Date:	Thu, 28 Mar 2013 19:50:47 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Rik van Riel <riel@...riel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	torvalds@...ux-foundation.org, davidlohr.bueso@...com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	hhuang@...hat.com, jason.low2@...com, lwoodman@...hat.com,
	chegu_vinod@...com, Dave Jones <davej@...hat.com>,
	benisty.e@...il.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 -mm -next] ipc,sem: fix lockdep false positive

On Thu, Mar 28, 2013 at 1:23 PM, Rik van Riel <riel@...riel.com> wrote:
> Subject: [PATCH -mm -next] ipc,sem: change locking scheme to make lockdep happy
>
> Unfortunately the locking scheme originally proposed has false positives
> with lockdep.  This can be fixed by changing the code to only ever take
> one lock, and making sure that other relevant locks are not locked, before
> entering a critical section.
>
> For the "global lock" case, this is done by taking the sem_array lock,
> and then (potentially) waiting for all the semaphore's spinlocks to be
> unlocked.
>
> For the "local lock" case, we wait on the sem_array's lock to be free,
> before taking the semaphore local lock. To prevent races, we need to
> check again after we have taken the local lock.
>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Signed-off-by: Rik van Riel <riel@...hat.com>

TL;DR: The locking algorithm is not familiar for me, but it seems
sound. There are some implementation details I don't like. More
below...

> ---
>  ipc/sem.c |   55 ++++++++++++++++++++++++++++++++++++++++---------------
>  1 files changed, 40 insertions(+), 15 deletions(-)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 36500a6..87b74d5 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -320,24 +320,39 @@ void __init sem_init (void)
>  }
>
>  /*
> - * If the sem_array contains just one semaphore, or if multiple
> - * semops are performed in one syscall, or if there are complex
> - * operations pending, the whole sem_array is locked.
> - * If one semop is performed on an array with multiple semaphores,
> - * get a shared lock on the array, and lock the individual semaphore.
> + * If the request contains only one semaphore operation, and there are
> + * no complex transactions pending, lock only the semaphore involved.
> + * Otherwise, lock the entire semaphore array, since we either have
> + * multiple semaphores in our own semops, or we need to look at
> + * semaphores from other pending complex operations.
>   *
>   * Carefully guard against sma->complex_count changing between zero
>   * and non-zero while we are spinning for the lock. The value of
>   * sma->complex_count cannot change while we are holding the lock,
>   * so sem_unlock should be fine.
> + *
> + * The global lock path checks that all the local locks have been released,
> + * checking each local lock once. This means that the local lock paths
> + * cannot start their critical sections while the global lock is held.
>   */
>  static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
>                               int nsops)
>  {
>         int locknum;
> + again:
>         if (nsops == 1 && !sma->complex_count) {
>                 struct sem *sem = sma->sem_base + sops->sem_num;
>
> +               /*
> +                * Another process is holding the global lock on the
> +                * sem_array. Wait for that process to release the lock,
> +                * before acquiring our lock.
> +                */
> +               if (unlikely(spin_is_locked(&sma->sem_perm.lock))) {
> +                       spin_unlock_wait(&sma->sem_perm.lock);
> +                       goto again;
> +               }
> +

So, there are a few things I don't like about spin_unlock_wait():

1- From a lock ordering point of view, it is strictly equivalent to
taking the lock and then releasing it - and yet, lockdep won't catch
any deadlocks that involve spin_unlock_wait. (Not your fault here,
this should be fixed as a separate change in lockdep. I manually
looked at the lock ordering here and found it safe).

2- With the current ticket lock implementation, a stream of lockers
can starve spin_unlock_wait() forever. Once again, not your fault and
I suspect this could be fixed - I expect spin_unlock_wait() callers
actually only want to know that the lock has been passed on, not that
it actually got to an unlocked state.

3- Regarding your actual use here - I find it confusing to call
spin_unlock_wait() before holding any other lock. The pattern I expect
to see is that people take one lock, then see that the other lock they
want is already taken, so they release the first lock and wait on the
second. So, I'd suggest we remove the sem_perm.lock checks here and
deal with this in a retry path later down.

>                 /* Lock just the semaphore we are interested in. */
>                 spin_lock(&sem->lock);
>
> @@ -347,17 +362,33 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
>                  */
>                 if (unlikely(sma->complex_count)) {
>                         spin_unlock(&sem->lock);
> -                       goto lock_all;
> +                       goto lock_array;
> +               }
> +
> +               /*
> +                * Another process is holding the global lock on the
> +                * sem_array; we cannot enter our critical section,
> +                * but have to wait for the global lock to be released.
> +                */
> +               if (unlikely(spin_is_locked(&sma->sem_perm.lock))) {
> +                       spin_unlock(&sem->lock);
> +                       goto again;

This is IMO where the spin_unlock_wait(&sma->sem_perm.lock) would
belong - right before the goto again.

Also - I think there is a risk that an endless stream of complex
semops could starve a simple semop here, as it would always find the
sem_perm.lock to be locked ??? One easy way to guarantee progress
would be to goto lock_array instead; however there is then the issue
that a complex semop could force an endless stream of following simple
semops to take the lock_array path. I'm not sure which of these
problems is preferable to have...

>                 }
> +
>                 locknum = sops->sem_num;
>         } else {
>                 int i;
> -               /* Lock the sem_array, and all the semaphore locks */
> - lock_all:
> +               /*
> +                * Lock the semaphore array, and wait for all of the
> +                * individual semaphore locks to go away.  The code
> +                * above ensures no new single-lock holders will enter
> +                * their critical section while the array lock is held.
> +                */
> + lock_array:
>                 spin_lock(&sma->sem_perm.lock);
>                 for (i = 0; i < sma->sem_nsems; i++) {
>                         struct sem *sem = sma->sem_base + i;
> -                       spin_lock(&sem->lock);
> +                       spin_unlock_wait(&sem->lock);
>                 }
>                 locknum = -1;
>         }

Subtle, but it'll work (modulo the starvation issue I mentioned).

Cheers,

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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