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Subject[RFC][PATCH 3/5] signal: Reduce sighand->siglock hold time in get_signal_to_deliver()
Date
From: Matt Fleming <matt.fleming@intel.com>

We don't need to hold sighand->siglock across large chunks of code in
the signal delivery path. Instead we can just acquire the lock when
we're modifying a data structure that is protected by it. In other
words, we're pushing all the locking down into the functions that need
it, which allows us to get rid of dequeue_signal_lock() and stop
playing asymmetric locking games, e.g. in do_signal_stop() where we
may or may not return with the siglock held.

This patch greatly reduces the contention on the per-process siglock.

Cc: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Anirudh Badam <abadam@cs.princeton.edu>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
---
drivers/block/nbd.c | 2 +-
drivers/usb/gadget/f_mass_storage.c | 2 +-
drivers/usb/gadget/file_storage.c | 2 +-
fs/jffs2/background.c | 2 +-
fs/signalfd.c | 5 -
include/linux/sched.h | 12 ---
kernel/signal.c | 157 +++++++++++++++++++++--------------
7 files changed, 100 insertions(+), 82 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index f533f33..06aa43e 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -199,7 +199,7 @@ static int sock_xmit(struct nbd_device *lo, int send, void *buf, int size,
siginfo_t info;
printk(KERN_WARNING "nbd (pid %d: %s) got signal %d\n",
task_pid_nr(current), current->comm,
- dequeue_signal_lock(current, &current->blocked, &info));
+ dequeue_signal(current, &current->blocked, &info));
result = -EINTR;
sock_shutdown(lo, !send);
break;
diff --git a/drivers/usb/gadget/f_mass_storage.c b/drivers/usb/gadget/f_mass_storage.c
index 5b93395..8688de2 100644
--- a/drivers/usb/gadget/f_mass_storage.c
+++ b/drivers/usb/gadget/f_mass_storage.c
@@ -2463,7 +2463,7 @@ static void handle_exception(struct fsg_common *common)
*/
for (;;) {
int sig =
- dequeue_signal_lock(current, &current->blocked, &info);
+ dequeue_signal(current, &current->blocked, &info);
if (!sig)
break;
if (sig != SIGUSR1) {
diff --git a/drivers/usb/gadget/file_storage.c b/drivers/usb/gadget/file_storage.c
index 639e14a..89206e5 100644
--- a/drivers/usb/gadget/file_storage.c
+++ b/drivers/usb/gadget/file_storage.c
@@ -2896,7 +2896,7 @@ static void handle_exception(struct fsg_dev *fsg)
/* Clear the existing signals. Anything but SIGUSR1 is converted
* into a high-priority EXIT exception. */
for (;;) {
- sig = dequeue_signal_lock(current, &current->blocked, &info);
+ sig = dequeue_signal(current, &current->blocked, &info);
if (!sig)
break;
if (sig != SIGUSR1) {
diff --git a/fs/jffs2/background.c b/fs/jffs2/background.c
index 404111b..127f511 100644
--- a/fs/jffs2/background.c
+++ b/fs/jffs2/background.c
@@ -122,7 +122,7 @@ static int jffs2_garbage_collect_thread(void *_c)
if (try_to_freeze())
goto again;

- signr = dequeue_signal_lock(current, &current->blocked, &info);
+ signr = dequeue_signal(current, &current->blocked, &info);

switch(signr) {
case SIGSTOP:
diff --git a/fs/signalfd.c b/fs/signalfd.c
index 492465b..728681d 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -144,7 +144,6 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, siginfo_t *info,
ssize_t ret;
DECLARE_WAITQUEUE(wait, current);

- spin_lock_irq(&current->sighand->siglock);
ret = dequeue_signal(current, &ctx->sigmask, info);
switch (ret) {
case 0:
@@ -152,7 +151,6 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, siginfo_t *info,
break;
ret = -EAGAIN;
default:
- spin_unlock_irq(&current->sighand->siglock);
return ret;
}

@@ -166,11 +164,8 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, siginfo_t *info,
ret = -ERESTARTSYS;
break;
}
- spin_unlock_irq(&current->sighand->siglock);
schedule();
- spin_lock_irq(&current->sighand->siglock);
}
- spin_unlock_irq(&current->sighand->siglock);

remove_wait_queue(&current->sighand->signalfd_wqh, &wait);
__set_current_state(TASK_RUNNING);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f067bbc..e4cd6bb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2145,18 +2145,6 @@ extern void ignore_signals(struct task_struct *);
extern void flush_signal_handlers(struct task_struct *, int force_default);
extern int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info);

