:
> On 01/18, Frederic Weisbecker wrote:
> > > Probably the best fix is to change this code so that the tracer owns
> > > ->ptrace_bps[], not the tracee. But this is not trivial, and needs a
> > > lot of changes in ptrace code.
> >
> > How much complicated would it be?
>
> The problem is, ptrace_detach/release_task can't sleep currently.
> The necessary changes are nasty.
Ok, let's forget that then :)
> > Because I see three solutions to solve this:
> >
> > - Have a mutex inside thread->ptrace_bps. The contention must be
> > rare and only concern ptrace and tracee exit. That's the simplest.
>
> I think we can reuse perf_event_mutex for this. Not very good too,
> but simple. But this depends on what can we do under this mutex...
That could work. I feel a bit uncomfortable to use a perf related
mutex for that though. I can't figure out any deadlock with the current
state, but if we are going to use that solution, perf events will be
created/destroyed/disabled/enabled under that mutex. May be that large
coverage might create some dependency problems in the future. I don't
know...
Dunno, that doesn't seem to be a good use of perf_event_mutex.
I had another idea based on a refcount. Can you have a look?
The drawback is to add an entry in task_struct. OTOH I can drop
more of them for the no-running-breakpoint case from thread_struct
in a subsequent task.
Note the problem touches more archs than x86. Basically every
arch that use breakpoint use a similar scheme that must be fixed.
(only compile tested yet)
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 45892dc..08d79f9 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -608,6 +608,9 @@ static int ptrace_write_dr7(struct task_struct *tsk, unsigned long data)
unsigned len, type;
struct perf_event *bp;
+ if (ptrace_get_breakpoints(tsk) < 0)
+ return -ESRCH;
+
data &= ~DR_CONTROL_RESERVED;
old_dr7 = ptrace_get_dr7(thread->ptrace_bps);
restore:
@@ -655,6 +658,9 @@ restore:
}
goto restore;
}
+
+ ptrace_put_breakpoints(tsk);
+
return ((orig_ret < 0) ? orig_ret : rc);
}
@@ -668,10 +674,17 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
if (n < HBP_NUM) {
struct perf_event *bp;
+
+ if (ptrace_get_breakpoints(tsk) < 0)
+ return -ESRCH;
+
bp = thread->ptrace_bps[n];
if (!bp)
- return 0;
- val = bp->hw.info.address;
+ val = 0;
+ else
+ val = bp->hw.info.address;
+
+ ptrace_put_breakpoints(tsk);
} else if (n == 6) {
val = thread->debugreg6;
} else if (n == 7) {
@@ -686,6 +699,10 @@ static int ptrace_set_breakpoint_addr(struct task_struct *tsk, int nr,
struct perf_event *bp;
struct thread_struct *t = &tsk->thread;
struct perf_event_attr attr;
+ int err = 0;
+
+ if (ptrace_get_breakpoints(tsk) < 0)
+ return -ESRCH;
if (!t->ptrace_bps[nr]) {
ptrace_breakpoint_init(&attr);
@@ -709,24 +726,23 @@ static int ptrace_set_breakpoint_addr(struct task_struct *tsk, int nr,
* writing for the user. And anyway this is the previous
* behaviour.
*/
- if (IS_ERR(bp))
- return PTR_ERR(bp);
+ if (IS_ERR(bp)) {
+ err = PTR_ERR(bp);
+ goto put;
+ }
t->ptrace_bps[nr] = bp;
} else {
- int err;
-
bp = t->ptrace_bps[nr];
attr = bp->attr;
attr.bp_addr = addr;
err = modify_user_hw_breakpoint(bp, &attr);
- if (err)
- return err;
}
-
- return 0;
+put:
+ ptrace_put_breakpoints(tsk);
+ return err;
}
/*
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 092a04f..519f03c 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -192,6 +192,9 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
child->ptrace = current->ptrace;
__ptrace_link(child, current->parent);
}
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+ atomic_set(&child->ptrace_bp_refcnt, 1);
+#endif
}
/**
@@ -352,7 +355,12 @@ static inline void user_single_step_siginfo(struct task_struct *tsk,
extern int task_current_syscall(struct task_struct *target, long *callno,
unsigned long args[6], unsigned int maxargs,
unsigned long *sp, unsigned long *pc);
-
-#endif
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+extern int ptrace_get_breakpoints(struct task_struct *tsk);
+extern void ptrace_put_breakpoints(struct task_struct *tsk);
+#else
+static inline void ptrace_put_breakpoints(struct task_struct *tsk) { }
+#endif /* CONFIG_HAVE_HW_BREAKPOINT */
+#endif /* __KERNEL */
#endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 777cd01..523a1c5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1526,6 +1526,9 @@ struct task_struct {
unsigned long memsw_bytes; /* uncharged mem+swap usage */
} memcg_batch;
#endif
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+ atomic_t ptrace_bp_refcnt;
+#endif
};
/* Future-safe accessor for struct task_struct's cpus_allowed. */
diff --git a/kernel/exit.c b/kernel/exit.c
index 8cb8904..5284464 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1015,7 +1015,7 @@ NORET_TYPE void do_exit(long code)
/*
* FIXME: do that only when needed, using sched_exit tracepoint
*/
- flush_ptrace_hw_breakpoint(tsk);
+ ptrace_put_breakpoints(tsk);
exit_notify(tsk, group_dead);
#ifdef CONFIG_NUMA
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 99bbaa3..23394f1 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -22,6 +22,7 @@
#include <linux/syscalls.h>
#include <linux/uaccess.h>
#include <linux/regset.h>
+#include <linux/hw_breakpoint.h>
/*
@@ -876,3 +877,19 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
return ret;
}
#endif /* CONFIG_COMPAT */
+
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+int ptrace_get_breakpoints(struct task_struct *tsk)
+{
+ if (atomic_inc_not_zero(&tsk->ptrace_bp_refcnt))
+ return 0;
+
+ return -1;
+}
+
+void ptrace_put_breakpoints(struct task_struct *tsk)
+{
+ if (!atomic_dec_return(&tsk->ptrace_bp_refcnt))
+ flush_ptrace_hw_breakpoint(tsk);
+}
+#endif /* CONFIG_HAVE_HW_BREAKPOINT */
--
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