linux-kernel - Re: [PATCH 1/2] ipc semaphores: reduce ipc

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Date:	Tue, 13 Apr 2010 19:15:30 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Chris Mason <chris.mason@...cle.com>
CC:	zach.brown@...cle.com, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop

Hi Chris,


On 04/12/2010 08:49 PM, Chris Mason wrote:
>   /*
> + * when a semaphore is modified, we want to retry the series of operations
> + * for anyone that was blocking on that semaphore.  This breaks down into
> + * a few different common operations:
> + *
> + * 1) One modification releases one or more waiters for zero.
> + * 2) Many waiters are trying to get a single lock, only one will get it.
> + * 3) Many modifications to the count will succeed.
> + *
>    
Have you thought about odd corner cases:
Nick noticed the last time that it is possible to wait for arbitrary values:
in one semop:
     - decrease semaphore 5 by 10
     - wait until semaphore 5 is 0
     - increase semaphore 5 by 10.

>   SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>   		unsigned, nsops, const struct timespec __user *, timeout)
>   {
> @@ -1129,6 +1306,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>   	struct sem_queue queue;
>   	unsigned long jiffies_left = 0;
>   	struct ipc_namespace *ns;
> +	struct sem *blocker = NULL;
> +	LIST_HEAD(pending);
>
>   	ns = current->nsproxy->ipc_ns;
>
> @@ -1168,6 +1347,14 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>   			alter = 1;
>   	}
>
> +	/*
> +	 * try_atomic_semop takes all the locks of all the semaphores in
> +	 * the sops array.  We have to make sure we don't deadlock if userland
> +	 * happens to send them out of order, so we sort them by semnum.
> +	 */
> +	if (nsops>  1)
> +		sort(sops, nsops, sizeof(*sops), sembuf_compare, NULL);
> +
>    
Does sorting preserve the behavior?
--
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