-static inline int dequeue_signal_lock(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
-{
- unsigned long flags;
- int ret;
-
- spin_lock_irqsave(&tsk->sighand->siglock, flags);
- ret = dequeue_signal(tsk, mask, info);
- spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
-
- return ret;
-}
-
extern void block_all_signals(int (*notifier)(void *priv), void *priv,
sigset_t *mask);
extern void unblock_all_signals(void);
diff --git a/kernel/signal.c b/kernel/signal.c
index 7f2de6d..b69c5a9 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -632,13 +632,14 @@ static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
/*
* Dequeue a signal and return the element to the caller, which is
* expected to free it.
- *
- * All callers have to hold the siglock.
*/
int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
{
+ unsigned long flags;
int signr;

+ spin_lock_irqsave(&current->sighand->siglock, flags);
+
/* We only dequeue private signals from ourselves, we don't let
* signalfd steal them
*/
@@ -672,8 +673,10 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
}

recalc_sigpending();
- if (!signr)
+ if (!signr) {
+ spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
return 0;
+ }

if (unlikely(sig_kernel_stop(signr))) {
/*
@@ -690,17 +693,12 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
*/
current->jobctl |= JOBCTL_STOP_DEQUEUED;
}
- if ((info->si_code & __SI_MASK) == __SI_TIMER && info->si_sys_private) {
- /*
- * Release the siglock to ensure proper locking order
- * of timer locks outside of siglocks. Note, we leave
- * irqs disabled here, since the posix-timers code is
- * about to disable them again anyway.
- */
- spin_unlock(&tsk->sighand->siglock);
+
+ spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
+
+ if ((info->si_code & __SI_MASK) == __SI_TIMER && info->si_sys_private)
do_schedule_next_timer(info);
- spin_lock(&tsk->sighand->siglock);
- }
+
return signr;
}

@@ -1997,16 +1995,15 @@ void ptrace_notify(int exit_code)
* If %JOBCTL_STOP_PENDING is not set yet, initiate group stop with @signr
* and participate in it. If already set, participate in the existing
* group stop. If participated in a group stop (and thus slept), %true is
- * returned with siglock released.
+ * returned.
*
* If ptraced, this function doesn't handle stop itself. Instead,
- * %JOBCTL_TRAP_STOP is scheduled and %false is returned with siglock
- * untouched. The caller must ensure that INTERRUPT trap handling takes
- * places afterwards.
+ * %JOBCTL_TRAP_STOP is scheduled and %false is returned. The caller
+ * must ensure that INTERRUPT trap handling takes places afterwards.
*
* CONTEXT:
- * Must be called with @current->sighand->siglock held, which is released
- * on %true return.
+ * Must be called with @current->sighand->siglock held, which may be
+ * released and re-acquired before returning with intervening sleep.
*
* RETURNS:
* %false if group stop is already cancelled or ptrace trap is scheduled.
@@ -2014,6 +2011,7 @@ void ptrace_notify(int exit_code)
*/
static bool do_signal_stop(int signr)
__releases(&current->sighand->siglock)
+ __acquires(&current->sighand->siglock)
{
struct signal_struct *sig = current->signal;

@@ -2105,6 +2103,8 @@ static bool do_signal_stop(int signr)

/* Now we don't run again until woken by SIGCONT or SIGKILL */
schedule();
+
+ spin_lock_irq(&current->sighand->siglock);
return true;
} else {
/*
@@ -2241,31 +2241,26 @@ static void unlock_action(struct sighand_struct *sighand, bool write_locked)
read_unlock(&sighand->action_lock);
}

-int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
- struct pt_regs *regs, void *cookie)
+/**
+ * __notify_parent - should we notify the parent of a stop/continue?
+ * @task: task that needs to notify parent
+ *
+ * Check to see if we should notify the parent, prepare_signal(SIGCONT)
+ * encodes the CLD_ si_code into %SIGNAL_CLD_MASK bits.
+ *
+ * RETURNS:
+ * %false if we didn't notify the parent
+ * %true if we did notify the parent
+ */
+static inline bool __notify_parent(struct task_struct *task)
{
- struct sighand_struct *sighand = current->sighand;
- struct signal_struct *signal = current->signal;
- int signr;
-
-relock:
- /*
- * We'll jump back here after any time we were stopped in TASK_STOPPED.
- * While in TASK_STOPPED, we were considered "frozen enough".
- * Now that we woke up, it's crucial if we're supposed to be
- * frozen that we freeze now before running anything substantial.
- */
- try_to_freeze();
+ struct sighand_struct *sighand = task->sighand;
+ struct signal_struct *signal = task->signal;
+ int why;

spin_lock_irq(&sighand->siglock);
- /*
- * Every stopped thread goes here after wakeup. Check to see if
- * we should notify the parent, prepare_signal(SIGCONT) encodes
- * the CLD_ si_code into SIGNAL_CLD_MASK bits.
- */
- if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
- int why;

+ if (signal->flags & SIGNAL_CLD_MASK) {
if (signal->flags & SIGNAL_CLD_CONTINUED)
why = CLD_CONTINUED;
else
@@ -2288,24 +2283,64 @@ relock:

if (ptrace_reparented(current->group_leader))
do_notify_parent_cldstop(current->group_leader,
- true, why);
+ true, why);
read_unlock(&tasklist_lock);

