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Date: Fri, 22 Mar 2013 16:01:05 -0700
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Subject: Re: [PATCH 7/7] ipc,sem: fine grained locking for semtimedop
From: Michel Lespinasse <walken@google.com>
To: Rik van Riel <riel@surriel.com>
Cc: torvalds@linux-foundation.org, davidlohr.bueso@hp.com,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        hhuang@redhat.com, jason.low2@hp.com, lwoodman@redhat.com,
        chegu_vinod@hp.com, Rik van Riel <riel@redhat.com>
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Sorry for the late reply; I've been swamped and am behind on my upstream mail.

On Wed, Mar 20, 2013 at 12:55 PM, Rik van Riel <riel@surriel.com> wrote:
> +static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
> +                             int nsops)
> +{
> +       int locknum;
> +       if (nsops == 1 && !sma->complex_count) {
> +               struct sem *sem = sma->sem_base + sops->sem_num;
> +
> +               /* Lock just the semaphore we are interested in. */
> +               spin_lock(&sem->lock);
> +
> +               /*
> +                * If sma->complex_count was set while we were spinning,
> +                * we may need to look at things we did not lock here.
> +                */
> +               if (unlikely(sma->complex_count)) {
> +                       spin_unlock(&sma->sem_perm.lock);

I believe this should be spin_unlock(&sem->lock) instead ?

> +                       goto lock_all;
> +               }
> +               locknum = sops->sem_num;
> +       } else {
> +               int i;
> +               /* Lock the sem_array, and all the semaphore locks */
> + lock_all:
> +               spin_lock(&sma->sem_perm.lock);
> +               for (i = 0; i < sma->sem_nsems; i++) {

Do we have to lock every sem from the array instead of just the ones
that are being operated on in sops ?
(I'm not sure either way, as I don't fully understand the queueing of
complex ops)

If we want to keep the loop as is, then we may at least remove the
sops argument to sem_lock() since we only care about nsops.

> +                       struct sem *sem = sma->sem_base + i;
> +                       spin_lock(&sem->lock);
> +               }
> +               locknum = -1;
> +       }
> +       return locknum;
> +}

That's all I have. Very nice test results BTW!

Reviewed-by: Michel Lespinasse <walken@google.com>

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