- goto relock;
+ return true;
+ }
+
+ spin_unlock_irq(&sighand->siglock);
+ return false;
+}
+
+int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
+ struct pt_regs *regs, void *cookie)
+{
+ struct sighand_struct *sighand = current->sighand;
+ struct signal_struct *signal = current->signal;
+ int signr;
+
+freeze:
+ /*
+ * We'll jump back here after any time we were stopped in TASK_STOPPED.
+ * While in TASK_STOPPED, we were considered "frozen enough".
+ * Now that we woke up, it's crucial if we're supposed to be
+ * frozen that we freeze now before running anything substantial.
+ */
+ try_to_freeze();
+
+ /*
+ * Every stopped thread goes here after wakeup.
+ */
+ if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
+ if (__notify_parent(current))
+ goto freeze;
}

for (;;) {
struct k_sigaction *ka;
bool write_locked;

- if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
- do_signal_stop(0))
- goto relock;
+ if (unlikely(current->jobctl & JOBCTL_STOP_PENDING)) {
+ bool stopped = false;
+
+ spin_lock_irq(&sighand->siglock);
+ if (current->jobctl & JOBCTL_STOP_PENDING)
+ stopped = do_signal_stop(0);
+ spin_unlock_irq(&sighand->siglock);
+
+ if (stopped)
+ goto freeze;
+ }

if (unlikely(current->jobctl & JOBCTL_TRAP_MASK)) {
- do_jobctl_trap();
+ spin_lock_irq(&sighand->siglock);
+ if (current->jobctl & JOBCTL_TRAP_MASK) {
+ do_jobctl_trap();
+ spin_unlock_irq(&sighand->siglock);
+ goto freeze;
+ }
spin_unlock_irq(&sighand->siglock);
- goto relock;
}

signr = dequeue_signal(current, &current->blocked, info);
@@ -2314,8 +2349,10 @@ relock:
break; /* will return 0 */

if (unlikely(current->ptrace) && signr != SIGKILL) {
+ spin_lock_irq(&sighand->siglock);
signr = ptrace_signal(signr, info,
regs, cookie);
+ spin_unlock_irq(&sighand->siglock);
if (!signr)
continue;
}
@@ -2367,6 +2404,8 @@ relock:
continue;

if (sig_kernel_stop(signr)) {
+ bool stopped;
+
/*
* The default action is to stop all threads in
* the thread group. The job control signals
@@ -2378,20 +2417,18 @@ relock:
* We need to check for that and bail out if necessary.
*/
if (signr != SIGSTOP) {
- spin_unlock_irq(&sighand->siglock);
-
/* signals can be posted during this window */

if (is_current_pgrp_orphaned())
- goto relock;
-
- spin_lock_irq(&sighand->siglock);
+ goto freeze;
}

- if (likely(do_signal_stop(info->si_signo))) {
- /* It released the siglock. */
- goto relock;
- }
+ spin_lock_irq(&sighand->siglock);
+ stopped = do_signal_stop(info->si_signo);
+ spin_unlock_irq(&sighand->siglock);
+
+ if (likely(stopped))
+ goto freeze;

/*
* We didn't actually stop, due to a race
@@ -2400,8 +2437,6 @@ relock:
continue;
}

- spin_unlock_irq(&sighand->siglock);
-
/*
* Anything else is fatal, maybe with a core dump.
*/
@@ -2427,7 +2462,7 @@ relock:
do_group_exit(info->si_signo);
/* NOTREACHED */
}
- spin_unlock_irq(&sighand->siglock);
+
return signr;
}

@@ -2795,7 +2830,6 @@ int do_sigtimedwait(const sigset_t *which, siginfo_t *info,
sigdelsetmask(&mask, sigmask(SIGKILL) | sigmask(SIGSTOP));
signotset(&mask);

- spin_lock_irq(&tsk->sighand->siglock);
sig = dequeue_signal(tsk, &mask, info);
if (!sig && timeout) {
/*
@@ -2804,6 +2838,7 @@ int do_sigtimedwait(const sigset_t *which, siginfo_t *info,
* they arrive. Unblocking is always fine, we can avoid
* set_current_blocked().
*/
+ spin_lock_irq(&tsk->sighand->siglock);
tsk->real_blocked = tsk->blocked;
sigandsets(&tsk->blocked, &tsk->blocked, &mask);
recalc_sigpending();
@@ -2814,9 +2849,9 @@ int do_sigtimedwait(const sigset_t *which, siginfo_t *info,
spin_lock_irq(&tsk->sighand->siglock);
__set_task_blocked(tsk, &tsk->real_blocked);
siginitset(&tsk->real_blocked, 0);
+ spin_unlock_irq(&tsk->sighand->siglock);
sig = dequeue_signal(tsk, &mask, info);
}
- spin_unlock_irq(&tsk->sighand->siglock);

if (sig)
return sig;
--
1.7.4.4


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